Formation of our Solar System

Exoplanets can help us understand our own solar system, how it formed, how it evolved and how it came to look the way it does today. Continue reading for a description of how our Solar System came to be.

The Solar System

The basic layout of our solar system has been known for centuries. We have the Sun in the center, surrounded by four rocky planets, two gas giants, and two ice giants. The planets can be seen even with small telescopes, and it was noted early in the history of astronomy that they move in orderly orbits, in near perfect circles, at a fixed distance from the Sun. Based on this observation, it is no surprise that the assumption of fixed orbits has been the bedrock of the study of the solar system since the beginning.

Over the years, this simple tale of the evolution of the solar system has progressed into a story of chaotic migration of the planets, bombardments of asteroids and comets, and potentially the exclusion of a ninth planet. Following the turn of the century, astronomers realized that their model of the formation of the solar system, which assumed that the planets formed exactly where they are now, doesn’t fully agree with what we see. Furthermore, observations of planetary systems around other stars (exoplanets) made us question the formation of our own.

A Star is Born

The story of the solar system starts 4.6 billion years ago with the collapse of a gigantic cloud of gas and dust, known as a giant molecular cloud. The majority of the collapsed cloud collected in the center to form the Sun, and the remaining material flattened out into a rotating disk around the star.  It is out of this disk, that the planets, moons, asteroids and comets formed.

Jupiter Leads the Way

There are two main categories of planets in our solar system: the rocky terrestrial planets in the inner solar system (Mercury, Venus, Earth, and Mars) and the giant gassy and icy planets in the outer reaches (Jupiter Saturn, Uranus, and Neptune). Even though these two ‘types’ of planets appear so different, their formation followed the same initial recipe.


Jupiter. Photo credit: NASA

Like for any recipe, you need ingredients. The main ingredient to build planets is dust that clumps together to build larger rocks. This clumping happens as the gas and dust in the disk orbit around the Sun in swirling and random motions, resulting in many collisions between the individual particles. As the particles collide, they stick together, and eventually, given enough time, form large bodies. Similar to the rolling of a snowball across a field of snow, the larger the ball, the more efficiently it picks up more material and therefore the faster it grows.

A second ingredient is ice. Due to the heat from the Sun, ice grains can only reside beyond a certain point in the solar system, known as the ‘ice line’. As the ice helps the dust grains to stick together, planets can grow more rapidly where this ingredient is available. This is how the gas and ice giants managed to become so large. Jupiter, for example, formed just beyond the ice line. It sucked up over half the material in the disk, and quickly outgrew the other planets, making it the dominant figure in the solar system.

The third main ingredient that went into making Jupiter was gas, which formed a blanket around the planet’s core. The new-born Sun would have heated up the inner solar system and over time blast away the gas in the disk. This means that in order for Jupiter to collect the amount of gas that it did, it must have formed before the gas was ejected by the Sun, in the astronomical blink of an eye of only five million years.

Saturn was also in a favourable position to become large, and grew into a gas giant similar to Jupiter. Further out, Neptune and Uranus, grew to smaller sized and are primarily made out of ice, making them the ice giants.

Unlike the gas and ice giants, the inner-most planets had no gas to claim as their own, and are therefore made out of dust. The so called terrestrial planets, formed through the slow process of colliding particles, becoming larger with every collision until all the material was assembled into four rocky bodies. The accompanying figure shows the relative sizes of the planets.

Other Worlds

This model of the formation of our planets can’t explain all the properties that we observe. Why is Mars so small? Why are Uranus and Neptune so far away from the Sun? Where did the craters on the moon come from? Further questions arose following the discovery of planets around other stars, known as exoplanets, which lead us to question some of our most fundamental assumptions of the solar system.

The first exoplanet was discovered in 1995, and since then we have been pointing our telescope at distant stars in order to study their solar systems. To everyone’s surprise, these other planetary systems looked nothing like our own, and we saw planets in places where they can’t have possibly formed. This was the first hint to suggest that planets are not fixed in their orbits, but take wild journeys around their stars.  It was this observation that made us question the fixed nature of our own planets, and astronomers went back to the drawing board to come up with new chaotic models of the evolution of our solar system (see the image below for our theory of the evolution of the Solar System).

Screenshot 2019-04-29 at 12.15.42.png

Evolution of the Solar System by Nora Eisner

The Grand Tack Model

A new model was proposed, which suggests that Jupiter started to migrate towards the Sun as soon as it formed, during a time when the terrestrial planets were still in their infancy. Saturn followed Jupiter’s inwards path, resulting in the two planets getting closer and closer until Jupiter was completing exactly three orbits for every two of Saturn’s. This alignment halted the inward motion of the giants and forced them back into larger orbits. This is known as the Grand Tack model.

