What Kind of World is LHS 1140 b?

One would be forgiven for thinking that it’s easy to tell different types of planets apart. Unfortunately, the most interesting worlds look awfully like comparatively boring ones to our telescopes. A well-studied planet, LHS 1140 b, nicely illustrates this tension: after a re-analysis of archival data, it’s unclear if the planet has a potentially habitable surface ocean, or just a thin layer of hydrogen over a rocky surface.

Previous Work

The star named LHS 1140 has become quite popular among exoplanet astronomers in the last seven years, and for good reason. While not quite as famous as its spotlight-stealing sibling TRAPPIST-1, LHS 1140 shares many of the same properties that make the former such a magnet for telescopes. At just under 50 light-years away, it’s nearby and fairly bright; at just a fifth the mass and radius of the Sun, it’s lightweight enough to be tugged around by tiny planets and small enough that those planets block a large fraction of its light when they transit. All of these characteristics combined make it an excellent host star for scientists looking to make sensitive measurements.

Consequently, LHS 1140 has been observed with a battery of instruments since the discovery of its two accompanying planets in 2017 and 2019. When analyzed one-by-one these datasets suggest that the two worlds, creatively christened in the typical exoplanet fashion as LHS 1140 b and LHS 1140 c, are a little bigger and heavier than our home world but basically made of the same combination of rock and metal. However, all of these observations hadn’t before been analyzed in a single, joint study.

Another Look

Charles Cadieux, University of Montreal, took up this challenge and led a team to reprocess nearly every byte of information collected on these worlds in an attempt to better constrain their masses and radii. They succeeded: as the authors put it, “the LHS 1140 planets are [now] among the best-characterized exoplanets to date, with relative uncertainties of only 3% for the mass and 2% for the radius.”

However, the team wasn’t aiming to shrink the old error bars just for precision’s sake. Instead, their ultimate aim was to constrain the composition of each planet. Although their analysis confirmed that LHS 1140 c is likely a generic super-Earth, their new measurements placed LHS 1140 b in a strange corner of parameter space.

They found that LHS1140 b must have a bulk density less than that of Earth but still much higher than those of the giant planets. After further modeling, the team was left with two very different scenarios for how a planet could arrive in this in-between gray zone. Either LHS 1140 b has a very light, puffy atmosphere of hydrogen and helium overlaying a rocky surface, or, more exotically, LHS1140 b is a “water world,” likely covered in ice with a pocket of liquid water.

A Path Forwards

While it’d be thrilling to have a potentially habitable planet in our galactic backyard, our current data, even when collected with our best instruments and processed with the most cutting-edge techniques, is frustratingly unable to distinguish between the two scenarios. There is technically a path forward to resolving the uncertainty: the authors boldly advocate for an 18-transit observational campaign with JWST to confirm a thick, water-friendly atmosphere. However, with many planets to point to and galaxies galore to observe, there’s no guarantee that the community will choose to dedicate so much time to one target. We may have to live with just the hint that there’s a nearby alien ocean: a bitter, but awe-inspiring, ambiguity.

Citation

“New Mass and Radius Constraints on the LHS 1140 Planets: LHS 1140 b Is either a Temperate Mini-Neptune or a Water World,” Charles Cadieux et al 2024 ApJL 960 L3. doi:10.3847/2041-8213/ad1691