It has been exactly a year since the first public TESS data release and the launch of Planet Hunters TESS! Thank you to everyone who has taken part and helped us classify all of the data so far. It has been a truly exciting year!
Since 6 December 2018, PHT has had over 14 thousand registered (and many more thousand unregistered) participants, and together you have completed almost 11 million classifications! Together, you have helped us find some exciting new planetary systems.
For example, your participation and dedication to the project over the past year have led to the detection and validation of the first PHT planet, TOI-813. TOI-813 is not only the longest period planet found in the TESS data to date, it is also in orbit around a subgiant star. Subgiant stars are stars in the later stages of their lives, meaning that studying these planets will help us understand the synergies between planetary and stellar evolution in the later stages of the stars life, in other words, it may help us understand what will happen to the Earth in the far far future.
But TOI-813 isn’t the only planet that PHT has found so far. You have brought many interesting targets to our attention and we are working hard to test whether these promising signals are indeed caused by planetary bodies. The targets that pass all of our initial vetting tests are being followed up using ground-based telescopes and we hope to validate them in the near future. This will allow us to contribute to the ever-growing population of known planets and bring us one step closer to findings a planet like Earth.
Here are some of the ones that we’re particularly excited about:
These are only some of the candidates that are currently being actively followed up using telescopes found across the globe, including Chile, France, Australia and the USA. We will be sharing the results of our findings soon!
In addition to the exciting planets that are being found by the project, we have also come across lightcurves of some puzzling stars. These are often brought to our attention via the talk discussion boards and I would like to thank you everyone for using this tool to post and highlight interesting signals and patterns in lightcurves there. Here are some of the ones that we haven’t been able to explain.
There appear to be more dips here than we would expect for a simple eclipsing binary?
This one appears to be two binaries, but could they be locked together making this a quadruple?
This is a beautifully long eclipsing binary!
Thank you so much for your participation over the past year! Here at Zooniverse we are celebrating Planet Hunter TESS’s first birthday with a sparkly cake!
Like many of you, I am extremely eager to find those tiny, elusive dips in the TESS lightcurves that reveal the existence of a distant, undiscovered, alien world. However, even though planets are my main focus, they are not the only interesting objects that we are able to find using TESS.
I have recently been talking to astronomers at the KU Leuven, in Belgium, who use the TESS lightcurves to study stars, their spots and pulsations as well as the architecture and behaviour of multiple star systems.
The lighcurves of these systems often boast beautiful patterns! Cole Johnston from the KU Leuven explains some of the systems behind these lightcurves:
Gamma Doradus variables are stars slightly more massive than the Sun with temperatures between 6,500K and 7,500K (the Sun is around 5,777K). You can recognise their lightcurves due their characteristic fluctuations in brightness that last from a few hours to several days. The fluctuations are caused by pulsations known as gravity-mode pulsations, which are waves that behave in the same way as surface waves in the ocean. These waves affect the surface temperature of the stars, changing the brightness that we observe.
Delta Scuti (also known as dwarf Cepheid variables) stars are hot, young (A-F type) stars that are ~1.5 to ~2.5 times the mass of our Sun. These stars can have a single strong pulsation or potentially hundreds of smaller pulsations with periods ranging from a couple of minutes to a few hours. The pulsations are caused by pressure waves, which behave identically to sound waves, that propagate near the surface of the star.
Cepheid Variables and RR Lyrae stars the older and more evolved cousins of delta scuti stars that also pulsate due to pressure waves. These stars are very exciting as they exhibit a pulsation period – luminosity (brightness) relationship, also known as the Leavitt law. This means that we can calculate the distances to these objects by studying the pulsations that we see in their lightcurves. Historically, these stars have helped us to calculate the distance to the Large Magellanic Cloud as well as to the centre of our Galaxy.
Slowly Pulsating B-stars
Slowly Pulsating B-stars (SPB stars) are very similar to Gamma Dor variables but are much more massive (2.5 to ~8 time as massive as the Sun). They are extremely hot with effective temperatures between ~11,000K and ~30,000K and typically show multiple gravity-mode pulsations that range from lasting a couple of hours to several days.
