Harvard scientists detect atmosphere on Earth-like planet for first time

By Stephen Beech

The search for alien life has been boosted after an atmosphere was detected surrounding an Earth-like planet for the first time.

The discovery provides the strongest evidence yet that worlds with conditions similar to our own in with the potential to support life could exist beyond our solar system, say American scientists.

Nearly a decade after discovering LHS 1140b, a rocky exoplanet in the "habitable zone" of a nearby low-mass star, a new study reveals the object may have its own atmosphere.

Study lead author Collin Cherubim, of Harvard University, said: "An atmosphere is essential for a planet to support life as we know it.

"This is the first time anyone has found an atmosphere on a rocky planet in the habitable zone of another star."

University of Florida assistant professor of astronomy Jason Dittmann, who first discovered LHS 1140b in 2016, is co-author of the new study, published in the journal Science.

He said: "The exciting part about this paper, and why I think it was accepted into Science, was that this is the first time that we're seeing a rocky, Earth-like planet that could still have an atmosphere."

The Magellan Clay telescope at Las Campanas Observatory in Chile showed proof of helium escaping from the planet.

But the planet's age signals it would have run out of helium unless it was replenishing its own supply, pointing to the potential of an atmosphere.

In 2016, Dittmann used ground-based surveys to search for stars whose light briefly dimmed as orbiting planets passed in front of them.

To understand if the stars were dimming due to wispy clouds or humidity blowing through on Earth, he trained a machine learning algorithm to decipher which signals were caused by our weather and which were because of a passing planet.

The method helped Dittmann find LHS 1140b.

Considering the type of star it orbits, he said the planet's temperature should be similar to Earth's.

It's made of rock instead of gas, is around 48 light-years away, and there is also a second planet in the system, LHS 1140c, which is located outside the habitable zone of its system.

Other rocky planets discovered within the past decade or so lost their atmospheres over time.

Considering the age of LHS 1140b and the lack of atmospheres by similar planets, Dittmann and his colleagues didn't expect to find helium.

He said: "We were getting to the point in the field where maybe all of these planets don't have an atmosphere, and we need to look at ones around sun-like stars instead of smaller stars.

"And then finally here is actually one with an atmosphere, and it happens to be the one that I had spent so many hours working on."

After originally discovering the planet, Dittmann requested X-ray data from the planetary system.

That gave him the energy input from the star into the planet, which became key to interpreting the helium signal years later.

The Magellan Clay telescope later revealed helium escaping from the planet, a process that also occurs on Earth.

LHS 1140b orbits a red dwarf star within the star's habitable zone — the region where temperatures and environmental conditions are within the range that could support liquid water on the planet's surface.

Astronomers have discovered thousands of exoplanets, including a few rocky worlds within their stars' habitable zones.

But determining whether those planets have atmospheres has remained a great challenge.

Harvard professor Robin Wordsworth said: "Twenty years ago we wondered whether other terrestrial-type planets even existed.

"Then we learned they're common, and found some in the habitable zone.

"The next question was whether any of them had managed to keep an atmosphere.

"Now we know at least one has."

Although other studies have found rocky planets in the habitable zones of their stars, this study is the first to clearly demonstrate the presence of an atmosphere, one that has existed for billions of years.

Cherubim and his colleagues' theoretical model predicted that LHS 1140 b has an upper atmosphere rich in helium that is slowly escaping into space.

To test their prediction, the team used the Warm Infrared Echelle (WINERED) Spectrograph on the Magellan Observatory.

They observed a rare alignment, where LHS 1140 b and another planet transited their star on the same night.

Although one planet showed no evidence of an atmosphere, the other, LHS 1140 b, showed helium escaping from around it, confirming that it retains an atmosphere.

The findings suggest that ground-based observations searching for escaping gases may become an important tool for studying atmospheres on rocky exoplanets.

Cherubim said he'd like to determine the atmosphere's full composition and eventually investigate whether the planet has surface oceans or other characteristics associated with habitability.

He and his colleagues will also use his model to search for similar worlds.

Cherubim added: "This has been a model validation, and hopefully it's just the first of many more observations to come."

LHS 1140b is one of the current selected targets under the Rocky Worlds Director's Discretionary Time (DDT) Program.

Rocky Worlds DDT is a joint initiative for the James Webb Space Telescope (JWST) and the Hubble Space Telescope that is dedicated to finding evidence of atmospheres on rocky exoplanets orbiting dwarf stars.

Dittmann said the project should be able to prove or disprove an atmosphere on LHS 1140b in the next four to five years.

He added: "Because there's helium there, and because the helium is escaping, the question is: is it a bare rock with no atmosphere that sometimes burps up some gas that then immediately escapes, or is there a steady-state atmosphere there that will leak out stuff like the Earth does from time to time?

"JWST data over the next four to five years will look for water, and if there's water in the atmosphere, then it's probably a stable atmosphere that will persist."