The latest report from ESO (European Southern Observatory):

Newly Discovered Exoplanet May be Best Candidate
in Search for Signs of Life 

An exoplanet orbiting a red dwarf star 40 light-years from Earth may be the new holder of the title “best place to look for signs of life beyond the Solar System”. Using ESO’s HARPS instrument at La Silla, and other telescopes around the world, an international team of astronomers discovered a “super-Earth” orbiting in the habitable zone around the faint star LHS 1140. This world is a little larger and much more massive than the Earth and has likely retained most of its atmosphere. This, along with the fact that it passes in front of its parent stars as it orbits, makes it one of the most exciting future targets for atmospheric studies. The results will appear in the 20 April 2017 issue of the journal Nature.

The newly discovered super-Earth LHS 1140b orbits in the habitable zone around a faint red dwarf star, named LHS 1140, in the constellation of Cetus (The Sea Monster) [1]. Red dwarfs are much smaller and cooler than the Sun and, although LHS 1140b is ten times closer to its star than the Earth is to the Sun, it only receives about half as much sunlight from its star as the Earth and lies in the middle of the habitable zone. The orbit is seen almost edge-on from Earth and as the exoplanet passes in front of the star once per orbit it blocks a little of its light every 25 days.

“This is the most exciting exoplanet I’ve seen in the past decade,” said lead author Jason Dittmann of the Harvard-Smithsonian Center for Astrophysics (Cambridge, USA). “We could hardly hope for a better target to perform one of the biggest quests in science — searching for evidence of life beyond Earth.”

“The present conditions of the red dwarf are particularly favourable — LHS 1140 spins more slowly and emits less high-energy radiation than other similar low-mass stars,” explains team member Nicola Astudillo-Defru from Geneva Observatory, Switzerland [2].

For life as we know it to exist, a planet must have liquid surface water and retain an atmosphere. When red dwarf stars are young, they are known to emit radiation that can be damaging for the atmospheres of the planets that orbit them. In this case, the planet’s large size means that a magma ocean could have existed on its surface for millions of years. This seething ocean of lava could feed steam into the atmosphere long after the star has calmed to its current, steady glow, replenishing the planet with water.

This artist’s impression video shows an imaginary trip to the exoplanet LHS 1140b, which orbits a red dwarf star 40 light-years from Earth and may be the new holder of the title “best place to look for signs of life beyond the Solar System”. Using ESO’s HARPS instrument at La Silla, and other telescopes around the world, an international team of astronomers discovered this super-Earth orbiting in the habitable zone around the faint star LHS 1140. This world is a little larger and much more massive than the Earth and has likely retained most of its atmosphere. Credit: ESO/spaceengine.org

The discovery was initially made with the MEarth facility, which detected the first telltale, characteristic dips in light as the exoplanet passed in front of the star. ESO’s HARPS instrument, the High Accuracy Radial velocity Planet Searcher, then made crucial follow-up observations which confirmed the presence of the super-Earth. HARPS also helped pin down the orbital period and allowed the exoplanet’s mass and density to be deduced [3].

The astronomers estimate the age of the planet to be at least five billion years. They also deduced that it has a diameter 1.4 times larger than the Earth — almost 18 000 kilometres. But with a mass around seven times greater than the Earth, and hence a much higher density, it implies that the exoplanet is probably made of rock with a dense iron core.

This super-Earth may be the best candidate yet for future observations to study and characterise its atmosphere, if one exists. Two of the European members of the team, Xavier Delfosse and Xavier Bonfils both at the CNRS and IPAG in Grenoble, France, conclude:

“The LHS 1140 system might prove to be an even more important target for the future characterisation of planets in the habitable zone than Proxima b or TRAPPIST-1. This has been a remarkable year for exoplanet discoveries!” [4,5].

In particular, observations coming up soon with the NASA/ESA Hubble Space Telescope will be able to assess exactly how much high-energy radiation is showered upon LHS 1140b, so that its capacity to support life can be further constrained.

Further into the future — when new telescopes like ESO’s Extremely Large Telescope are operating — it is likely that we will be able to make detailed observations of the atmospheres of exoplanets, and LHS 1140b is an exceptional candidate for such studies.

Notes

[1] The habitable zone is defined by the range of orbits around a star, for which a planet possesses the appropriate temperature needed for liquid water to exist on the planet’s surface.

[2] Although the planet is located in the zone in which life as we know it could potentially exist, it probably did not enter this region until approximately forty million years after the formation of the red dwarf star. During this phase, the exoplanet would have been subjected to the active and volatile past of its host star. A young red dwarf can easily strip away the water from the atmosphere of a planet forming within its vicinity, leading to a runaway greenhouse effect similar to that on Venus.

[3] This effort enabled other transit events to be detected by MEarth so that the astronomers could nail down the detection of the exoplanet once and for all.