Beta Cephei stars are the high mass (8 to 20 times more massive than the Sun) stars that oscillate due to pressure waves, similar to Delta Scuti stars. The iron interior of these stars reaches extremely high temperatures of 200,000 K. At this temperature, the metal starts to behave strangely, resulting in a build up energy deep within the interior of the star. This causes the star to expand, resulting in an increase in surface area and thus an increase in observed brightness. The expansion of the star, however, uses up the stored energy and eventually it runs out of ‘expansion fuel’. At this point, the star begins to contract again due to gravity, resulting in a decrease in surface area and a decrease in brightness. This cycle repeats resulting in pulsations on time-scales of a couple of hours.
Stars are not simple, and many of them exhibit both pressure and gravity mode pulsations. Those lower mass stars which exhibit both are hybrid delta scuti / gamma dor pulsators, while the higher mass stars which exhibit both are hybrid SPB-Beta Cep pulsators.
Many stars are in binary (double) systems. Heartbeat stars are a class of binary stars that are so eccentric (not circular) that at the point when the stars are closest together, their gravity is so strong that the spherical stars morph into rugby-ball shapes. This increases their visible surface area, and hence increases the total amount of light that we see. Depending on the orientation of the system, we might see either a single brightening, a dip then a brightening, visa versa, or a brightening and eclipse.
We often like to visualise different types of stars on a plot of brightness (luminosity) versus their temperature, an example of which is shown in the figure below.
I love looking through the TESS data and coming across these beautiful light curves of fascinating stars. Maybe some of them even host a planet or two…
We have some exciting news! We have validated the first Planet Hunters TESS planet, TOI-813b, where validated means that we can say, beyond reasonable doubt, that it is a planet! TOI-813b is around 7 times larger than the Earth, on an 84 days orbit around a 3.7 billion year old star. The paper has been submitted to the Monthly Notices of the Royal Astronomical Society (MNRAS) journal and you can find a version of it on arXiv at: https://arxiv.org/abs/1909.09094
So how did we go from detection to validation?
You initially spotted the transit events that occurred in sector 5 and brought it to our attention via Talk. This put the candidate on our ‘to watch’ list until a second transit was discovered in sector 8 a couple of months later. Additional transits were also identified in sectors 2 and 11. With multiple transit events we could be much more certain that the signal was real and, therefore, began to invest more time looking into studying it.
Initial validation checks
We carried out a number of vetting tests on the TESS data in order to validate it as a planet. First we made sure that the signal wasn’t caused by a jolt in the satellite or a background event. Next, we verified that it wasn’t an ‘astrophysical false positive’ – signals caused by other astrophysical phenomena such as an eclipsing binary (two stars orbiting around one another). To do this, we first compared all four transit events to make sure that the shape and depth of the signals were consistent with one another, as alternating transit depths are characteristic of eclipsing binaries. Next, made sure that when extracting the lightcurve with different aperture sizes the depth of the transits didn’t change. A change in depth could indicate that the signal is caused by an eclipsing binary in the background. Furthermore, to make sure that the light wasn’t coming from a background object, we subtracted images from when the star was in transit to when it was out of transit in order to make sure that the change in light (causing the dip in the light curve) is centred on the star. Finally, we looked at nearby stars in order to make sure that their light curves did not have transit-like dips at the same time as TOI-813.
Spectroscopy. In order to determine whether TOI-813b really is a planet, we had to find out as much as possible about the host star. This can be done using spectroscopy, whereby we split light up into its individual wavelengths, much like a prism splits light into a rainbow. From this we can derive properties such as temperature and composition of the host star. We obtained these observations using the Wide Field Spectrograph instrument on the 2.3-m Australian National University telescope and the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph on the ESO 3.6-m telescope in Chile.
High Resolution Imaging. We also wanted to check for nearby stars that could contaminate the light curve. This was done using a technique known as ‘speckle’ imaging, which takes thousands of consecutive images with extremely short exposure times. When the images are combined in a particular way, we are able to essentially ‘freeze’ our the effects of the atmosphere (which usually makes our images blurry) and obtain high resolution images. This was done using the Zorro instrument on the 8.1-m Gemini South telescope in Chile.
With this data in hand, we were able to run a program, known as VESPA, to statistically validate the planet. This clever program models the data assuming a number of different astrophysical scenarios and returns the likelihood of each one being true. Based on this analysis, we can be 99.7% sure that he signal is caused by a planet! Yay!
