Exoplanet LHS 1140b, a potentially rocky world that is 1.7 to over five times larger than Earth and orbits a faint red star in the constellation of Cetus, has been the subject of reports of escaping helium gas from its atmosphere. This volatile loss suggests the possible presence of a stable, long-lived secondary atmosphere.
Detecting Volatiles on a Distant Super-Earth
Exoplanet science took a step forwards this summer. Researchers tracking worlds beyond our solar system zeroed in on LHS 1140b. Orbiting a red dwarf roughly one-fifth the mass of our Sun and 300 times fainter than our home star, the planet reveals its presence through a regular brightness dip every 24.7 days. Initial discovery metrics from 2017 estimated the world at 1.7 times larger than Earth. Subsequent observations indicated a planetary body just over five times bigger than Earth.
Classifying these intermediate bodies presents a problem for astronomers. In our local celestial neighborhood, the next biggest planet compared to Earth is Uranus, which is equal to 14 Earths. Then comes Neptune at 17 Earth masses. Worlds sitting in the sub-Neptunian gap remain largely mysterious because we lack examples of such worlds close to home. Yet, these sub-Neptunian worlds appear to be the most common type of planet out there.
The Diagnostic Value of Helium Gas Escaping LHS 1140b
The core excitement centers on atmospheric composition. Previous observations of LHS 1140b with the James Webb Space Telescope ruled out a hydrogen-rich primary atmosphere. Astronomers think that all rocky planets may possess a primary atmosphere and then lose it shortly after they form during a transient phase.
The reported observation of helium may indicate a secondary atmosphere. Unlike primary envelopes, secondary atmospheres persist around their planets. On Earth, our secondary atmosphere has endured for billions of years, driven by the combined effects of geology, chemistry and, ultimately, life.
Thermal Equilibrium and the Habitable Zone Puzzle
LHS 1140b sits within the habitable zone of its host star. This orbital sweet spot theoretically maintains surface temperatures that could allow for the existence of liquid water on its surface.
The mathematical models, however, require calibration. If the exoplanet absorbs all of the stellar radiation that falls upon it, it reaches an equilibrium temperature of -30 C. Introduce a more realistic mathematical treatment, and projections drop the surface thermal baseline to -90 C—a regime comparable to present day Mars. Without a robust greenhouse effect driven by a thick atmosphere, the planet might be cold.
Tenuous Signals and the Limits of Current Instrumentation
Verifying these atmospheric signatures demands precision. Capturing data on the outer gaseous envelopes of stars launched the field of astrophysics 150 years ago. Today, applying similar spectroscopic techniques to planetary envelopes represents an exciting area in modern astronomy.
LHS 1140b marks only the second time astronomers have captured hints of an atmosphere around a remotely Earth-like world, following previous observations of Gliese 1214b. Even so, the result is tenuous and nuanced. A second observation of the planet within the study did not show the presence of helium. Why that discrepancy exists remains an open question.