[4] The planet around Proxima Centauri (eso1629) is much closer to Earth, but it probably does not transit its star, making it very difficult to determine whether it holds an atmosphere.

[5] Unlike the TRAPPIST-1 system (eso1706), no other exoplanets around LHS 1140 have been found. Multi-planet systems are thought to be common around red dwarfs, so it is possible that additional exoplanets have gone undetected so far because they are too small.

An announcement from the Max Planck Institute for Astronomy:

Atmosphere around low-mass Super-Earth detected

Astronomers have detected an atmosphere around the super-Earth GJ 1132b. This marks the first detection of an atmosphere around a low-mass Super-Earth, in terms of radius and mass the most Earth-like planet around which an atmosphere has yet been detected. Thus, this is a significant step on the path towards the detection of life on an exoplanet. The team, which includes researchers from the Max Planck Institute for Astronomy, used the 2.2 m ESO/MPG telescope in Chile to take images of the planet’s host star GJ 1132, and measuring the slight decrease in brightness as the planet and its atmosphere absorbed some of the starlight while passing directly in front of their host star.

While it’s not the detection of life on another planet, it’s an important step in the right direction: the detection of an atmosphere around the super-Earth GJ 1132b marks the first time an atmosphere has been detected around a planet with a mass and radius close to that of Earth (1.6 Earth masses, and 1.4 Earth radii).

Astronomers’ current strategy for finding life on another planet is to detect the chemical composition of that planet’s atmosphere, on the look-out for certain chemical imbalances that require the presence of living organisms as an explanation. In the case of our own Earth, the presence of large amounts of oxygen is such a trace.

We’re still a long way from that detection though. Until the work described in this article, the (few!) observations of light from exoplanet atmospheres all involved planets much more massive than Earth: gas giants – relatives of our own Solar System’s Jupiter – and a large super-Earth with more than eight times the Earth’s mass. With the present observation, we’ve taken the first tentative steps into analyzing the atmosphere of smaller, lower-mass planets that are much more Earth-like in size and mass.

The planet in question, GJ 1132b, orbits the red dwarf star GJ 1132 in the Southern constellation Vela, at a distance of 39 light-years from us. Recently, the system has come under scrutiny by a team led by John Southworth (Keele University, UK). The project was conceived, and the observations coordinated, by Luigi Mancini, formerly of the Max Planck Institute for Astronomy (MPIA) and now working at the University of Rome Tor Vergata. Additional MPIA team members were Paul Mollière, and Thomas Henning.

The team used the GROND imager at the 2.2 m ESO/MPG telescope of the European Southern Observatory in Chile to observe the planet simultaneously in seven different wavelength bands. GJ 1132b is a transiting planet: From the perspective of an observer on Earth, it passes directly in front of its star every 1.6 days, blocking some of the star’s light.

The size of stars like GJ 1132 is well known from stellar models. From the fraction of starlight blocked by the planet, astronomers can deduce the planet’s size – in this case around 1.4 times the size of the Earth. Crucially, the new observations showed the planet to be larger one of the infrared wavelengths than at the others. This suggests the presence of an atmosphere that is opaque to this specific infrared light (making the planet appear larger), but transparent at all the others. Different possible versions of the atmosphere were then simulated by team members at the University of Cambridge and the Max Planck Institute for Astronomy. According to those models, an atmosphere rich in water and methane would explain the observations very well.

The discovery comes with the usual exoplanet caveats: while somewhat larger than Earth, and with 1.6 times Earth’s mass (as determined by earlier measurements), observations to date do not provide sufficient data to decide how similar or dissimilar GJ 1132b is to Earth. Possibilities include a “water world” with an atmosphere of hot steam.

The presence of the atmosphere is a reason for cautious optimism. M dwarfs are the most common types of star, and show high levels of activity; for some set-ups, this activity (in the shape of flares and particle streams) can be expected to blow away nearby planets’ atmospheres. GJ 1132b provides a hopeful counterexample of an atmosphere that has endured for billion of years (that is, long enough for us to detect it). Given the great number of M dwarf stars, such atmospheres could mean that the preconditions for life are quite common in the universe.

In any case, the new observations make GJ 1132b a high-priority target for further study by instruments such as the Hubble Space Telescope, ESO’s Very Large Telescope, and the James Webb Space Telescope slated for launch in 2018.

Background information

The team members are John Southworth (Keele University), Luigi Mancini (Max Planck Institute for Astronomy [MPIA], Universita die Roma Tor Vergata, INAF – Osservatorio Astrofisico di Torinio), Nikku Madhusudhan (University of Cambridge), Paul Mollière (MPIA), Simona Ciceri (Stockholm University), and Thomas Henning (MPIA).

The work described here has been published as J. Southworth et al., “Detection of the atmosphere of the 1.6 Earth mass exoplanet GJ 1132B” in the Astronomical Journal.

• Article page at IOP Publishing
• E-Print on arXiv