Why is TOI-813b so great?
TOI-813b is interesting for many reasons. First because it is orbiting around an evolved, subgiant star. The subgiant phase of a star occurs when a star runs out of its nuclear fuel source and, in a desperate attempt to find another source of energy, expands its outer layers and contracts its core. Our Sun has not yet reached this stage of its life – it still has around half of its fuel source left – but it will undoubtedly one day also become a subgiant star. There is a noticeable lack of well-studied planets around these types of stars, however, they may be able to help us predict what will happen to the planets within our own solar system in the (very very distant) future. Do planets survive this stage of a star’s life? And if so, how do their characteristics change? We investigated this further for TOI-813 by modelling the size of the star over time. This analysis showed that our newly discovered 7 Earth radii planet will sadly be engulfed by its evolving host in approximately 780 million years (mark your calendars)!
TOI-813 is also interesting as it currently has the longest orbital period (84 days) of any validated TESS planet. This is largely because most of the TESS targets are only observed for ~30 days, making the discovery of longer period planets challenging. Nonetheless, the PHT project has already shown to be extremely good at finding these longer period planets. Even though TOI-813b is currently the longest period planet found by TESS, this will likely change very soon as TESS continues to observe thousands of stars every night.
Finally it’s a great planet because it was discovered by you. The discovery and validation of TOI-813b has shown that we are able to find planets that the pipelines miss! This is the first validated PHT planet, but we are actively following up more targets that you have helped us identify! Thank you so much to everyone for helping us search for unique and exciting new systems that will help us understand the Universe that we live in. Also a special thank you to Frank Barnet, Stewart J. Bean, David M. Bundy, Zbigniew Chetnik, Jamie L Dawson, Judy Garstone, Andrés Guillermo Stenner, Marc Huten, Scott Larish, Larry D. Melanson, Thomas Mitchell, Christopher Moore, Klaus Peltsch, David John Rogers, Claudia Schuster, Dean J. Simister, Daniel Shane Smith, Christopher Tanner, Ivan Terentev and Alexander Tsymbal, the PHT volunteers who helped us find TOI-813b and are now co-authors of the validation paper.
We are very excited to announce a new Planet Hunters TESS interface! Whilst the idea and the aim of the project has remained the same, we have added some additional features that will hopefully improve your overall planet hunting experience.
One of these new features is the option to zoom in on the lightcurve, giving you the opportunity to explore different parts of the data in more detail. This close up view will give you a better insight into the shape and depth of a potential transit-event and may be able to reveal more about the nature of a dip.
The new interface will be launched with data from Sector 9 – the sector that we have just finished classifying with the old interface. Not only will this give us a more accurate and careful look at this current sector, it is also important for us to have this data overlap so that we can quantify any differences in the output and allow us to fairly compare and combine our findings across all observational sectors.
We hope to have the new interface up and running later today. In order to make the switch the site will have to be down for a short while – but don’t worry, we’ll be back as soon as possible to continue our search for planets!
EDIT: The new interface is now live! It is not supported on some older browsers such as Internet Explorer 11, but will work on up-to-date versions of all major browsers.
PHT may well have found its first planets! They are not yet confirmed, but we have taken the big step of uploading the candidates to ExoFOP, the website used by the worldwide exoplanet community to contribute to the follow-up TESS planet candidates. If all goes well, the additional ground-based observations that are needed to confirm whether our candidates are really planets will be made soon.
It all started when a very exciting planet was brought up and discussed on Talk by Dolorous Edd, mhuten, davidbundy77, zbish and Vidar. This is the first of three candidates we uploaded so far, and is currently my favourite. TIC 55525572.01 is a long period planet candidate that appears in multiple observational sectors. The fact that the dips don’t repeat in any one sector is likely to be the reason why the official TESS pipeline didn’t find it (yet!). The widely separated transits suggest that the planet candidate completes an orbit every 83.4 days, making this the longest period planet found in the TESS data so far (as far as we know)!