On this adventure, Jupiter travelled inwards to approximately the current orbit of Mars and back out to where we see it now. This movement caused large scale disruption in the solar system. First, by entering the orbit of Mars, Jupiter stole material that would have otherwise contributed to building the small red planet. This explains Mars’s smaller than expected size. Second, it is thought that Jupiter could have halted the formation of an entire other planet by scattering rocks that would have otherwise grown into a full sized planet. Instead of clumping together, a band of rocks remained to orbit the Sun, known as the Asteroid Belt. Finally, as Jupiter migrated into the inner solar system, it snowploughed gas and ice rich bodies across the ice line. These bodies would have been scattered in all directions, bombarding the young terrestrial planets and seeding them with water and other gases. Without this water brought to Earth by Jupiter, we would likely not be here.

Bombardment of the Planets

Once Jupiter and Saturn were back in larger orbits, the inner planets were able to fully grow by accumulating all the matter in their local area of the disk. Uranus and Neptune also grew to their full size, at a distance from the Sun significantly closer than where we see them now. Furthermore, contrary to where they are today, Neptune was in a smaller orbit than Uranus. Beyond the orbit of all the outermost planet resided a thick belt of icy bodies.

These icy bodies tugged at the large planets, one by one over hundreds of millions of years, once again making the orbits of the giants more and more unstable. A point came where the orbits of Jupiter and Saturn were so disrupted that they no longer remained in their stable orbits. Jupiter moved slightly inwards, pushing the other giants outwards in a violent motion that could have propelled one or even two unknown planets out of the solar system entirely. This is known as the Nice model. Neptune was thrown beyond the orbit of Uranus through the belt of ice and rocks, scattering material across the solar system. This attack, called the late heavy bombardment, left scars on the surface the planets and moons.  Our Moon is no exception to this, with many of the craters that we see on it today still remaining from this violent era 4.1 to 3.8 billion years ago.

The Calm

 Following the late heavy bombardment, the planets settled into stable orbits around the Sun where they have remained ever since. Over the course of 20 years, our view of the formation of the solar system changed drastically. As we continue to explore solar systems around distant stars, we will learn more about our own, and continue to solve the mystery of how we got here.

Exploring the impacts of Planet Hunters TESS


With the help of the Public Engagement with Research team at the University of Oxford, we’re conducting a short survey in order to explore the impacts of Planet Hunters TESS and to better understand who takes part and why. We’d love to hear your thoughts at

Thank you to those who have already taken the time to complete the survey.

Evaluating Planet Hunters

By Annaleise Depper

To date, thousands of volunteers worldwide have contributed their time to classify millions of light curves on Planet Hunters in the search for extrasolar planets. With the help of citizen scientists, the Planet Hunters team have been able to find out more about the diversity of planets and to understand what kind of solar systems exist.

But what we don’t know is… What impact does Planet Hunters have on its volunteer community? Who and why do people take part? What are the benefits and barriers?

I’ve been given the task of exploring these questions.

As Public Engagement with Research Evaluation Officer, my role is to support researchers at the University of Oxford to evaluate the impact of their public engagement with research activities.

I was particularly interested in collaborating with the Zooniverse Planet Hunters team as they are working on an innovative citizen science project that is constantly expanding and making scientific research more accessible. I have been inspired by the work the Planet Hunters team are doing to involve, engage and collaborate with citizen scientists worldwide in a very unique way.

Over the coming months I will be working with the TESS team to launch our evaluation survey in order to explore the views and experiences of volunteers engaging with Planet Hunters. Our aim is to find out:

  • what impact does the platform have on its volunteer community?
  • what are the benefits and potential challenges?
  • how can Planet Hunters become even more inclusive of its growing, diverse community?

Please share your thoughts about Planet Hunters by completing our short online survey:

This should take no longer than 5-10 minutes to complete and your responses will be anonymised. We will share our overall findings with the Planet Hunters community on this blog and the University of Oxford webpages.

If you would like to add any additional comments or thoughts please feel free to email me

We look forward to sharing the results!


Planet or no Planet?

You continue to bring extremely exciting planet candidates to our attention! When we find promising targets there are many steps that we have to go through in order to determine their planetary nature – sometimes the planets pass all our tests, and sometimes they turn out to be false positives. Read on for an awesome summary of how we investigated the possible planet around τ Ceti written by Benjamin Pope from New York University.

But first, none of this would be possible without all your amazing help, your classifications and your Talk discussions.  I would like to say a special thanks to the 15 volunteers who classified this target: Anchel, LarryW, JobiMine, EEZuidema, lvwarren, ElisabethB, TaxiCab1729, LAIS_IONUT_ANTONEL, Jose-Adao, DanielRA37, baconsteven, bugge, gulpfumetti, adam211 and Vidar87, and to andrey373 who brought this candidate up on Talk. We couldn’t have done it without you all!

Notes on τ Ceti

by Benjamin Pope

Outside of our solar system, the nearest solar-like stars are only a few light-years away: the two bright components of the binary system α Centauri AB (orbited by a third component Proxima Centauri, a dim red dwarf). But to find a star like the sun with no companion, you have to look a little further away to τ Ceti (tau Ceti), a G8 dwarf (which means it is a little less massive and cooler than the G2 Sun) which is the second-closest star system visible to the naked eye at a distance of only about 12 light-years.

