Escaping helium hints at a long-lived atmosphere on LHS 1140b

The case for air on LHS 1140b rests on elimination. Earlier JWST observations ruled out a hydrogen-rich primary atmosphere ^[3]. That is the kind astronomers think rocky planets hold briefly and lose soon after they form ^[4]. With that envelope excluded, an explainer published on phys.org reads the reported helium as a possible sign of a secondary atmosphere, the kind thought to be more stable and long-lived ^[1][5]. The hedge comes from the piece itself: it calls the result "tenuous and nuanced" ^[15]. The account does not name the team or instrument behind the helium report, give a detection significance, or say how thick the implied atmosphere would be.

The record so far is too thin to say how often planets around red dwarfs keep their air. This is only the second time astronomers have caught even a hint of an atmosphere around a remotely Earth-like planet. The first was Gliese 1214b ^[14].

Then there is the question of what is holding the gas. At 1.7 times Earth's size ^[8] and just over five Earth masses ^[9], LHS 1140b has a mean density of roughly Earth's ^[1] and about a third of Uranus's mass ^[2]. Those figures do not settle what kind of planet it is ^[11]. Asked whether it is a super-Earth or a mini-Neptune, the explainer's author wrote, "We have little idea," mostly because there are no such worlds close to home to compare it with ^[11]. In the solar system the next planet up from Earth is Uranus, at 14 Earth masses ^[10]. Sub-Neptune worlds appear to be the most common planets there are ^[11].

The habitable-zone label needs the same care ^[2]. If the planet absorbed all the starlight falling on it, its equilibrium temperature would be minus 30 C. That is cold, but within the range where a thick greenhouse atmosphere could warm it to milder temperatures ^[12]. A more realistic treatment puts it as low as minus 90 C, comparable to present-day Mars ^[13]. The two estimates sit 60 degrees apart ^[3].

I think the evidence supports "may" and not yet "probably." Escaping helium is gas being lost. If the signal is real, it shows the planet has an atmosphere to lose, and it says nothing yet about how long that atmosphere will last ^[1]. The host is a red dwarf about a fifth of the sun's mass and some 300 times fainter, and the planet dims it every 24.7 days ^[7]. LHS 1140b becomes the leading case for red-dwarf planets holding air only if the helium holds up on a second look and the planet turns out to be rock. If both happen, it would have the persistent, studiable kind of atmosphere Earth has, one whose chemical balance reflects billions of years of geology, chemistry and life ^[5][6].