'Best place to look for life': Atmosphere discovered on Earth-like planet

Astronomers say they are one step closer to finding a habitable, Earth-like planet beyond our Solar System with the discovery of an atmosphere on a rocky planet.

They detected traces of helium in the atmosphere of LHS 1140 b, an exoplanet that orbits a dim star about 49 light-years away.

"This [planet] is the best place currently to look for life outside of our Solar System because it has all the main ingredients that we think are essential for life," said Collin Cherubim, who led the study while at Harvard University.

Modelling in the study published today in the journal Science suggests the planet, which is a bit larger than Earth and five times its mass, could potentially be a water world.

Atmospheres are thought to be vital to life on rocky exoplanets as they provide a more stable climate, protect the alien world from radiation, and trap water.

LHS 1140 b also sits within its star's Goldilocks zone, an area that has the right temperatures to support water on a planet's surface.

"I hope this is just the first [discovery] of many to come," Dr Cherubim said.

An unlikely discovery

First discovered in 2017, LHS 1140 b orbits a small star known as a red dwarf.

Scientists look to rocky planets within the Goldilocks zone of their star because of their similarities to Earth.

"[LHS 1140 b] is mostly rocky. We think it's the right temperature to have liquid water," Dr Cherubim said.

"It just happens to be really nearby, relatively. And it orbits a star that's very old and quiet. It's never been seen to flare … which are all really good things for habitability."

While Earth-like rocky planets within the Goldilocks zone are not particularly rare, that doesn't necessarily make them habitable.

Until now, it's been extremely hard to determine whether a planet like LHS 1140 b had an atmosphere.

Recent research using the James Webb Space Telescope looked for signs of different types of chemicals around some rocky planets, including LHS 1140 b, but was inconclusive.

But Dr Cherubim and his colleagues used a relatively new technique and ground-based telescopes to look directly for the specific fingerprint of helium escaping an atmosphere.

Helium can be found much further away from the planet when compared to some other atmospheric gases, which makes it easier to find.

This technique has mostly been used to detect atmospheres around large planets, particularly gassy giants, because it was assumed smaller planets wouldn't have the telltale signature.

"Nobody bothered looking for helium on a rocky, Earth-like planet, especially at Earth-like temperatures as well," Dr Cherubim said.

"People thought it would be a waste of time because you wouldn't expect a lot of hydrogen or helium … because they're such light gases that can evaporate to space over time."

But Dr Cherubim and the team had developed a model looking at how planetary atmospheres evolve over long time frames, particularly atmospheres with high levels of helium.

When he looked at potential planets that could fit the bill, LHS 1140 b was on the list.

"LHS 1140 b [had] a relatively high probability of having this helium-dominated atmosphere," he said.

"And that target jumped out at me because it's a rocky planet, it's in the habitable zone. It would be crazy if it had a helium-dominated atmosphere."

When they went looking, they found the planet had the telltale sign of helium

"Lo and behold, there it was," he said.

Potential search for life

Tom Evans-Soma, an astronomer at the University of Newcastle who specialises in exoplanets and who was not involved in the research, said the signal looked convincing.

"It's very exciting," he said.

Dr Evans-Soma was part of an international team of researchers who used the same technique to make the first-ever detection of helium in the air of an alien world — albeit not a rocky Earth-like one.

"All of the rocky planets that had hints of atmospheres detected so far have been much hotter and less hospitable planets."

Although he noted signal from LHS 1140 b was only detected once, and couldn't be found on a second observation, he said this was not that unusual for helium.

Dr Evans-Soma said modelling done by Dr Cherubim and the team also suggested what this planet might look like.

There are two options — one with a low amount of water and a thick atmosphere, and one with significantly more water than Earth.

"[The model] seems to favour this water world scenario where you've got a mostly Earth-like rocky composition plus 10 per cent water by mass," Dr Evans-Soma said.

"And I think that's very exciting — to imagine what such a planet might be like, especially in the habitable zone, where that water could be in the liquid form on the surface of the planet."

Both Dr Evans-Soma and Dr Cherubim noted that more research was needed to confirm what type of atmosphere the planet has.

The model suggested that along with a large amount of helium, the planet might have water vapour, carbon dioxide, carbon monoxide and small amounts of oxygen.

"It's still a bit tentative," Dr Cherubim said.

"It's not like a smoking gun that it's a helium-dominated atmosphere, but it's consistent with the prediction."

While he hoped there would be more research on LHS 1140 b, he also planned to look for signatures of helium in other planets that lie within their star's habitable zone.

"I have already been awarded time on the same [telescope] to observe a planet that's like a LHS 1140 b twin," he said.

"It's a very similar star that it's orbiting, it's a very similar size, it's a little bit smaller."

"That one I'm really excited about."

Email address