NASA’s Transiting Exoplanet Survey Satellite, or TESS, has discovered three new worlds that are among the smallest, nearest exoplanets known to date. They orbit a star 73 light-years away, which researchers have designated TOI-270, and include an Earth-sized planet (TOI-270b) with a solid, rock structure and two planets (TOI-270c and TOI-270d) similar to Neptune, but about half the size.
“There are a lot of little pieces of the puzzle that we can solve with this system,” says Maximilian Günther, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research. “You can really do all the things you want to do in exoplanet science, with this system.”
The sub-Neptune furthest out (TOI-270d) from the star appears to still be within a “temperate” zone – sometimes called the “Goldilocks” zone. It gets this name because it’s an area that’s not so close to the star that water just boils away, or too far so that everything’s frozen, but in the middle, where it’s just right.
This means that the very top of the planet’s atmosphere is within a temperature range that could support some forms of life. However, scientists say the planet’s atmosphere is likely a thick, ultradense heat trap that renders the planet’s surface too hot to support liquid water or life.
An exoplanet is any planet that exists outside of our solar system. As it stands, scientists have discovered and confirmed more than 4,000 exoplanets in the Milky Way, including gas giants like Jupiter, rocky planets like Earth, and even some dark planets that glide through the universe untethered by the gravity of a star. This milestone, achieved in June, has been recorded by NASA’s amazing Exoplanet Archive.
"TOI-270 is a true Disneyland for exoplanet science, and one of the prime systems TESS was set out to discover” — Maximilian Günther
Despite the possibility of a toxic atmosphere, this new planetary system is proving to have other curious qualities. For instance, all three planets appear to be relatively close in size. In contrast, our own solar system is populated with planetary extremes; from the small, rocky worlds of Mercury, Venus, Earth, and Mars, to the much more massive Jupiter and Saturn, and the more remote ice giants of Neptune and Uranus.
There’s nothing in our solar system that resembles an intermediate planet, with a size and composition somewhere in the middle of Earth and Neptune, but the star TOI-270 appears to host two such planets.
Astronomers believe TOI-270’s two sub-Neptunes may be a “missing link” in planetary formation, as they are of an intermediate size and could help researchers determine whether small, rocky planets like Earth and more massive, icy worlds like Neptune follow the same formation path or evolve separately.
“There are a lot of little pieces of the puzzle that we can solve with this system,” says Maximilian Günther, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research and lead author of a study published in Nature Astronomy that details the discovery. “You can really do all the things you want to do in exoplanet science, with this system.”
To search for exoplanets, TESS uses four large cameras to watch a 24-by-96-degree section of the sky for 27 days at a time. Some of these sections overlap, so some parts of the sky are observed for almost a year. TESS is concentrating on stars closer than 300 light-years from our solar system, watching for transits, which are periodic dips in brightness caused by an object, like a planet, passing in front of the star.
TOI 270 is a M3V-type star (a red dwarf) and is roughly 40 percent the size and mass of our Sun and is roughly located between the constellations of Pictor and Dorado in the southern night sky. Just to really confuse things, it’s also known as TIC 259377017, 2MASS J04333970-5157222 and L 231-32. Different astronomical projects tend to name objects according to that project – for instance, TOI is short for TESS Object of Interest and 2MASS is short for Two Micron All-Sky Survey, a project that took place between 1997 and 2001.
TOI-270’s discovery initially caused a stir of excitement within the TESS science team, as it seemed, in the first analysis, that planet D might lie in the star’s habitable zone, a region that would be cool enough for the planet’s surface to support water, and possibly life. But the researchers soon realized that the planet’s atmosphere was probably extremely thick, and would therefore generate an intense greenhouse effect, causing the planet’s surface to be too hot to be habitable.
“TOI-270 is a true Disneyland for exoplanet science, and one of the prime systems TESS was set out to discover,” Günther says. “It is an exceptional laboratory for not one, but many reasons, it really ticks all the boxes.”
But Günther says there is a good possibility that the system hosts other planets, further out from planet D, that might well lie within the habitable zone. Planet D, with an 11-day orbit, is about 10 million kilometers out from the star. Günther says that, given that the star is small and relatively cool – about half as hot as the Sun – its habitable zone could potentially begin at around 15 million kilometers.
This marks the end of the southern portion of the survey and the spacecraft has now turned its cameras to the north. When it completes the northern section in 2020, TESS will have mapped over three quarters of the sky.
“Kepler discovered the amazing result that, on average, every star system has a planet or planets around it,” TESS project scientist Padi Boyd said in a statement. “TESS takes the next step. If planets are everywhere, let’s find those orbiting bright, nearby stars because they’ll be the ones we can now follow up with existing ground and space-based telescopes, and the next generation of instruments for decades to come.”
On Friday, August 2, the TESS Science Conference at MIT will host a live stream of the Future of Exoplanet Research symposium, featuring panels by leading astronomers in the field on the vision for better understanding exoplanets, distant solar systems and the path to get there. Access the live stream on Facebook, Twitter or at this link. You can follow TESS on Twitter and there is a wealth of information at the Goddard Media Studios that’s really cool and free to use.