Such a close system is one of the first targets for bold proposals for interstellar travel and contact, and for science fiction: closest to my heart, in Ursula Le Guin’s The Dispossessed, τ Ceti is home to the twin habitable worlds of Urras and Anarres; the former capitalist, the latter the home for anarchist exiles. But to astronomers since then it has become increasingly exciting as a host for real exoplanets: from radial velocity observations (measuring the red and blue shift in the star’s spectrum as it is tugged back and forth by planets) it has been suspected since 2012 that it hosts a number of exoplanets, with orbital periods of a few weeks to a few years. If one of these transits, it would be a huge discovery – both by independently confirming the existence of these planets, but also because it would open up an unprecedented opportunity to study their atmospheres as they are illuminated by the star behind during transit. τ Ceti is ten times brighter than the next-brightest transiting planet host star and the extra light would be a significant boon to photon-starved spectrographs trying to detect chemistry (and life!) in its atmosphere.

When the data from the Transiting Exoplanet Survey Satellite (TESS) covering τ Ceti came out two weeks ago, I received an email from Prof. Suzanne Aigrain at Oxford, my former DPhil supervisor, saying that the Planet Hunters team had noticed evidence of a transit in the light curve of τ Ceti and asking if I could check this – without knowing when the transit they found occurred, so that I had to replicate the result blind! One of the difficulties is that τ Ceti is very bright, a third magnitude star in a telescope that saturates (overexposes – just like in other cameras) on stars three magnitudes (fifteen times) fainter. In my DPhil, I had worked with Dr Tim White (ANU) to develop the method of ‘halo photometry’ (the code halophot) to deal with this problem for similar data obtained by the previous mission Kepler, which we used to look at the Seven Sisters and the planet-hosting red giant Aldebaran. The way it works is by discarding the unusable ‘saturated’ pixels but looking at the broader distribution of light (the ‘halo’) around the star and teasing out a signal from these many good pixels. So I used this code to look at τ Ceti (and if you want to see how it’s used and the plots below were made, check out the Jupyter notebooks on the GitHub repo!). Running halophot, it produces a huge signal that looks just like a transit (light curve on the left – standard ‘PDC’ pipeline in orange, new halo light curve in blue, halo map on the right):


When I told Suzanne, she confirmed this was exactly when they thought the transit was. So we were on: time to check if it’s real! Suzanne’s postdoc Dr Oscar Barragán modelled the transit signal in the standard PDC data, assuming the transit was equatorial, the planet was in a circular orbit and using the stellar mass and radius to try to estimate a range of valid periods. The depth of the transit signal gives you a planet to star ratio of 0.0108835, translating to a planet radius of 0.94 Earth radii. So this would be by far the closest Earth-sized planet to be known to transit. Meanwhile from the transit duration of ~ 11 hours we get that the minimum orbital period should be ~ 230 days, corresponding to an orbital radius of ~0.7 AU (though somewhat worryingly, not to the known periods of any of the planets found by radial velocity – though it could just be because it has too low a mass to detect). At this distance from the star the planet’s equilibrium temperature would be ~225 K. At nearly fifty degrees C below freezing this is quite cold, but there is a lot of uncertainty about the effects of planetary atmospheres, and to me this is quite exciting – no cool transiting Earths are known around such a nearby star!

Oscar produced this great visualization of his model:


So with such high stakes we had to be very careful. In comparison to the PDC data, the transit in the halo light curve I made was much higher signal-to-noise, but seemed much deeper (a few percent rather than a fraction of a percent). This isn’t necessarily a killer, in that neither the absolute normalization of PDC nor halo light curves of saturated stars is completely accurate, but they are usually much closer than this. First thing to check: halophot doesn’t do anything obviously wrong, and generates a model (on the right of the figure above) that looks rather like the expected pattern of light from a star as seen by TESS. The light curve you get from this has a deep and clean transit, which is maybe a bit long and deep, but looks ok.

What is immediately suspicious, though, is that it occurs just before perigee: TESS has an eccentric orbit in a 2:1 resonance with the moon, which means that twice a month it approaches very close to the Earth briefly (perigee) and then swings back out again to spend most of its orbit far away. When it is at perigee it is subject to a lot of reflected light from the Earth – Earthshine! This is why there is a gap right in the middle of the light curve. So to me it immediately raised alarm bells that this signal happened just when the telescope was most vulnerable to contamination from background light.

To figure out what is going on with the Earthshine, we produce a ‘background’ light curve for τ Ceti using only pixels far away from where the star is contributing much light. Let’s plot this with a vertical line to note the midpoint of the ‘transit’ we found earlier:

tau_ceti_backgroundUh-oh! There is a ‘transit’ signal in the background light, a little later than the transit. This isn’t an absolute killer – τ Ceti is very bright, and it isn’t implausible that its light could have directly contaminated the background or done so via some electronic chip effect (‘cross-talk’). It is also not quite at the same time as our putative planet. But it is pretty alarming.

