NASA has been searching for exoplanets, which are planets outside our solar system, for several decades. The search intensified when the Kepler mission was launched in 2009. Scientists made thousands of discoveries from the data gathered from the Kepler telescope, including a type of exoplanet that orbits binary stars. Binary stars are when two stars orbit each other, and the planets that go around these systems are called “circumbinary planets.” Kepler got things started by finding a handful of these planets and demonstrating that they existed by looking at a small patch of sky. But, in order to really understand these far away worlds, NASA needed to perform continuous observations of half a million of these star systems by looking at most of the sky. That’s where TESS comes in.
Transiting Exoplanet Surveying Satellite (TESS) is a new space telescope launched on April 18, 2018 that will search for exoplanets of all kinds, including those closer to us than Kepler could see. Recently, TESS found it’s first circumbinary planet, called TOI-1338. It is about the size of Saturn and orbits its double sun every 95 days (so it’s year is like a third of ours). These kinds of planets are millions of light years from Earth, too far to visit. So how can we see them?
Part of TESS’s name “transiting” is a clue into how it works. TESS is outfitted with four sensitive cameras that collect light data from distant stars. When the star light dims, it is because something has passed in front of it. This is called a transit. If the transit occurs in a pattern, this object is considered to be orbiting the star. The amount of dimming indicates whether it is a planet or something else. The trouble with observing planets around binary stars is that while the planet causes dimming, so do the stars as they pass in front of each other. This creates messy data that is hard to analyze, because you need to first be able to tell the difference between a star transit and a planet transit.
One large research team, combining scientists at NASA Goddard, The SETI Institute, San Diego State, University of Chicago, numerous other research institutes, volunteer “citizen scientists” called Planet Hunters, and even a high school intern sifted through these binary star datasets and uncovered TOI-1338. Interestingly, TOI-1338 had been discovered before by ground-based telescopes, but the only thing known at that point was that a dimming pattern occurred every 15 days. It was thought that TOI-1338 was just a binary star system, not a planet. TESS uncovered an additional dimming event which occurred every 93-95 days, and it seemed much more like a planet’s orbit than a binary star.
As extraordinary claims call for extraordinary measures, the research team performed intense statistical calculations to verify that the planet was not a false positive, along with the characteristics of the planet, and the characteristics of the star. Because there are so many competing signals when you observe space, the scientists needed to do this work to convince the planet-finding community that they had indeed found a planet.
Two of the best arguments against it being a false positive were 1) it’s irregular orbit, more consistent with that of a planet than a star, and 2) their chances of finding this type of planet. That chance, called false positive probability, was calculated by how much of the sky was searched, how long it was searched, and how common these types of planets are known to be. By their calculations, there is a 0.0000001% chance that this discovery is false. Also, the scientists were able to take their new data and see if it still made sense when compared to the old, ground-based telescope data from 2009.
One question on everyone’s mind might be — does this Saturn-sized planet have life? Unfortunately, it probably does not. It is too close to its binary sun, so it is probably too warm. However, the scientific community now knows that TESS is capable of finding exoplanets orbiting binary stars, which means it will surely find more. Perhaps one of those planets will be Earth-sized and just the right distance away from its star to be a candidate for life, and our technology will be ready and waiting to learn more.