The discovery of the exoplanet LHS 1140 b has captivated scientists since its detection in 2017— mainly because it’s relatively close at a distance of only 48 light-years. This places it among one of the closest potentially habitable worlds, and future findings might substantially boost curiosity. Recent studies using the James Webb Space Telescope indicate that LHS 1140 b may not be a gas giant as earlier presumed; rather, it might be an icy or watery planet with a dense atmosphere. An appropriate target for seeking alien life forms is this curious celestial body which could be neither fish nor fowl.
An oversized planet around an undersized, cool star: LHS 1140 b dwarfs Earth by a factor of about 5.6 in mass while out-sizing our planet by 70% in radius. It was the initiative of a group under the leadership of Charles Cadieux from Université de Montréal to take advantage of ‘director’s discretionary time’ on Webb to study LHS 1140 b using Near-Infrared Imager and Slitless Spectrograph (NIRISS). Earlier observations had labeled LHS 1140 b as a mini-Neptune but this new research presented almost as if it were an icy ocean world— dared to suggest otherwise.
The study, accepted for publication in The Astrophysical Journal Letters, indicates that 10 to 20% of LHS 1140 b’s mass could be water. Although the planet is on the edge of its star’s habitable zone, most of its surface would be frozen. However, models predict a significant “bull’s-eye” ocean where the planet receives maximum solar radiation. This open water patch, potentially half the size of the Atlantic Ocean, could reach temperatures up to 20 degrees Celsius (68 degrees Fahrenheit). “Of all currently known temperate exoplanets, LHS 1140 b could well be our best bet to one day indirectly confirm liquid water on the surface of an alien world beyond our Solar System,” said Cadieux.
Webb’s data builds upon research from Spitzer, Hubble, and TESS, revealing intriguing possibilities such as a nitrogen-rich atmosphere on LHS 1140 b. If future observations confirm this, LHS 1140 b would be the first exoplanet known to have formed a secondary atmosphere after its initial formation. It might also possess high hydrogen levels, categorizing it as a Hycean world, which Webb should be able to verify.
Despite Webb’s power, it faces limitations in observing exoplanets. The hint of nitrogen requires confirmation, and detecting other gases like carbon dioxide will take longer. Observations are only possible when LHS 1140 b transits its star, allowing for a maximum of eight transits per year due to Webb’s location. Therefore, confirming whether LHS 1140 b is an icy world instead of a gas giant could take several years. The preprint study is available on the arXiv server.