Let’s look at some less highly processed data. What we have been looking at so far has been a ‘target pixel file’ (TPF) produced with a frame every 2 minutes and a lot of sophisticated calibration. Insted let’s use the TESSCut tool to grab part of the Full Frame Image (FFI) data, which has 30 minute frames and a lot less processing but of a much wider field. If you use lightkurve.interact() and look at the individual pixel time series in the FFI, they all show this dip. But in the pipeline TPF this is different: above the mid-axis of the star, they go up during ‘transit’ and below they go down. You can actually see this at a global level if you use the slider and the right scalings: it seems that as a whole the background flux shifts upward on the detector for a few hours and then shifts back down. So something funky has happened to the spatially-varying background during processing.

So let’s look at another very bright star in the field: the giant star ζ Ceti. It shows the same background dip – but as Tim White pointed out, at a slightly different time! If you look at individual pixel time series from ‘above’ and ‘below’ the star midlines, above the line they go up and below they go down – features with the same midpoint and similar duration to the ‘signal’ at τ Ceti – they are something to do with TESS and not a planet around τ Ceti. The difference between the top and the bottom, Tim realized, can be ascribed to the fact that these pixel cutouts are very elongated in that axis, so if we have a spatially-varying background but subtract only a constant background, we will find this asymmetric pattern. This poor background subtraction may therefore have contaminated all the pixels and created the appearance of a transit where in reality there is none.

So to look at the spatial detail in the background, I downloaded all TPFs on the same camera as τ Ceti, extracted their background light curves, and made a video of their background flux over time. Each point below is coloured by the logarithm of the background flux, clipped at the top and bottom to bring out the features best. τ Ceti is a blue star bang in the middle and ζ Ceti orange to the top right, and the ‘transit’ occurs at day 1394.3 or about 11 seconds into the video. (1)

As you can probably just make out, there is a lot of spatial structure there, mostly in the lower left. Just around the transit, there is a spur through the middle towards the top right that lights up a little, and then it switches back to the lower left, and then everything gets brighter overall towards perigee. When we contacted Dr Chelsea Huang (MIT) about this, she was able to dig into the huge full frame images and make a ‘difference image’, subtracting one from the next to look for where the background might be changing. In her image below, τ Ceti is highlighted with a red arrow:


The fuzzy blobs pick out bright stars (e.g. τ Ceti itself, or ζ Ceti top right). The vertical streaks are probably ‘straps’ on the back of the detector that reflect back some of the light that passes through. There is probably also some CCD smear like you get with cheap cameras at night, and it runs up and down every column with a sufficiently bright star in it (such as τ Ceti). But more importantly are the ripply concentric rings which are lens flare from the Earth just out of shot, and you can see τ Ceti lies smack bang in the middle of one of these rings. As the Earth seems to move and get brighter this ring runs over τ Ceti and causes this apparent transit effect.

Regrettably we have been ‘dispossessed’ of this potentially very exciting planet candidate. But this is how science is: something that is too good to be true very often turns out that way, but it doesn’t make it any less worth investigating. In digging through the data on τ Ceti we were forced to fix bugs in our code and grapple with unfamiliar systematics in TESS that we didn’t see in Kepler. Personally, I am concerned that systematics which affect so many pixels in common over such a wide field are going to pose a serious problem to the approach we had been taking to bright stars in K2, and playing with the τ Ceti data has been a valuable learning experience. While we haven’t yet detected her home planet’s transits, we should keep in mind the maxim of Le Guin’s Anarresti scientist Takver:

There was process: process was all. You could go in a promising direction or you could go wrong, but you did not set out with the expectation of ever stopping anywhere.

Kepler vs. TESS

Kepler and TESS are both amazing space telescopes that have and will revolutionise our understanding of exoplanets. But let’s have a look at how these two telescopes differ?


Artist impression of NASA’s planet-hunting Kepler spacecraft (left) and TESS satellite (right). Image credit: NASA Ames/JPL-Caltech/T Pyle

Kepler was launched in March 2009 and used a 1.4-m primary mirror that observed a 12×12 degree patch of sky (for reference the Moon covers half a degree on the sky). The sensitivity of Kepler was significantly better than that of any other instruments at the time, thus enabling Kepler to find exoplanets as small as half the size of the Earth.

Conversely, TESS will survey the entire sky, looking at 400 times more stars than Kepler did throughout its lifetime. TESS will do this with four identical telescopes, which, combined, observe a 24-degree patch of sky at any one point. Each 27 days, TESS changes direction and looks for planets around a different set of stars in a new ‘sector’. The entire sky has been split into 26 overlapping sectors, and TESS will visit each one over the course of the next 2 years.


