Another Hycean Planet Found? TOI-270 d

Hycean planets may be able to host life even though they’re outside what scientists consider the regular habitable zone. Their thick atmospheres can trap enough heat to keep the oceans warm even though they’re not close to their stars.

Astronomers have found another one of these potential hycean worlds named TOI-270 d.

The word hycean is a portmanteau of ‘hydrogen’ and ‘ocean’ and it describes worlds with surface oceans and thick hydrogen-rich atmospheres. Scientists think that they may be common around red dwarfs and that they could be habitable, although any life that exists on a hycean world would be aquatic.

Because they contain so much water, scientists think they’re larger than comparable non-hycean planets. Their larger size makes them easier targets for atmospheric study by the JWST. Though hycean worlds are largely hypothetical now, the JWST is heralding a new era in planetary science and may be able to show that they do exist.

The telescope’s ability to characterize exoplanet atmospheres could be the key to confirming their existence. Using transmission spectroscopy, the space telescope can watch as starlight travels through their atmospheres, revealing the presence of certain important chemicals and even biosignatures.

The exoplanet TOI-270 d could be a hycean world, and a new paper presents evidence supporting that. The paper is “Possible Hycean conditions in the sub-Neptune TOI-270 d,” and it’s published in the journal Astronomy and Astrophysics. The authors are Måns Holmberg and Nikku Madhusudhan, both from the
Institute of Astronomy at the University of Cambridge.

“The JWST has ushered in a new era in atmospheric characterizations of temperate low-mass exoplanets with recent detections of carbon-bearing molecules in the candidate Hycean world K2-18 b,” the authors write. That was an important discovery, and the authors of this paper say the JWST has more to show us about exoplanet atmospheres. In this work, the pair of researchers examined two sub-Neptunes in the TOI-270 system as they transited their M-dwarf. “We report our atmospheric characterization of the outer planet TOI-270 d, a candidate Hycean world, with JWST transmission spectroscopy…,” they write.

TOI-270 is an M-dwarf (red dwarf) star about 73 light-years away. Red dwarfs are known to sometimes flare violently, ruling out habitability on nearby planets. However, the authors describe TOI-270 as a quiet star. It hosts three sub-Neptune planets, and the pair of outermost planets, TOI-270 c and d, are both candidate hycean worlds. TOI-270 d is considered the strongest candidate.

TOI-270 d is about 4.2 Earth masses and measures about 2.1 Earth radii. It takes just over 11 Earth days to complete an orbit, a fact that aids atmospheric study. The Hubble Space Telescope looked at TOI-270 d recently, and its observations suggested a hydrogen-rich atmosphere with some evidence of H₂O. Those results warranted further examination with the more powerful JWST.

Though scientists still haven’t proven that hycean worlds exist, they know something about their atmospheric chemistry. On an ocean world with a thick, hydrogen-rich atmosphere, scientists expect to find strong signatures of CH₄ (methane) and CO₂ and no evidence of NH₃ (ammonia.) This is what the JWST found at K2-18b, though there is still uncertainty if that exoplanet is a hycean world.

Every planet is different, but each type should have things in common. “For Hycean worlds, the presence of an ocean below a thin H2-rich atmosphere may be inferred by an enhancement of CO2, H2O, and/or CH4, together with a depletion of NH3,” the authors write. Since TOI-270 d is a candidate hycean world, its spectroscopy should be similar to other hycean candidates like K2-18b. “Therefore, for the Hycean candidate TOI-270 d, observations of these key carbon-, nitrogen-, and oxygen- (CNO) bearing molecules are required to assess whether or not it is a Hycean world,” the paper’s authors explain.

In October of 2023, the JWST observed TOI-270 b and d during two transits. The observations amounted to a total exposure time of 5.3 hours. “This rare event allows for transmission spectroscopy of both planets,” the authors write.

“Our atmospheric retrieval results support the inference of an H2-rich atmosphere on TOI-270 d and provide valuable insights into the abundances of dominant CNO molecules,” the authors explain. Furthermore, the abundances are similar to what the JWST found on K2-18 b, another suspected hycean world.

But when it comes to water, the results are less certain. “We found only tentative evidence of H2O, with the detection significance and abundance estimates varying…,” the authors write. The detection and abundance of H₂O were more strongly dependent on what method the researchers used to analyze the data.

The appearance of CS² (carbon disulphide) in TOI-270 d’s atmosphere is intriguing. It’s considered a detectable biomarker in hycean world atmospheres, as well as in hydrogen-rich atmospheres of rocky worlds, although the direct sources could also be volcanic or photochemical.

The atmospheric spectrum also contains hints of C₂H₆ (ethane.) Ethane can be a byproduct of photochemical reactions involving methane and other gases, including biogenic ones. Its presence is another indication that methane is present. The researchers also point out that the abundances of ethane and carbon disulphide are well above theoretical predictions. “More observations are required to robustly constrain the presence and abundances of both molecules,” they write.

All the researchers can conclude is that TOI-720 d is a candidate hycean world. But while the previous HST observations that hinted at its status showed the presence of H₂O in an H₂-rich atmosphere, the JWST observations provide more depth. The JWST’s more robust detections of CH₄ and CO_2, along with its non-detection of NH₃, makes it an even stronger hycean world candidate.

“The planet stands out as a promising Hycean candidate, consistent with its initial predictions as a world with the potential for habitable oceans beneath an H2-rich atmosphere,” the authors conclude.