It counts as one of the great goals of exoplanet research: to find an Earth-like rocky planet that not only orbits within its star's life-friendly zone but also holds a protective envelope of gas. At the planet LHS 1140 b, 48 light-years away, exactly that has now been achieved — a finding that advances the hunt for habitable worlds by a notable margin.
Of the more than 6,300 exoplanets confirmed so far, few are as revealing as this one. LHS 1140 b is a rocky planet and circles its star in the so-called habitable zone — the distance at which liquid water could persist on the surface. Whether such a planet could hold an atmosphere at all was long unclear, because small rocky worlds easily lose their gas envelopes to space.
Helium as a telltale signal
The proof came from an international team that observed helium escaping from the planet's upper atmosphere. The escaping gas reveals that LHS 1140 b is wrapped in a helium-rich envelope — strong evidence that even small rocky planets can hold on to an atmosphere. The results appeared in the journal Science.
Astronomers detect such envelopes by analysing the starlight that passes through a planet's atmosphere as it transits: the gases present measurably alter the light's colour composition. Thin atmospheres are especially intriguing, because they may resemble Earth's.
"It's been a major goal in the field of exoplanets to find atmospheres on rocky exoplanets," says planetary scientist Collin Cherubim of Harvard University. For Earth-like life, he adds, an atmosphere is one of the basic prerequisites.
LHS 1140 b is not a second home just yet: the precise makeup of its envelope must await further measurements, and whether water vapour exists there remains open. But the planet now ranks among the most promising candidates for studying the conditions under which distant worlds become hospitable — giving astronomy a rare, concrete address for the search ahead.