How to Find an Exoplanet from Home: A Beginner's Guide to Real NASA TESS Data
Most of the 6,000-plus planets known outside our Solar System were never seen directly. They were found by noticing a star grow very slightly dimmer, again and again, as a planet passed in front of it. That is the transit method, and the data it uses are public. With a browser and a little patience, you can look for planets in the same measurements professional astronomers use.
This guide explains how a transit hunt works, step by step, with the real stars in our Planet Hunter.
Where the data come from: NASA's TESS
NASA's Transiting Exoplanet Survey Satellite (TESS), launched in April 2018, watches a strip of sky 24° by 96° for about 27 days at a time (one "sector"), then turns to the next. For hundreds of thousands of selected stars it records a brightness measurement every two minutes. The result for each star is a light curve: brightness plotted against time.
All of it is released to the public through NASA's MAST archive. TESS has flagged thousands of possible planets, called TESS Objects of Interest (TOIs), and hundreds have been confirmed so far. Many more signals are sitting in the data, waiting for someone to look.
How big is a planet's dip?
When a planet crosses its star, it blocks a fraction of the star's disc equal to the ratio of their areas, (planet radius / star radius)². That gives a feel for what you are looking for:
- A Jupiter in front of a Sun-like star: about 1% dimmer.
- An Earth in front of a Sun-like star: about 0.008% (84 parts per million).
- A small planet in front of a small red dwarf: much deeper, because the star is small. This is why many small TESS planets orbit red dwarfs.
The planets in Planet Hunter span that whole range, from WASP-18 b, a giant whose dip is about 1% deep and comes back every 22.6 hours, down to Pi Mensae c, the first planet TESS announced, whose transit dims its bright star by less than 0.03%.
Step 1: find a repeating dip
Real light curves are noisy. Stars flicker, the spacecraft jitters, and every measurement has a little scatter. A single dip proves nothing. What you want is a dip that repeats, with the same depth and the same length each time.
In Planet Hunter you mark each dip you see. Once you have two or three, the gaps between them should be whole multiples of one number: that is the planet's orbital period. LHS 3844 b, a rocky planet around a red dwarf, goes round in just 11 hours, so a single 27-day sector shows dozens of its transits.
Step 2: fold the light curve
Once you have a period, you can cut the light curve into pieces one period long and stack them on top of each other. This is called phase folding. If the period is right, all the transits line up and a weak dip that was buried in the noise suddenly stands out clearly. If the period is wrong, the dips smear out. Folding is how astronomers find planets far too shallow to see by eye in the raw data.
Step 3: read the planet from the dip
The folded transit tells you a lot:
- Depth gives the planet's size relative to the star. Knowing the star's radius, you get the planet's radius.
- Duration depends on the orbit's size and the star's size: a planet far from its star moves slowly and takes longer to cross.
- Shape matters: a planet makes a flat-bottomed dip with sloping edges, because it is completely in front of the star for a while.
Step 4: make sure it is not two stars
The most common impostor is an eclipsing binary: two stars orbiting each other and taking turns to block each other's light. Warning signs:
- The dips are V-shaped rather than flat-bottomed.
- Alternate dips have different depths (odd and even dips do not match), because the two stars are different.
- There is a smaller second dip halfway between the main ones, when the brighter star hides the fainter one.
- The dip is too deep for any planet: a planet cannot block more than a few percent of a Sun-like star.
Eclipsing binaries are not always bad news, though. In 2020, a 17-year-old NASA intern, Wolf Cukier, was checking Planet Hunters TESS data for exactly this kind of binary, the pair TOI-1338, when he noticed an extra dip that did not fit. It turned out to be TOI-1338 b, a planet about 6.9 times the size of Earth orbiting both stars. You can study the same system in Planet Hunter.
Step 5: is it already known?
Before you celebrate, check whether the star already has a TOI or a confirmed planet. Planet Hunter does this for you by comparing your period with NASA's catalogues. If it does not match anything known, you may have spotted a genuine new candidate. Volunteers on the Planet Hunters TESS project (on Zooniverse) have found many such candidates, and some have been confirmed as planets.
Try a real search
The ten training stars in Planet Hunter are known systems, so you can check your answers. When you are ready, the Real search mode loads light curves from the newest TESS sector straight from NASA's archive. Most signals in it have not been examined by anyone yet.
Open Planet Hunter, start with WASP-18 b, and see how small a dip you can find.
Frequently asked questions
Can amateurs really discover exoplanets? Yes. TESS and Kepler data are public, and volunteers in projects such as Planet Hunters TESS have found planet candidates that professional astronomers later confirmed, including the circumbinary planet TOI-1338 b.
What is a light curve? A graph of a star's brightness over time. A planet passing in front of the star shows up as a small, repeating dip.
How small a planet can TESS find? TESS has found planets smaller than Earth, mostly around small, nearby red dwarfs, where an Earth-sized planet blocks a larger share of the starlight.
Why do most transiting planets have short orbits? Because a planet has to pass in front of its star at least twice, preferably more, during the few weeks TESS watches a patch of sky. Close-in planets transit often; a planet with a one-year orbit would usually show at most one dip.
Explore it yourself: Planet Hunter · Second Earth · Exoplanets and the Milky Way in 3D · How big are the stars?
Sources
- Ricker et al. (2015), Transiting Exoplanet Survey Satellite, JATIS 1, 014003. doi:10.1117/1.JATIS.1.1.014003
- Kostov et al. (2020), TOI-1338: TESS's First Transiting Circumbinary Planet, AJ 159, 253. doi:10.3847/1538-3881/ab8a48
- NASA, NASA's TESS Mission Uncovers its 1st World with Two Stars (2020): Wolf Cukier's discovery. nasa.gov
- Huang et al. (2018), TESS Discovery of a Transiting Super-Earth in the Pi Mensae System, ApJL 868, L39. doi:10.3847/2041-8213/aaef91
- Vanderspek et al. (2019), TESS Discovery of an Ultra-short-period Planet around the Nearby M Dwarf LHS 3844, ApJL 871, L24. doi:10.3847/2041-8213/aafb7a
- NASA Exoplanet Archive and MAST (Mikulski Archive for Space Telescopes). exoplanetarchive.ipac.caltech.edu · archive.stsci.edu
Transit depths are (planet radius / star radius)², using Earth 6,371 km, Jupiter 69,911 km and the Sun 695,700 km. Periods and depths of the Planet Hunter stars are the values measured in their TESS light curves.