NASA Promised More Smaller, Earth-size Exoplanets. TESS is Delivering.

When NASA launched TESS (Transiting Exoplanet Survey Satellite) in 2018, it had a specific goal. While its predecessor, the Kepler spacecraft, found thousands of exoplanets, many of them were massive gas giants. TESS was sent into space with a promise: to find smaller planets similar in size to Earth and Neptune, orbiting stable stars without much flaring. Those constraints, astronomers hoped, would identify more exoplanets that are potentially habitable.

With this discovery of three new exoplanets, TESS is fulfilling its promise.

The three new exoplanets are not what we’re used to. During the Kepler spacecraft’s mission, we grew accustomed to exoplanet discoveries. But the planets were mostly larger than Earth, and many of them were gas giants, hot Jupiters and such. But these three new ones are different: one is slightly larger than Earth, and the other two are roughly twice the size of Neptune, which is small in exoplanet terms.

This scattergram of Kepler first three years of exoplanets shows that Kepler mostly found planets much larger than Earth. Many of the Kepler candidate exoplanets were orbiting very active stars, with so much stellar flaring and other activity that habitability is doubtful. Image Credit: NASA Ames/W. Stenzel

All three orbit a faint, cool star called TOI (TESS Object Of Interest) 270, about 73 light years away in the constellation Pictor. TOI 270 is an M-type dwarf star that’s about 40% smaller than our Sun in both mass and size. It’s also about one-third cooler than the Sun. The three planets are temperate in planetary terms, but the planets are still pretty hot compared to Earth, because they’re close to their star.

The three planets are named TOI 270 b, c, and d, from innermost to outermost planet, following convention.

Because these planets are so close to their star, it’s natural to compare them with Jupiter and its moons. <Click to Enlarge> Image Credit: NASA’s Goddard Space Flight Center

TOI 270 b is probably a rocky world like Earth, but about 25% larger. It takes only 3.4 days to orbit its star, at a distance of 0.03 AU, about 13 times closer to its star than Mercury is to the Sun. The TESS team surmises that it’s about 1.9 times more massive than Earth.

Unfortunately, 270 b is scorching hot. Not as hot as many of the hot Jupiters that Kepler discovered, but still too hot for life as we know it. It’s equilibrium temperature—the temperature before any atmospheric effects—is 254 Celsius (490 F.) It’s also tidally locked to the star.

The middle planet is called TOI 270 c, and its 2.4 times larger than Earth. It orbits the star every 5.7 days. The outermost planet, 270 d, is 2.1 times larger than Earth and orbits the star every 11.4 days. They’re calling these two planets ‘mini-Neptunes’ because they’re composed mostly of gas, and they’re about half the size of Neptune. Both of them are tidally locked to their star, too.

Compare and contrast worlds in the TOI 270 system with these illustrations of each planet. Temperatures given for TOI 270 planets are equilibrium temperatures, calculated without taking into account the warming effects of any possible atmospheres.
Credits: NASA’s Goddard Space Flight Center

The outermost planet is of particular interest to scientists because it’s the most temperate planet of the three. TOI 270 d’s equilibrium temperature is about 66 Celsius (150 F,) still very hot in Earthly terms, but temperate enough to be rare for exoplanets.

The new paper outlining these findings is published in the journal Nature Astronomy. The paper is titled “A super-Earth and two sub-Neptunes transiting the nearby and quiet M dwarf TOI-270.”

“This system is exactly what TESS was designed to find — small, temperate planets that pass, or transit, in front of an inactive host star, one lacking excessive stellar activity, such as flares,” said lead researcher Maximilian Günther, a Torres Postdoctoral Fellow at the Massachusetts Institute of Technology’s (MIT) Kavli Institute for Astrophysics and Space Research in Cambridge. “This star is quiet and very close to us, and therefore much brighter than the host stars of comparable systems. With extended follow-up observations, we’ll soon be able to determine the make-up of these worlds, establish if atmospheres are present and what gases they contain, and more.

These findings are interesting because our Solar System contains nothing like the two mini-Neptunes, or sub-Neptunes. In fact, planets with sizes between 1.5 and 2 times Earth size are very rare in the exoplanet population, too, as far as we know.

<CLick to Enlarge> This infographic illustrates key features of the TOI 270 system, located about 73 light-years away in the southern constellation Pictor. The three known planets were discovered by NASA’s Transiting Exoplanet Survey Satellite through periodic dips in starlight caused by each orbiting world. Insets show information about the planets, including their relative sizes, and how they compare to Earth. Temperatures given for TOI 270’s planets are equilibrium temperatures, calculated without the warming effects of any possible atmospheres. Credit: NASA’s Goddard Space Flight Center/Scott Wiessinger

“An interesting aspect of this system is that its planets straddle a well-established gap in known planetary sizes,” said co-author Fran Pozuelos, a postdoctoral researcher at the University of Liège in Belgium. “It is uncommon for planets to have sizes between 1.5 and two times that of Earth for reasons likely related to the way planets form, but this is still a highly controversial topic. TOI 270 is an excellent laboratory for studying the margins of this gap and will help us better understand how planetary systems form and evolve.”

Astronomers were hoping that TESS would find these types of planets so that other telescopes could do follow-up observations. In particular, the James Webb Space Telescope will be able to identify the atmospheres of some of these planets.

“TOI 270 is perfectly situated in the sky for studying the atmospheres of its outer planets with NASA’s future James Webb Space Telescope,” said co-author Adina Feinstein, a doctoral student at the University of Chicago. “It will be observable by Webb for over half a year, which could allow for really interesting comparison studies between the atmospheres of TOI 270 c and d.”

The James Webb Space Telescope inside a cleanroom at NASA’s Johnson Space Center in Houston. Launch the darn thing already! Credit: NASA/JSC

The team behind these results thinks that, although these results are exciting, there be more to this solar system. Additional observation may reveal other planets. Perhaps, there’s another rocky planet further out from the star than TOI 270 d is. If so, it may be even more temperate. And if it has a rocky core and an atmosphere, it could harbor liquid water on its surface.

These are exciting times in exoplanet research. With results like these, and with the James Webb coming online eventually, our understanding of the exoplanet population will grow in leaps and bounds.


Evan Gough

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