A New Potentially Habitable Planet from TESS

Illustration of the orbits of the planets in the TOI-700 system. Credit: NASA.

This week is what many call the “Super Bowl of Astronomy”: the winter meeting of the American Astronomical Society (AAS), held this year in Honolulu. I am not there, sadly, but I am involved in one of the big press stories to come out of there.

Extrasolar planets are one of the many fields of astronomy that are announcing discoveries at the AAS, and one of the biggest stories is that NASA’s Transiting Exoplanet Survey Satellite (TESS) has discovered its first “potentially habitable”* planet, TOI-700 d. As it happens, I am one of the coauthors (along with 92 others) on the first of the three papers about this planet that were announced yesterday, which is not something you get to say often about a paper with its own press conference, even at NASA.

This paper is about the discovery of TOI-700’s three planets and a little bit about what we can infer about their atmospheres. For the record, the second paper was about using the Spitzer Space Telescope to observe the planets, both to prove that they’re really there (a fraction of these “detections” are actually binary stars in the background) and to potentially measure some actual data about the atmosphere. (As an aside, Spitzer is being retired on January 30, so this was a really last-minute thing.) And the third paper wasn’t about observing the planet directly, but about making theoretical models for the atmosphere to figure out what it will take to measure the composition of the atmosphere.

TESS is tasked with scanning the half million brightest stars in the sky for transiting planets. These are planets that pass directly in front of their stars. They’re are only a small fraction of all the planets out there, but they are the easiest to study, and by looking at the brightest stars, TESS finds planets that are closer and even easier to study than the ones found by Kepler.

TOI-700 is a red dwarf star—type M2 with a brightness only 2.3% that of our Sun. This is important because TESS mostly looks only at short-period planets, and you need a small, cool star for short-period planets to be cool enough to be habitable. TOI-700 has three planets: b, c, and d, but it’s planet d that everyone is talking about. Planet d orbits its star in 37 days; its size is such that it should have a solid surface, and (if it has a similar atmosphere) its temperature should be very close Earth’s.

Unfortunately, it will be 5-10 years before we can get good enough observations to tell what’s in the atmosphere of this planet. What I think is more interesting is the middle planet, planet c. Planet c orbits in 16 days. At that distance, we would expect it to be a lot like Venus, but it’s not. It’s a mini-Neptune with a radius 2.7 times as large as Earth, which almost certainly means it has a deep hydrogen atmosphere.

This is very interesting because to find a planet with this kind of hydrogen atmosphere in between two planets that don’t have one is unusual. Planets in the same solar system are usually about the same size, like Venus and Earth, and it’s uncommon to see such a large difference in planets that are right next to each other.

I caution that this part is not in the paper, and I didn’t do a detailed analysis because it wasn’t the subject of the paper, but TOI-700 c is close to the boundary where you would expect to see a deep hydrogen atmosphere evaporated away by the starlight. It will take more observations to tell, but there may be something unusual about its atmosphere. Getting good observations of TOI-700 c will be much easier, and it think they could give us valuable information about how planetary atmospheres form and evolve.

One very important measurement is to find the masses of all three planets. This is much easier to do with radial velocities and can probably be done within the year. Once we know the exact masses of the planets, we will be able to estimate the properties of their atmospheres much better. TOI-700 is ongoing proof that it is a very exciting time in the field of exoplanets.

I do want to take some time to explain the name of the star because it confused even me, and I work in this field.

TOI-700 means “TESS Object of Interest #700.” For Kepler, candidate planets were calls KOIs, while confirmed planets got their own Kepler numbers, like Kepler-22. (Like I mentioned, a lot of them were false alarms.) But there’s not much point in doing this for TESS because all of the stars TESS is watching have their own catalog numbers. Most of these are at least semi-memorable, like Pi Mensae or GJ 357. However, we use TOI-700 for this one because its proper catalog number is 2MASS J06282325-6534456. 2MASS stands for the Two-Micron All Sky Survey, the most important infrared survey of the sky, done from 1997-2001. And that long string of numbers are the coordinates of the star in the sky. Usually, they shorten it to something like 2M0628 for quick reference, such as if you’re writing a paper about many 2MASS objects, but TOI-700 is easier to remember and work with.

* To many of us astronomers, “potentially habitable” isn’t very good terminology. It implies more than it really means. What it means is only that the planet has a solid surface and is a temperature where it could have liquid water, but there’s no proof that it does have liquid water, let alone life. I might write more on this later.

About Alex R. Howe

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