The combined field of view of the four TESS cameras (left) and the sub-division of the celestial sphere into 26 observation sectors (right). Image credit: NASA 

The two satellites also differ in their observing strategy and the types of stars that they focus on. Whilst Kepler observed one patch of sky for a long period of time, TESS will only spend a month looking at each sector. The long exposure times of Kepler allowed it to find the dimmer and more distant stars, whereas TESS will monitor the nearby, and brightest targets. In fact, the stars observed by TESS are 10 times closer and 100 times brighter than the Kepler target stars! Observing brighter and closer stars has the advantage that any planet candidates that we find will be easier to observe using ground based telescopes.


Artist impression of a burning exoplanet. Image credit: NASA 

The main Kepler mission ended in 2013, when the telescope lost its ability to change orientation without the use of fuel. Luckily, engineers and astronomers quickly realised that the pressure from the Sun could be used to steer the telescope in order to keep it pointing at one patch of the sky. This new era of observations became known as the K2 mission.

K2 ran out of fuel in mid 2018, bringing the mission to a close. Luckily, by this point NASA’s new satellite TESS had already been launched. We now have brightness measurements of around 45,000 stars from the first three sectors, and we are already finding some promising planet candidates within the TESS data!

Will you help us find the planets hidden within the TESS data? Click here to give it a go!




More TESS Data and More Planets Waiting to be Found

Planet Hunters TESS is back with brand new data!  The Sector 3 lightcurves have just been released and we are ready to find the planets hidden within them. This new data set consists of brightness measurements of 16 thousand bright stars that were observed by TESS between 22 September to 17 October, 2018.


Artists impression of a Distant Alien World. Photocredit: Nora Eisner

You may notice some difference between this data release and the last one. During the Sector 3 observations TESS underwent some test in order to improve the data quality. This meant that the data collected during the first four and the last three days of the scheduled observations are not usable, leaving us with around 21 days, as opposed to the usual 28 days, of data. These tests are necessary as they give the amazing TESS system engineers and scientist the opportunity to learn more about how the satellite operates, allowing them to advance the system and improve the pointing stability. Due to this the TESS data will improve with every new sector. Each 21 day lightcurve has been split into three sections, providing you with higher resolution data and making it easier to spot even the smallest dips in brightness.

There are sure to be many planets hidden within this data, ranging from Earth-sized rocky planets to Jupiter-like gas giants orbiting around various different types of stars. What kind of planet will you find?

We hope you enjoy the new TESS data. Happy Planet Hunting!

Data Reduction Guide

Think you’ve found a great transit candidate? Can’t wait for us researchers to look into it? Here are a few things that you can do yourself to check whether your candidate could be a real planet. These are the first steps that we would do ourselves, so it’s a great help to us if you have the time or inclination to make a start yourself – and a great opportunity to learn a few cool things in the process. Note you can do as many or as few of the steps on this list as you like – it’s completely up to you!

1. Is it a TOI (Tess Object of Interest)?

TOI is the name used by the TESS team for good planet candidates that they have checked carefully and consider worthy of follow-up observations.


In order to check whether the candidate is a TOI you need to find the TIC number (you can view it by clicking the “i” icon below the subject image in Talk) and check if it appears on the TESS data alerts page: TIC ID is the first column in the big table. If the candidate is on the TOI list, well done – you have found a candidate that the TESS team have identified as a planet candidate.


If the candidate you found is a TOI you’re doing really well. However, it’s already being looked into by the TESS team, so we won’t duplicate their efforts – we want to focus on objects that they haven’t already found. Before you leave the talk page for that subject though, please tell everyone else what you’ve found – you can say “This is Tess Object of Interest (TOI) XXX” where XXX is the number that appears in the 2nd column on the data alerts table.

2. Is it a TCE (Threshold Crossing Event)?

All of the TESS data are passed through the TESS transit search pipeline, which automatically flags any lightcurves that might contain a planet. TCEs are the raw flagged candidates of this pipeline (prior to any vetting done by the TESS team).

In order to check whether a candidate is a TCE you can download a CSV file, for each sector, where they are all listed:

Alternatively you can check if a given candidate is a TCE using EXOMAST ( On EXOMAST, simply enter “TIC ” followed by the TIC number, and click ‘search’. If the candidate you are looking into is a TCE, you will  be taken to a page containing some information about the host star and the potential planetary system.


If the candidate is not a TCE, you will see a notification below the search bar stating “No planet found”.


If you find a TCE, once again, you’re doing really well – it means that you’re as good at finding (some) transits as the pipeline that professional astronomers developed over a number of years!

Please flag such an object as a #TCE on the talk page (if possible including a link to the EXOMAST page for that TCE).

3. It’s a TCE but not TOI?

A candidate that is a TCE but not a TOI is an object that the TESS pipeline flagged, but the TESS team decided wasn’t a good enough planet candidate to be promoted to TOI status. Finding these is really great, not least because – in some cases – we might take a different view to the TESS team and consider them to be likely planet candidates. So if you find a TCE that isn’t a TOI, please let us know by including “@researchers” in your comment on talk. We will get notified automatically and – time permitting – we will look at it more closely.