From Talk to Telescope
It was exactly three weeks ago today when I first saw the lightcurve of TIC 55525572, a subgiant star which is potentially hosting a beautiful, distant world. Prepped with strong coffee, awesome data and many pages of code we spent the afternoon pulling together figures, parameters, models and plots in order to find out everything we could about the transit events. If this candidate was going to pass the scrutiny of the planet-jury we would need a whole file of evidence. The initial checks included looking at the plots of the background flux and stability of TESS at the time of the transits; checking whether the lightcurve extraction aperture size had an effect on the size and shape of the dips; and making sure that the brightest points in the aperture didn’t move during the transit. All these tests were passed with flying colours, which urged us to move on to modelling the transit event, to see what kind of a planet it would be, if it really is a planet. Amongst other things the models showed that all three of the observed transits have the same depth and width; and revealed that, if the planet is real, it has a radius that is approximately 7 times greater than that of the Earth. At this point we were happy to call it bona fide planet candidate and upload it to ExoFOP as a “community TESS Object of Interest”, or cTOI. The candidate is now known as cTOI 55525572.01.
Next, we wanted to gain a better understanding of the entire system, and thus we needed to obtain a spectrum of the host star in order to accurately determine properties such as its mass, radius and temperature. Due to the very Southern location of this object, we turned to our Australian friends over at Australian National University who kindly observed the star for us. We are still in the process of analysing this spectrum.
As one of the final steps in the verification process we will need to obtain images of the system that are sharper than those TESS gathered, to see if the star being obscured is really TIC 55525572 and not some fainter, neighbouring star. In an ideal world we would make these observations around a transit. In fact, there was one just this past weekend, but sadly it was only observable from Antarctica… We don’t want to wait 3 months until the next transit, so for now we will settle for just having a good, sharp image the target area. Once we have all that information in hand we should be in a position to validate the candidate statistically, and if all goes well that’s when we will be able to give it its proper, planet name: will cTOI 55525572.0 one day become PHT-1b???
This candidate got the ball rolling, and within a matter of days we found two more excellent candidates that surface on Talk. All three are now on ExoFOP and awaiting follow-up. More planet candidates can be expected to appear on there soon once they pass all our initial vetting tests!
We will soon know more about these exciting candidates and I can’t wait to share that information with you. None of this would be possible without your incredible help and dedication in finding these distant, alien worlds within our galaxy!
An earlier version of this post was briefly live at the weekend; I’m so sorry for the confusion.
By Oscar Barragán
The night has finally arrived at the Roque de Los Muchachos Observatory. The blue sky has turned into a deep ocean full of stars which eclipses the beautiful horizon that is scattered with pink clouds. The telescopes are ready to hunt for starlight. At first sight, all the stars seem static in the night sky which is victim to the Earth’s rotation. However, this is a misconception, as all the stars that shine at night are moving within our galaxy, the Milky Way. Our mission for the night is is to detect their subtle movement which may tell us about the existence of faraway worlds.
The motion of the stars manifests itself in two ways. The first one is their movement in the plane of the sky – also known as their proper motion- which slowly re-shapes the constellations. The second one, and the one that we are searching for, is the movement of the stars with respect to us. This receding and approaching velocity of the stars is known as their radial velocity. This stellar motions, however, is so small that is is imperceptible to our naked eyes, meaning that we need to use big telescopes and state-of-the-art instruments in order to detect it.
You may be familiar with the acoustic version of the Doppler effect: the change in sound as a car first moves towards and then away from you. This change in sound is caused by the compression and elongation of the car’s sound waves caused by the motion of the car. In the same vein, light travels as a wave, and the Doppler effect results in an apparent change in color. If a light-emitting astronomical object moves towards us the waves are compressed and appear redder. Conversely, if the object moves away from us the waves are elongated and appear bluer. This effect is extremely small, and thus we have to use specifically designed instruments, known as spectrographs, to measure it. These devices work by dividing starlight into all the colors of the rainbow. The resultant colourful decomposition of light -called a spectrum– is imprinted with strange dark lines which, combined, make up a signature conveying information about the building blocks of the star. This is because the dark lines are a result of the emitted light travelling through the atmosphere of the star which absorbs specific colours depending on its composition. Astronomers have been using this technique to learn about stars for centuries. Additionally, we can look at these dark spectral features to study how the star dances across the sky. The position of the lines, with respect to where we expect them to be if the star were not moving, allows us to measure the Doppler effect and therefore the radial velocity of the star. It is this effect that hints at the presence of exoplanets around stars.
