This summer, exoplanet science — the study of planets orbiting stars beyond our Solar System — took a significant step forward. Reports of helium escaping from the atmosphere of LHS 1140b brought this exoplanet out of the pages of scientific journals and into the attention of mainstream media.
An Exoplanet in the “Habitable Zone”
As phys.org reports, scientists continue to study the exoplanet LHS 1140b. It orbits a dim red star in the constellation Cetus — named after a sea monster from Greek mythology — located directly next to the easily recognizable “W” shape of the constellation Cassiopeia. But do not squint trying to see it — the star appears approximately 10,000 times dimmer than the faintest star visible to the naked eye.
This exoplanet is of particular interest because it is a potentially rocky world located in the “habitable zone” of the star it orbits. This means that the temperature on its surface could be suitable for liquid water to exist. It is entirely possible that life could exist there.
A “Secondary” Atmosphere
Previous observations of LHS 1140b with the James Webb Space Telescope ruled out the presence of a hydrogen-rich atmosphere — a so-called “primary” atmosphere, which is thought to form together with young planets and disperse into space soon afterward.
Astronomers believe that all rocky planets may have primary atmospheres that they lose shortly after formation. This is a brief, transient phase in their history.
However, the reported detection of helium may indicate the presence of a “secondary” atmosphere, which is believed to be more stable and long-lived. Secondary atmospheres remain around planets and can be studied by astronomers.
On Earth, our secondary atmosphere has existed for billions of years, and its chemical balance reflects the combined influence of geology, chemistry, and, ultimately, life.
The Problem of Determining What Type of Planet LHS 1140b Is
LHS 1140b orbits a red dwarf whose mass is approximately one-fifth that of our Sun, but whose brightness is about 300 times lower. However, as seen from Earth, the brightness of this star regularly decreases every 24.7 days.
Based on the magnitude of the drop in brightness, the team that discovered the planet in 2017 estimated its size to be 1.7 times that of Earth. Further observations showed that the planet’s mass is slightly more than five times that of our planet.
Such “super-Earths” are among the least massive planets we can currently detect and are among the best candidates in the search for life beyond our Solar System. However, precisely characterizing such worlds remains a challenge for astronomers.
So is LHS 1140b a “super-Earth” or a “mini-Neptune”? We have almost no idea, mainly because of the lack of examples of such worlds near our Solar System. This gap in our understanding of planetary physics is an urgent problem because worlds smaller than Neptune appear to be the most common type of planet in the Universe.
A rocky, Earth-like exoplanet with a detected atmosphere in the habitable zone, even around a dim red star, would be an important scientific discovery because of its potential to support life. However, as with any cutting-edge science, the devil is in the details.
Determining the Conditions Necessary for Life on the Surface of an Exoplanet
As already noted, LHS 1140b is of particular interest because it lies within its star’s “habitable zone” and may have a surface temperature between 0°C (32°F) and 100°C (212°F), which could allow liquid water — and possibly life — to exist on its surface.
However, such conditions should be treated with caution. If LHS 1140b absorbs all the stellar radiation that reaches it, its equilibrium temperature would be -30°C (-22°F). At first glance this may seem cold, but it is still well within the range in which the greenhouse effect of a dense atmosphere could warm the planet to more moderate temperatures.
However, if a more realistic mathematical approach is applied to the planet, the temperature of LHS 1140b could be as low as -90°C (-130°F), comparable to modern-day Mars.
This is only the second time astronomers have managed to detect even a hint of an atmosphere around a planet that remotely resembles Earth. The first occurred around the exoplanet Gliese 1214b.
Like the probable atmosphere itself, this result is faint and ambiguous. During the second observation of the planet conducted as part of this study, no helium was detected. The reasons for this remain unclear. In addition, the figures underlying calculations of the conditions on the surface of LHS 1140b involve significant, untested assumptions.
Nevertheless, this is a step forward, even if a small one, toward the goal of conducting atmospheric spectroscopy of potentially habitable worlds. And we should not forget that this single step forward was the result of years of persistent work by a dedicated scientific team.
An Important Step Toward Understanding the Rules Governing Planetary Atmospheres
But what about the higher goal — detecting life on such worlds through traces of gases in their atmospheres? Which potentially biogenic molecules would constitute unequivocal evidence — a so-called “smoking gun” — for the existence of life? Could the answer lie in our own atmosphere, enriched with oxygen and methane molecules?
The answer to these questions is that we will probably gain a much clearer understanding once we observe the atmospheres of thousands of exoplanets, rather than just one or two. We need nature to teach us the rules by which planetary atmospheres function, and that work has only just begun.
But if the journey to a thousand planets begins with a single step, then thanks to these recent observations of LHS 1140b, we have just taken the second one. The journey continues.
