NASA's TESS mission announces a roundup from its first year of exoplanet-hunting in the southern sky, introducing us to some of our planetary neighbors.

Scientists with NASA’s Transiting Exoplanet Survey Satellite (TESS) mission have announced a roundup of the mission’s first year of exoplanet hunting at the meeting of the American Astronomical Society in Honolulu.

From July 2018 to July 2019, TESS hunted for planets in the southern sky, revealing more than 1,100 planet candidates. (Including data from the second year, that number climbs to more than 1,500.) Of these, 37 — so far — have been confirmed via follow-up observations and published in peer-reviewed journals. The first-year catalog will be available in an upcoming issue of Astrophysical Journal Supplements, says Natalia Guerrero (MIT).

The public, used to the treasure trove uncovered by TESS’s predecessor, NASA’s Kepler mission, may not be impressed by these numbers. After all, we’ve seen announcements of thousands of planet candidates before. But this time is different.

Like Kepler, TESS finds its planets by watching for dips in starlight caused when a planet transits across the face of its star. But unlike Kepler, TESS watches stars close to Earth. All of the planets it has found are within about 3,000 light-years, and some (such as those below) are much closer, as close as 25 light-years away. With these discoveries, astronomers can do more than tally an exoplanet census — we can finally start getting to know our planetary neighbors, one by one.

Planet radius vs. distance
All planets TESS discovers are within 1,000 parsecs, or about 3,000 light-years, and many are much closer than that. The close distances make the planets easier to observe with other telescopes to characterize their properties, such as their masses and atmospheres (if any).
Natalia Guerrero / Astrophysical Journal Supplements 2020

Meet the Neighbors

Period vs. Radius for TESS planets and planet candidates
The planet candidates TESS has revealed so far (open circles) have a wide variety of size and distance from their star. Confirmed planets are shown as filled circles. (There are 37 confirmed planets, but only 33 have confirmed radii and are plotted here. The other four were discovered via other methods, such as transit timing variations, that do not provide an estimate of the planet's radius.)
Natalia Guerrero et al. / Astrophysical Journal Supplement Series

Earth-size Planet in the Habitable Zone

The Earth-size planet TOI 700d circles its cool, red star every 37 days, putting it in the star’s habitable zone, the region where (under the right conditions) liquid water could exist on its surface. TOI 700d is one of only a few Earth-size planets discovered in their stars’ habitable zones so far, and the first one to be discovered by TESS.

The star, 100 light-years away in the southern constellation Dorado, hosts two other planets as well, both circling the star too closely to be habitable. One is Earth-size; the other, between the size of Earth and Neptune, is likely a gaseous world. All three planets are likely tidally locked, like the Moon, meaning they orbit close enough that they only face one side to the star at all times.

TESS's first Earth-size planet in habitable zone
Diagram of TOI 700 planetary system.
NASA Goddard

Interestingly, the star TOI 700 appears to be relatively quiet compared to other small stars. Red dwarf stars are known for their roiling magnetic activity and outbursts of radiation, which could be harmful to life as we know it and might even strip a planet of its atmosphere. However, over the period of 11 months that TESS watched this star, it did not flare once. While this bodes well for potential habitability, it doesn’t guarantee that the star didn’t flare more often during its youth.

The Thirteenth Tatooine

TOI 1338b is TESS’s first planet found to be orbiting two stars, first identified by high school student and NASA summer intern Wolf Cukier. The discovery rounds out the number of known circumbinary planets to a baker’s dozen.

Star with two suns
TESS's first circumbinary planet
NASA Goddard

The planet, 1,300 light-years away in the southern constellation Pictor, is 6.9 times Earth’s size, between the size of Neptune and Saturn. The stars it orbits are yellow (10% more massive than the Sun) and red (one-third the Sun’s mass). As the planet orbits in the same plane as the two stars, any inhabitants would regularly see one of the stars eclipse the other.

A Planet with Three Red Suns

Jennifer Winters (Center for Astrophysics, Harvard & Smithsonian) raised the stakes a bit higher when she used TESS data to find a planet in a system of three cool, red stars known as LTT 1445ABC. The star designated A orbits the close pair B and C, and follow-up observations by the MEarth Project show that it’s the one that’s hosting the planet. This star, too, is remarkably quiet for a red dwarf — 13 years of monitoring data show that the star varies very little in brightness.

Three red suns
An artist's rendering shows the three red suns that would be visible from LTT 1445Ab, a planet that may be rocky.
ESO / M. Kornmesser

If the planet has the same density as Earth, it would be about 2.5 times Earth’s mass, so it’s not yet clear if it’s rocky. Nevertheless, at just 22.5 light-years away, it’s the second-closest transiting planet known. Astronomers are already itching to use the James Webb Space Telescope, due to launch in March 2021, to measure the planet’s atmosphere.

The system’s unique configuration may also help astronomers better understand planet formation. The planet appears to be transiting the A star in the same plane as stars B and C orbit. In other words, the whole system might be orbiting on the same plane, which could shed light on how planets come to be born in such complicated families.

Some preliminary results announced before the release of the first-year catalog can be found here and here.




Image of dgjohnsonstein


January 10, 2020 at 11:35 pm

What does TOI mean? It would be nice when referring to acronyms to write out what they mean at least once in any article.

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January 11, 2020 at 3:23 pm

TESS Object of Interest.

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