Let’s picture a planet orbiting a star as a gravitational tango where one of the dancers, the planet, isinvisible. By analysing the movements of the visible dancer, we can reconstruct the choreography, the song and even the nature of the hidden companion. We, the planet hunters, search for the periodic changes in the stellar pulsations, fluctuating between red and blue, which can last anywhere from hours to years. These changes indicate perturbations in the stellar velocity, suggesting that there is a planet affecting the galactic dance of one of the stars which illuminates our night sky.
Changes in stellar radial velocity are not only useful to learn about the existence of exoplanets, but can also be used to determine the minimum mass of the planets. This is because the effect of the ‘wobble’ of the star is larger when the difference in mass of the star and the planet is higher. We can, therefore, use the the spectra of a star to understand if a planet is massive like Jupiter, or relatively light like the Earth. The problem with this method is that these changes in velocity are very small. Jupiter, for example, causes the Sun to wobble with a mere velocity of 13 m/s every 10 years, while the Earth does it with an almost insignificant 9 cm/s each year. Hence, we need instruments with extremely high levels of precision and stability if we want to be able detect the effect that exoplanets have on their stars.
We are now in the Telescopio Nazionale Galileo, which hosts one of the best exoplanet hunter instrument in the northern hemisphere: HARPS-N. This spectrograph is a copy of the original HARPS (High Accuracy Radial Velocity Planet Searcher) which is located in the Southern hemisphere, in Chile. Both of these instruments allow us to measure the stellar velocity with a mean precision of 1 m/s, which is approximately equivalent to the speed of a crawling baby. Our mission here is to follow-up exoplanets discovered by the Kepler space telescope, TESS’ predecessor. If we combine our radial velocity measurements with the transits observed by Kepler we are able to obtain the real planet mass (and not just a lower limit). This gives us a first approximation of what the planet is made of, and paves the the first step along the way of testing for habitability. Perhaps the next time we are here we will be measuring the mass of an exoplanet discovered by you via Planet Hunters…
As a matter of symmetry, the end of the night is announced on the horizon with the same colors that we saw at the beginning of the night, 9 hours ago. The vibrant colors mark the time to close to telescope before the Sun is back as a protagonist in the bright blue sky. We leave the telescope in the early hours of the day after having successfully measured the radial velocity of tens of potential planet-hosting stars. Each datum taken this night will help us to decode, step by step, a gravitational choreography, which will tell us about the existence of faraway worlds.
An Observer’s start to the Day
Our ‘day’ here in La Palma starts around 4 pm. After a quick breakfast and a much needed coffee we head up to the mountain to the telescope where the telescope operator has already started setting up the equipment for the night. The Sun is still high in the sky so the telescope dome stays closed while we carry out the calibrations of the instrument that we plan to user throughout the night.
Once all the calibrations are done we have to wait for the sunset, giving us time to have dinner, visit other telescopes, or have a quick snooze in preparation for the long night ahead. The telescopes around here are incredible, and we have bee lucky enough to get a tour of three of the most impressive ones.
MAGIC and CTA
MAGIC (Major Atmospheric Gamma Imaging Cherenkov Telescopes) and CTA (Cherenkov Telescope Array) are the first telescopes that you see when you drive up the mountain from sea-level and their impressive mirrored structures make you feel like you have entered into another world. MAGIC is a system of two Cherenkov telescoped which detect particle showers in the atmosphere released by gamma rays. The twin-telescopes each consist of a 17-m diameter dish that is covered with smaller mirrors that reflect the light into a highly sensitive camera. Next to MAGIC lies the newly built CTA which has a similar design and beautifully reflects the stars at night the sunrise at the start of the day. All three of these telescopes are sensitive to galactic and extragalactic gamma-rays, allowing us to study high energy events in the Universe such as active galactic nuclei, gamma-ray bursts, pulsars and supernova remnants. Without any domes, these telescopes proudly dominate the hillside, making the scenery look slightly surreal (or MAGICal).