When vetting the TCEs, the TESS team perform a long list of checks. These tests are designed to weed out instrumental false positive (the signal isn’t real) and astrophysical false positives (the signal is real but isn’t caused by a planet, but something else). The results of these tests are saved in a DV (data validation) report, which they have helpfully made publicly available – so we can use them to understand why the TCE didn’t become a TOI. This is a really quick way to look through candidates and to avoid repeating the hard work that the TESS team have already done. The DV reports are long and complex, and currently a little tricky to access for TCEs that aren’t TOIs, so we are not including instructions on downloading and using DV reports in this post (though we hope to do so at a later date).

Importantly, there are already a few TCE (and not TOI) candidates found by volunteers for which we have examined the DV reports and come to the conclusion that the candidates are promising. This mainly happens because the TESS pipeline requires at least two transits for a detection, so it only searches for transits that repeat with periods up to the duration of a TESS sector (~28 days). If there is only one real transit, it might be missed altogether (this is where you volunteers come in!) or it might be wrongly paired up with an artefact or noise feature somewhere else in the light curve. In that case, the diagnostics in the DV report, which are based on all the transits combined, might be misleading.

4. Create a cutout or movie of the TESS data

There is a fun tool at which allows you to extract a time-series of cutout images around a given target. You can use these to look at what is in the vicinity of the target, or even to make a movie! If the transit is deep enough, you might even see the star “blink” (this can be a fun thing to try out on variable stars or eclipsing binaries too).


Sometimes, what appears to look like a transit is actually due to some weird artefacts, affectionately dubbed “fireflies” or “fireworks” by the TESS team, that sweep through the field of view. These are probably due to scattered light from bright stars or moving objects inside the telescope and camera optics. If you notice that a promising candidate is actually due to such an artefact, please let everyone know on talk!

5. Want to play with the TESS data products yourself?


If you’re really keen and want to examine the TESS data in more detail, you can easily get your hands on them. Go to, enter “TIC” followed by the TIC number of the subject in the search box, and hit “search“. This should bring up a list of datasets stored by MAST (Mikulski Archive for Space Telescopes), including two that will have “TESS” in the “Project” column. The lightcurve is the one that lists the TIC number (rather than “TESS FFI”) under “Target name”.


You can download the data to your computer by clicking on the little floppy disk icon in the corresponding row. You can find more information on the format of these datasets in the TESS Science Data Products Description Document:

What to do with the data when you have it is a long story, far too long for this post… but again, we hope to provide a separate, dedicated article with some examples at a later date.

Initial Planet Candidates

Thank you so much for all your amazing work! The next data release is just around the corner so hopefully everyone is ready to find some more planets. Until then, we have some preliminary results from the sector one data.

Over the past month the science team has been working hard on putting together a list of some of the most promising planet candidates. We find these by carefully looking at the lightcurves where many of you marked planets in the same location. With a careful eye we filter out lightcurves that show eclipsing binaries or that have transit-like events due to systematic effects. We  can identify these by looking at features such as the shape and depth of the dips, as well as the time of the transit.

The candidates that withstand this initial filtering process have to go through a further screening before they can be promoted to be a high priority planet candidate. This screening involves looking at the variability of nearby stars, the depths of the alternating transits (if the lightcurve shows multiple transits), and stellar parameter of the host star.

So far, we have identified five high priority candidates, three of which are TCEs (you can see their lightcurves below). Even though these lightcurves have passed all of our tests up to this point, we cannot confirm that these transits are due to planets without further observations. As a next step we will, therefore, look to observe these targets with ground based telescopes in order to find out more about these fascinating systems.

image from ios

We are very excited about these initial five candidates and look forward to finding many more as we finish looking through the sector one data. Stay tuned for more results!

Planet Hunters TESS

Welcome to our new Planet Hunters TESS project!

TESS is NASA’s new Transiting Exoplanet Survey Satellite, which will spend the next two years looking for distant alien worlds. The first batch of data has only just been released by NASA and we are ready to find planets around other stars in our Galaxy. Will you join the search?

We are proudly following in the footsteps of the tremendously successful Planet Hunters project. You may have heard of this project, or even contributed to some of its amazing discoveries. Over the course of eight years they found over two-thousand planet candidates, including planetary systems that we didn’t even think possible!

The original Planet Hunters project used data from the Kepler mission, which came to an end earlier this year. But with the end of Kepler comes the beginning of TESS, NASA’s Transiting Exoplanet Survey Satellite, and with that an exciting new project: Planet Hunters TESS. Throughout the mission, TESS will point its four cameras at two-hundred-thousand bright nearby stars, four-hundred times more than Kepler observed throughout its lifetime. These TESS target stars will be closer and brighter than the Kepler targets, which will allow us to more easily observe planet candidates using Earth-based telescopes. The discovery of many more worlds will further our knowledge of planet formation and evolution, and will allow us to better understand the galaxy in which we live.

