Planets Unlike Any in Our Solar System

TOI-270 Star System

 

“The tiny star TOI-270 (less than half the size and temperature of our Sun) hosts one super-Earth and two sub-Neptunes. In our solar system, there is absolutely nothing that resembles such intermediate planets, with a size and composition somewhere in the middle of Earth and Neptune (4 Earth radii); we directly jump from rocky worlds (like Earth) to gas-dominated worlds (like Neptune),” astrophysicist Maximilian Gunther, an ESA Fellow at the European Space Research and Technology Centre (ESTEC), told The Daily Galaxy.

In contrast, Gunther continued, “TOI-270 does seem to only host these kinds of intermediate planets! And despite knowing that such intermediate worlds are plentiful around other stars, we still do not know what they might be made out of – do they have big rocky cores with thick atmospheres? Are they water worlds? Or are they tiny gas balls, like scaled-down versions of Neptune?

“The special thing about TOI-270,” Gunther explained is: “the host star is bright and the system is close to us (only 73 light years away), so we can actually study the masses, atmospheres, and potential compositions of these planets! That is why TOI-270 can be our “missing link” in understanding planetary formation, as its worlds can help determine whether small, rocky planets like Earth and more massive, icy worlds like Neptune follow the same formation path or evolve separately.”

“We’ve found very few planets like this in the habitable zone, and many fewer around a quiet star, so this is rare,” said astronomer Stephen Kane, at UC Riverside’s NASA-funded Alternative Earths Astrobiology Center about super-Earth rocky planet, dubbed TOI-270b, closely orbiting an M-dwarf star with 40% the mass of our Sun. The planet takes only 3.4 days to complete one orbit.  “We don’t have a planet quite like this in our solar system.”

Unlike Any in Our Solar System

Launched in 2018, the TESS discovery of three new worlds around this nearby M-dwarf. TOI-270 is exactly what the satellite was designed to find. Of the three new exoplanets,  one is rocky and slightly larger than Earth, while the two others are gaseous and roughly twice Earth’s size.

Not only is the smaller planet in the habitable zone—the range of distances from a star that is warm enough to allow liquid-water oceans on a planet—but the TOI-270 star is nearby, making it brighter for viewing. It’s also “quiet,” meaning it has few flares and allows scientists to observe it and its orbiting planets more easily, says Kane.

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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 rather than terrain. We don’t have planets about half the size of Neptune, though these are common around other stars.

“Missing Link” Planet

“Our solar system has a significant size gap between the largest rocky planet (Earth) and the smallest gas giant (Neptune, 3.9 times the size of Earth),” Kane wrote in an email to The Daily Galaxy. “The known planets in the TOI-270 system range in size between 1.2 and 2.4 times the size of the Earth, and so straddle a fascinating boundary between smaller rocky planets and larger gas giant planets. An important step to understanding these planets,” Kane continued, “is to measure the composition of their atmospheres, which will teach us how they formed and evolved, and why we don’t have similar such planets in our system. These observations will be carried out by JWST, since the star is relatively close and is bright enough to provide an excellent opportunity to study these ‘missing link’ planets.

“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,” said lead researcher Gunther.

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Around solar-type stars, the Kepler spacecraft discovered that rocky super-Earths are generally observed at shorter orbital periods compared to larger, gaseous sub-Neptunes. This trend possibly suggests that photoevaporation or stellar winds from the host stars strip the gaseous envelopes of the closer planets. The TESS discovery of both a rocky super-Earth and two gaseous sub-Neptunes orbiting a cool, faint M-dwarf star will now allow astronomers to test this hypothesis. 

Follow-up observations on the system have been planned with the James Webb Space Telescope, which was launched less than two weeks ago. It will be able to measure the composition of the TOI-270 planets’ atmospheres for oxygen, hydrogen, and carbon monoxide.

Kane says these kinds of observations can help determine whether a planet has ever had a liquid water ocean, and whether any of the planets has conditions suitable for life as we know it.

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While TOI-270 is far enough away that no one living will likely ever travel there, at 73 light-years away it is still considered close. “The diameter of our galaxy is 100,000 light years, and our galaxy is just one of millions of galaxies,” Kane said. “So, 73 light years means it’s one of our neighboring stars.”

The Last Word from Maximilian Gunther

“We already secured and obtained observations of the atmospheric spectra for the two sub-Neptunes with the Hubble Space Telescope (HST) and proposed for observations with the James Webb Space Telescope (JWST). Most exciting is that to compare the atmospheres of two planets formed in the same system, as this has only been possible for a couple of other targets to date,” Gunther concluded in his email. “For example, any existing methane might react with UV radiation to produce photochemical smogs, similar to what we see in the atmosphere of Titan. Because the TOI-270 sub-Neptunes orbit the same star, we can check if one or both of them have this smog in their atmosphere, and check if it correlates with the distance (and thus UV input) from their star.
“We were of course thrilled to see the hints of three such small exoplanets popping up in our data back in 2019,” Gunther observed. “It was exactly what TESS was designed to find! Twice a week our team comes together at MIT to vet and discuss the latest signals that our machine learning algorithms identify. If we agree a signal has a high potential of being a real exoplanet, we post it publicly as a “TESS Object of Interest”. This way, astronomers and citizen scientists worldwide can observe the target with their telescopes. Only by pooling all our resources we can gain such collaborative results.
“I think it is safe to say that none of the three planets themselves will host life-as-we-know-it on their surface,” notes Gunther. “The outermost planet does, in fact, lie in a temperate zone, where liquid water could exist. However, its surface is likely hidden under a gigantic gas atmosphere. The atmospheric pressures and resulting heat would thus hinder any surface biology.
“Nevertheless,” he ends his email, “one might wonder about whether simple extraterrestrial life (for example, microbial) could find a way to originate and stay aloft in the clouds of such planets – and idea, that Carl Sagan and others already discussed decades ago. Additionally, there is a theoretical chance that this outermost planet might host a small moon with a thinner atmosphere, whose surface temperature might thus be similar to ours on Earth.”

Image of TOI 270 at the top of the page was, taken by the NaCo (NAOS-CONICA) instrument on ESO’s Very Large Telescope. Credit: Günther et al / ESO.

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Maximilian Gunther, Stephen Kane and UC Riverside

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