Isaac Newton Telescope
The next telescope that we visited was the 2.54-m optical Isaac Newton Telescope (INT). It was initially built in 1967 at Herstmonceux Castle in Sussex, England (the initial site of the Royal Greenwhich Observatory) but was moved to La Palma in 1984, due to light pollution and the less-than-ideal British weather. The INT is the oldest telescope on the mountain, and walking into the impressive building gave me the slight feeling of going back in time. The control room is filled with computers from the 70s with a control deck that exhibits analog dials and manual knobs that control various aspects of the telescope.
The telescope is located on the third floor and sits on a warm wooden floor along with tanks of liquid nitrogen that are used to manually cool the electronics. But it’s not just the telescope that’s impressive at this observatory. Leaving the dome of the telescope we set off on a tour to explore the rest of the building, which felt like a beautiful combination of a museum and a 70s royal bunker. It is fully equipped with office spaces, sleeping rooms, rooms and cupboards filled with various electrical equipment and spare telescope parts, and an incredible library that hosts books and journals that date back to the eighteen hundreds.
We also headed to the roof of the INT, which presented us with a good view of the 4.2-m William Herschel Telescope (WHT) that is currently shining an extremely powerful orange laser into the atmosphere (see top image). Simultaneous observations of this laser and the targets throughout the night allows us to correct for the effects of turbulence in the atmosphere, transforming fuzzy observations into sharp images.
Gran Telescopio Canaria
Our final telescope tour was of the Gran Telescopio Canaria (GTC), the largest optical and infrared telescope in the world. As we walked into the silver dome we were overarched by the huge structure of the telescope – with a height of over 25 m from top to bottom it truly is gigantic! The primary mirror is made up 36 individual hexagonal segments that perfectly piece together to act as a single 10.4 m mirror. The segments, which are made of a ceramic material similar to that used for modern kitchen hobs, are polished to perfection, conforming to a 15 nanometer (millionths of a millimetre) margin of error. But not only does each mirror have to be perfectly uniform, the individual segments must also fit together smoothly with no more than 90 nanometers difference between each. To put this into perspective, if the primary mirror were scaled up to the size of Texas, the ‘unevenness’ would have to be kept to less one millimetre. Automatic sensors are used in order to ensure this degree of accuracy throughout the observations.
Its immense scale combined with the perfect location makes the GTC the ideal telescope to study the nature of black holes, the formation and evolution of stars and galaxies in the early universe, the nature of exoplanets and the mysteries of dark matter and dark energy that fills our Universe. The 400 tonne instrument was truly amazing to see.
Back to our own Telescope
At sunset we drive back up to our own telescope, slightly overwhelmed by the beauty of the sunset that is reflected off the sea of clouds that lies beneath us. So far we have been lucky and have always remained above the clouds, leaving us with beautifully clear skies that allow us to obtain spectra of Kepler and K2 targets. But what are Spectra and what can they tell us? Stay tuned for the types of observations that we are obtaining during our time here.
Oscar and I have just arrived in La Palma, one of the Spanish Canary Islands, where we will be spending the next few days taking radial velocity follow-up observations of Kepler and K2 exoplanet candidates. As this tiny island is situated north of the equator, we are not able to observe any TESS targets from here (yet!), as TESS is currently observing stars in the southern hemisphere. Even though these are not TESS targets, this is great training for when our targets are observable from here.
We will be using the Italian owned 3.58-metre Telescopio Nazionale Galileo (TNG) with the HARPS-N instrument, located at the Roque de los Muchachos Observatory. At it’s highest point of 2,400 m above sea level, the observatory lies above a beautiful ‘sea of clouds below which we find ourselves surrounded by the vast extent of the Atlantic ocean. It is this body of water that ensures that the air at the observatory is very stable, providing us with the perfect conditions to look at stars, galaxies, and our own solar system. There’s good reason as to why they call this one of the best places on Earth to observe the night sky.
We will start to use the telescope tonight, at which point I’ll be able to tell you much more about how this exciting instrument works and about the thrilling process of discovering distant worlds.
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.
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.
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).
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.
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.
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 https://oxford.onlinesurveys.ac.uk/zooniverse-planet-hunters-feedback.
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: https://oxford.onlinesurveys.ac.uk/zooniverse-planet-hunters-your-feedback-needed-
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 firstname.lastname@example.org.
We look forward to sharing the results!