Archive by Author | Nora Eisner

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?

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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.

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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.

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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!

 

 

 

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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.

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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.

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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: https://archive.stsci.edu/prepds/tess-data-alerts. 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.

toi_list

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:

 https://archive.stsci.edu/tess/bulk_downloads/bulk_downloads_tce.html.

Alternatively you can check if a given candidate is a TCE using EXOMAST (https://exo.mast.stsci.edu/). 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.

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

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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 planethunters.org 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 https://mast.stsci.edu/tesscut/ 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).

tesscut

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?

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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 https://archive.stsci.edu/, 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”.

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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:

https://archive.stsci.edu/missions/tess/doc/EXP-TESS-ARC-ICD-TM-0014.pdf

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!