But we can’t find them alone! We’ll be hunting for planets by monitoring the light given off by a star. When an planet passes, or “transits”, in front of its host star, the star momentarily dims and we see a dip in the light. This method of detecting planets has already proven to be extremely effective! Even though we can train machines to find some planets, studies have shown that human brains are excellent at detecting patterns and finding planets that automated routines miss. Together we can find the most complex, unusual and exciting planetary systems.

We’re the new Planet Hunters TESS core science team, and we’re very excited to have you join our hunt for distant worlds at

We hope you enjoy the project!

Nora & the Planet Hunters TESS team

Planet Hunters: a new beginning!

In 2010, the Planet Hunters website began showing light curves from the Kepler mission to Citizen Scientist volunteers. This project was different from other Zooniverse projects like the successful “Galaxy Zoo” because instead of showing beautiful images, we were serving up “boring graphs” of brightness measurements for 150,000 stars.
• Would people be interested in spending time sifting through these data to find changes in brightness from transiting planets?
• Would humans beat out the sophisticated computer algorithms designed by NASA scientists?
• Would Planet Hunter volunteers contribute unique scientific discoveries that would otherwise be undiscovered today?

We now know the answers to these questions: yes, yes and yes! In retrospect, the right question would have been: can the scientists keep up with the wave of discoveries from Planet Hunter volunteers? Barely!

Planet Hunters has been a game-changer and showcases the ability of Citizen Scientists to make important discoveries. Here are some highlights of the ways that you have changed our understanding of exoplanets:
1. Planet Hunters independently identified about 2000 of the planets found by the NASA Kepler team.
2. Excluding cases where a discovery was also made by science teams working on the Kepler data, Planet Hunters uniquely contributed 120 unique discoveries that would otherwise still be buried in the Kepler data today. For transiting planets with orbits longer than 2 years, Planet Hunters detected 50% of the planets that are known today.
3. You identified hundreds of eclipsing binary stars, and most surprising, planets orbiting outside of eclipsing binary systems! Who knew that these objects could even exist? …you found them!
4. You identified the most mysterious star in the galaxy: “Tabby’s Star,” which gained notoriety when Jason Wright at Penn State suggested that this could be the sign of an alien megastructure. We did not promote that intriguing explanation, but Tabby Boyajian gave an amazing TED talk about this discovery and has an ongoing campaign to study that star.

This scientific legacy could not have happened without the many, many hours that all of you put into this project. Your patient and persistent clicks on prospective transit events have changed our understanding of exoplanets. Thank you for your hard work!  We also owe a debt of thanks to the dedicated Zooniverse team, the postdocs, grad students and undergrads who worked tirelessly on Planet Hunters!

Over the past 8 years, we became friends. We chatted through the Talk site, met each other at Kepler meetings and our most active users wrote data analysis tools and helped to guide new volunteers. Many of you were co-authors on Planet Hunter discovery papers. In 2012, Planet Hunter Kian Jek won the American Astronomical Society Chambliss prize for “achievement in astronomical research by an amateur astronomer.” In 2016, that prize was awarded to Daryll LaCourse. In 2017, we were saddened by the passing of Gerald Green, a co-author on science papers and one our most active volunteers. In May 2018, Smith College students Rebekah and Jennifer Kahn, who became interested in astronomy while volunteering as high school students, arrived at Yale to work on summer research, modeling of the Kepler-150 system.

Now, it’s time for a transition – a new beginning for Planet Hunters. There will be a new look and feel to the website, and the light curves will come from the NASA TESS mission (launched in April 2018), instead of Kepler. We need you more than ever and hope that you’ll continue with the search for transiting exoplanets and other weird things in the galaxy!

“ I wanted to contribute to the world of planetary science in a meaningful way”

-an interview with Al Schmitt by Jennifer and Rebekah Kahn

Al Schmitt is a long-time Planet Hunters member. Having lived during the space program era and been an avid follower of subsequent space missions, Al found that Planet Hunters enabled him to actively participate in planetary science research. In fact, he used his job experience as a software engineer to develop LcTools, a light curve display and signal analysis toolset designed specifically for the Planet Hunters community. Al is also a researcher on the HEK team “The Hunt for Exomoons with Kepler” whose goal is to determine the occurrence rate of exomoons. Al has co-authored many published research papers in association with Planet Hunters and the HEK project. We thought that it would be of interest to other fellow Planet Hunters members to learn more about LcTools and how Al is able to pursue his passion as a citizen scientist.

You can learn more about LcTools on his website:
More Information on the HEK project can be viewed here:

PH: Would you tell us a bit about your background?
AS: Career wise, I was a software engineer for 35 years developing applications in various engineering fields including computer diagnostics, computer aided design (integrated circuits and printed circuit boards), and medical software (heart pacemakers and defibrillators). In 2010, I retired early in part to pursue science on an amateur basis.

