NASA’s TESS planet hunter has discovered three new exoplanets around a nearby star. Two of which, described as ‘sub-Neptunes’ are unlike anything found in our solar system.
NASA’s newest planet hunter, the Transiting Exoplanet Survey Satellite (TESS), has discovered three new exoplanets — one slightly larger than Earth and two of a type not found in our solar system — orbiting a nearby star about 73 light-years away in the southern constellation of Pictor.
The planets straddle an observed gap in the sizes of known planets and promise to be among the most curious targets for future studies.
The star hosting these exoplanets is TESS Object of Interest (TOI) 270–a faint, cool star also known as UCAC4 191-004642.
The M-type dwarf star is about 40% smaller than the Sun in both size and mass, and it has a surface temperature that is about one-third cooler than the Sun’s.
Maximilian Günther, a Torres Postdoctoral Fellow at the Massachusetts Institute of Technology’s (MIT) Kavli Institute for Astrophysics and Space Research in Cambridge is the lead researcher of the study that found the exoplanets.
He says; “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.
“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.”
A paper describing the system was published in the journal Nature Astronomy and is now available online.
Finding the expected, and the not-so expected
TOI 270 b–the innermost planet– is a rocky world roughly 25% larger than Earth which orbits the star every 3.4 days at a distance about 13 times closer than Mercury orbits the Sun. TOI 270 b has a mass around 1.9 times greater than Earth’s based on estimates using existing exoplanets.
Its close proximity to the star means that planet b is an oven-hot world. Its temperature based only on the energy it receives from its star and ignoring warming effects from a possible atmosphere–a quantity known as equilibrium temperature– is around 254 degrees Celcius.
The other two planets, TOI 270 c and d, are, respectively, 2.4 and 2.1 times larger than Earth and orbit the star every 5.7 and 11.4 days. Although only about half its size, both may be similar to Neptune in our solar system, with compositions dominated by gases rather than rock, and they likely weigh around 7 and 5 times Earth’s mass, respectively.
All of the planets are expected to be tidally locked to the star, which means they only rotate once every orbit and keep the same side facing the star at all times–just as the Moon does in its orbit around Earth.
Planet c and d might best be described as ‘mini-Neptunes’–a type of planet not seen in our own solar system.
The ‘missing link’ planets
In our own solar system, there are either small, rocky planets like Earth, Mercury, Venus, and Mars, or much larger planets like Saturn, Jupiter, Uranus, and Neptune that are dominated by gasses.
What our solar system doesn’t have is planets about half the size of Neptune, even though these are common around other stars.
Gunther says: “TOI-270 will soon allow us to study this “missing link” between rocky Earth-like planets and gas-dominant mini-Neptunes because here all of these types formed in the same system.”
The researchers hope that further exploration of TOI 270 may help explain how two of these mini-Neptunes formed alongside a nearly Earth-size world.
Fran Pozuelos, a postdoctoral researcher at the University of Liège in Belgium and co-author of the paper, says: “An interesting aspect of this system is that its planets straddle a well-established gap in known planetary size.
“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.”
The outermost planet — TOI 270 d — is of particular interest to the team. They estimate the planet’s equilibrium temperature to be about 66 degrees C. This makes it the most temperate world in the system — and as such, a rarity among known transiting planets.
Adina Feinstein, a doctoral student at the University of Chicago and one of the paper’s other co-authors, says: “TOI 270 is perfectly situated in the sky for studying the atmospheres of its outer planets with NASA’s future James Webb Space Telescope.
“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.”
Such studies are planned for 2021 when the James Webb space telescope launches. Researchers will measure the composition of the TOI-270 planets’ atmospheres for oxygen, hydrogen, and carbon monoxide. These kinds of observations should allow them to determine if the planets ever had liquid water and thus, were able to support life.
Gunther concludes: “TOI-270 is a true Disneyland for exoplanet science, and one of the prime systems TESS was set out to discover.