Dr. Guenther said the TESS team was initially very excited, when the outermost of the three planets, one of the sub-Neptunes, seemed to orbit in the star’s habitable zone; that would have been a first for TESS. But as the analysis advanced, the team concluded that the planet likely had a thick greenhouse atmosphere, with suffocating surface temperatures.
But any planets orbiting farther from the star could be habitable, Dr. Guenther and his colleagues said. Locating any such planets is made easier by the fact that the star is relatively quiet, free of outbursts and noise that could interfere with searches.
“Chances are good that we will find more planets further out in the habitable zone,” he said.
The new system could shed light on a looming planetary mystery: Why are there no planets in the size range between 1.5 and 2 times that of Earth?
Planets below that size range, including Mars and Venus (and, of course, Earth), are rocky worlds. Planets more than twice the size of Earth have thick gas atmospheres, presumably surrounding rocky cores — like Neptune, but smaller. Our own solar system does not contain any sub-Neptunes; the only known examples are far away, found in the growing catalogs of exoplanets.
The worlds of TOI-270 crowd either side of this missing-link gap.
It is intriguing that the innermost planet is also the small rocky one, Dr. Guenther said. Perhaps, he suggested, it was once a gas giant like its siblings, but lost its atmosphere when, in the ceaseless shift of orbits and worlds, it moved too close to its star. If that notion bears out, it could have consequences far beyond the TOI-270 system, including for our own solar system.
Follow-up observations are already being planned with NASA’s upcoming James Webb Space Telescope to probe the atmospheres of these planets and see what they are made of.