LHS 1140b, an Exoplanet orbiting a red dwarf star 40 light-years away may be the new holder of the title “best place to look for signs of life beyond the Solar System”, according to an international team of astronomers. eso.org/...
LHS 1140b is a rocky planet, a little larger and much denser than Earth and has likely retained an atmosphere. It orbits in the habitable zone around its faint red dwarf star LHS 1140, located in the constellation of Cetus. The orbit is seen almost edge-on from Earth, making detection and analysis of its atmosphere, possible during transits.
A few vital statistics, as reported by the authors of the paper at www.eso.org/… -
Parameter |
Value |
Comparision With Earth/Sun |
Diameter |
18,000 km
|
1.43x that of Earth |
Mass |
3.97 x 1025
|
6.65x that of Earth
|
Average Density |
13 g/cm³ |
2.3x denser than Earth
|
Surface Gravity |
31.8 m/s2 |
3.25x that on Earth |
Core |
Iron, rock |
|
Atmosphere |
Likely exists |
|
Average Surface Temperature |
? |
14o C |
Insolation (Solar radiation) |
|
0.46x that of Earth |
Distance to Star |
15 million km |
1/10th of Earth-Sun distance |
Orbital period |
24.7 days
Rather short seasons
|
1/15th that of Earth |
Rotation period |
Unknown
Is it tidally locked to the star?
|
24 hours |
Age |
~5 billion years |
4.5 billion years |
Distance from Earth |
39.12 light years |
|
Star mass |
3 x 1029 kg |
1/6th that of our Sun |
Star Effective Temperature |
A cool 3131 K |
5777 K for our Sun |
Star age |
~5 billion years |
4.6 billion years for our Sun |
Star rotation period |
131 days
|
24 days for our Sun |
Inhabitability
Although the planet is potentially habitable now, it might have faced a more challenging past. When red dwarf stars are young, they are known to emit harsh ultra-violet radiation that can strip off planetary atmospheres.
However, given LHS 1140b’s large size, it is possible that a magma ocean could have existed on its surface for millions of years after the star’s luminosity decreased to present levels. This seething ocean of lava could have fed steam into the atmosphere long after the star calmed to its current, steady glow. This process could have replenished the planet with water, making it capable of supporting life.
The present conditions of the red dwarf are much more benign — LHS 1140 spins more slowly and emits less high-energy radiation than other similar low-mass stars.
According to the authors, a recent study found that a planet orbiting an M dwarf could have surface temperatures that allow liquid water if it receives between 0.2 and 0.8 times the insolation that Earth receives from the Sun. LHS 1140b currently receives 0.46 times Earth’s insolation, thereby supporting expectations that there is liquid water on its surface.
Discovery
LHS 1140b was found using the transit method, in which the light from the star reduces in intensity as a planet crosses in front of it as seen from Earth. This allows measurement of the orbital period and the planet’s size.
The researchers have calculated that the planet very likely consists of a dense iron core surrounded by a magnesium silicate mantle. They conclude that LHS 1140b is a rocky planet without a substantial gas envelope.
No other planets were detected during the search, implying that other planets, if any, are likely smaller than LHS 1140b.
Observatories
LHS 1140b was discovered using the MEarth-South Observatory, an array of eight 40 cm telescopes, located at Cerro Tololo, Chile, which monitor small stars (less than 33% the size of the Sun) located within 100 light-years of the Sun. The telescopes are robotically controlled and can take hundreds of observations each night.
The ESO HARPS instrument (High Accuracy Radial Velocity Planet Searcher) along with the 3.6m telescope, located at La Silla, Chile, was used to make follow-up observations and measurements. HARPS helped measure the orbital period, velocity and estimate the exoplanet’s mass and density.
Here is a cool time-lapse video of the MEarth telescopes in action, hopping from star-to-star, looking for and detecting exoplanet transits in real-time.
Comparison with Other Promising Exoplanets
The discovery of Proxima b, only 4.25 light years away, was announced with great fanfare last year. But it probably does not transit its M dwarf star Proxima Centauri, and its mass is not known accurately, making analysis difficult.
Seven Earth-sized planets were discovered recently in the TRAPPIST-1 system, 39 light years away, which do transit their very low-mass star, but the authors of this discovery state that their masses and, particularly, their densities are poorly constrained.
The researchers claim that “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.”
Note: The term super-Earth is used for an exoplanet with a mass higher than Earth's, but substantially below the masses of the Solar System's ice giants, Uranus and Neptune, which contain 15 and 17 Earth masses respectively.
The Future
The researchers state that because LHS 1140b is nearby, additional measurements by Hubble and by telescopes currently under construction (e.g., the James Webb Space Telescope and ESO’s Extremely Large Telescope) would be able to identify specific atmospheric gases in the near future.
As the authors state, this has been a remarkable year for exoplanet discoveries!
References
- ESO announcement — eso.org/…
- A temperate rocky super-Earth transiting a nearby cool star (Journal paper) — www.eso.org/…
- The Ultracool Dwarf (Star) and the Seven Exoplanets — www.dailykos.com/…
- Exoplanet Proxima b - Water, Ocean, Life? — www.dailykos.com/…
- Enceladus - Plumes, Oceans, Hydrogen, Life? — www.dailykos.com/…
- Is There Life on Mars? — www.dailykos.com/...
(In the poll below, “we are not alone” is meant as “some life-form will be found, microbial or more”; also, don’t forget that our solar system holds some promise too — esp. Mars, Europa and Enceladus).