PH: When did you first become interested in Astronomy?
AS: I grew up in the 1950s and 1960s closely following our manned space program which I found extremely exciting. Planetary science became more important starting in the mid-1970s with the Viking missions to Mars. My interest in planetary science continued to grow with the Galileo and Cassini missions. As I approached retirement, I knew that I wanted to contribute to the world of planetary science in a meaningful way.

PH: Is this why you joined Planet Hunters?
AS: Yes. I joined in April 2011 a few months after the website became fully operational. Over the next couple of months, I spent a great deal of time classifying stars and identifying transit candidates. This was a big learning period for me since I didn’t have any prior experience in this area.

PH: How did you become involved with the HEK project and the search for exomoons?
AS: In June 2011, Gerald Green started a thread in the PH science forum for discussing potential moon and ring signals seen in Kepler lightcurves. I joined the discussion and quickly decided that exomoon research would be my new focus area. At about the same time, I read a research paper by David Kipping which showed model exomoon signals based on his LUNA algorithm. Armed with this knowledge, I performed my own visual exomoon survey for several hundred KOIs and then sent him the results. A few months later, David asked me to join his new research project called HEK – The Hunt for Exomoons with Kepler.

PH: Was your work on the HEK project related to your development of LcTools?
AS: Very much so. LcTools was born out of necessity. I needed a software tool to perform large-scale visual surveys of Kepler lightcurves in a fast and efficient manner with the ability to record candidate signals of any type. In early 2012, I developed an application called LcViewer to accomplish this. LcViewer formed the basis for a much larger system of applications called LcTools developed over the next six years.

PH: What are the other applications in LcTools?
AS: In addition to LcViewer, there are five other major applications. LcSignalFinder automatically detects and records periodic signals found in a set of lightcurve files. LcGenerator builds lightcurve files in bulk for use by LcViewer and LcSignalFinder. LcReporter generates an Excel spreadsheet showing all the user defined signals recorded by LcViewer. The last two applications in the system are LcStacker and LcStackAnalyzer.
LcTools is described in detail on my website. The website also includes links to Kepler and K2 lightcurves designed to work with LcViewer and LcSignalFinder. Over 200,000 files are available for the Kepler project and about 350,000 files for the K2 project.

PH: Can LcTools be used by other individuals?
AS: Indeed it can. Initially I built the system for myself but I quickly realized that it could be extremely useful to other serious researchers especially those with a strong science, technology, or astronomy background (LcTools is not designed for entry level users). Currently, I have 63 registered users spanning the citizen science, academic, and professional domains.

PH: What are some of the notable highlights of LcTools?
AS: First and foremost, the system is designed to be fast and easy to use. Operation is simplified wherever possible. The system is optimized for high volume processing of lightcurves.
Second, the system supports signals of any type whether astrophysical in origin or not. Signals may be periodic, quasi-periodic, or non-periodic. This permits a wide range of phenomena to be studied.
Third, signals can be imported into LcViewer and LcSignalFinder from various external sources when a lightcurve file is loaded. For example, project based signals such as KOIs, K2OIs, and TCEs can be imported from the NASA Exoplanet Archive. Signals can also be imported from public signal libraries designed to be shared between individuals or groups across the network via shared Google Drive folders. Use of public signal libraries opens the door to collaborative research projects. Fourth, the system supports a comprehensive set of high-level features typically found in professional lightcurve analysis packages. Major features include detrending of lightcurves, automatic detection of periodic signals, and phase folding of periodic signals.

PH: What new features are planned for LcTools?
AS: The most important upcoming feature is support for the TESS project. My goal is to have a product ready for customers shortly after MAST releases the first batch of lightcurves to the public hopefully sometime in November or December. The high volume capabilities of LcTools will be essential for handling the large number of lightcurves expected.

PH: You are listed as co-author on a number of research papers. Which one do you consider most significant and why?
AS: The most significant would be the last HEK paper published a year ago. In it, we mention a strong exomoon candidate dubbed KOI-1625b I. Over the past eight months, David Kipping and Alex Teachey have been very busy vetting the candidate (I have not been part of this effort since it’s well outside my field of expertise). If the candidate can pass all of the vetting tests, then we will have discovered the first confirmed exomoon. Such a discovery would usher in a new era of research, similar to what the first confirmed exoplanet did back in the mid-1990s.

PH: What advice would you give veteran PH citizen scientists moving forward?
AS: Don’t be afraid to investigate new phenomena! Kepler and K2 lightcurves may be host to a wide variety of intriguing phenomena such as moons, rings, trojans, and comets. By no means has everything been found! There are important discoveries still to be made if you’re willing to search.
Thank you Al for discussing your personal involvement with research in the field of Planetary Science.  We are excited to see what the new release of LcTools will entail, and hope that your experience in this field will also inspire other citizen scientists.