LHS 1140 b Transiting rocky super-Earth found in habitable zone of quiet red dwarf star

ESO press release :

http://www.eso.org/public/unitedkingdom/news/eso1712/?lang

Newly Discovered Exoplanet May be Best Candidate in Search for Signs of Life
Transiting rocky super-Earth found in habitable zone of quiet red dwarf star

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 paper (free) : https://www.eso.org/public/archives/releases/sciencepapers/eso1712/eso1712a.pdf

Another habitable exoplanet? *yawn*

Actually no, good to have a nearby transiting HZ planet around a more luminous star than TRAPPIST-1, will be interesting to see how the conditions on the planets compare.

Though it's still something of an anomaly, given the apparent high rate of "habitable" terrestrial planets around M dwarf stars, that the habitable planet we are on (so far, the only confirmed planet with life) is located around what would appear to be an unusually massive host star.

Is the measured M dwarf "eta-Earth" actually higher than the G dwarf "eta-Earth"?
I suspect that final statement is probably prone to a number of observational biases.

Now on arXiv.
https://arxiv.org/abs/1704.05556

Even if the rate of HZ terrestrials around red dwarfs is comparable to that around G dwarfs, the Earth would still be atypical because there are more low mass stars than high mass ones. You'd need a ridiculously high G-dwarf eta-Earth to counteract that, or alternatively the eta-Earth for M dwarfs is substantially lower than the apparent occurrence rate for HZ terrestrials around M dwarfs, i.e. the vast majority of these "habitable" planets are not actually habitable.

Oh! I get what you were saying now. Yeah I agree that does seem odd.

Interesting... https://twitter.com/MartianColonist/status/1013781221191634946

That is definitely interesting!

Also, LHS 1140b is pushed closer to the inner HZ edge by the Gaia data's distance determination for the star (as is Kepler-186f).

The Impact of Stellar Distances on Habitable Zone Planets
https://arxiv.org/abs/1807.00378

Minimizing the bias in exoplanet detection - application to radial velocities of LHS 1140
https://arxiv.org/abs/1807.02483

Quote :

A rocky planet orbiting LHS 1140 with a period of 24.7d has been found based on the discovery of transits in its light and high precision radial velocity data (Dittmann et al. 2017). This discovery by two independent methods is an observational tour-de-force, however, we find that a conservative analysis of the data gives a different solution. A three planet system is apparent in the radial velocity data based on our diagnosis of stellar activity. We encourage further targeted photometric and radial velocity observations in order to constrain the mini-Neptune and super-Earth mass objects apparently causing the 3.8 and 90 day radial velocity signals. We use our package Agatha (this https URL) to provide a comprehensive strategy to disentangle planetary signals from stellar activity in radial velocity data.

This appears to be independent from the Charbonneau study presented upthread. This study finds evidence of 3 planets -- the already detected planet, a second planet at 92 days, and a third at 3.8 days, but they're less confident about the third planet. So since we have independent detection (and transits, most notably) of the 3.8 day planet from Charbonneau et al, this would seem to secure LHS 1140 as a 3-planet system.

If inner 3.8 days planet is transiting and second 24 days also so maybe third will transit too?

A second planet with an Earth-like composition orbiting the nearby M dwarf LHS 1140
https://arxiv.org/abs/1808.00485

Quote :

LHS 1140 is a nearby mid-M dwarf known to host a temperate rocky super-Earth (LHS 1140 b) on a 24.737-day orbit. Based on photometric observations by MEarth and Spitzer as well as Doppler spectroscopy from HARPS, we report the discovery of an additional transiting rocky companion (LHS 1140 c) with a mass of 1.81±0.39 MEarth and a radius of 1.282±0.024 REarth on a tighter, 3.77795-day orbit. We also obtain more precise estimates of the mass and radius of LHS 1140 b to be 6.98±0.98 MEarth and 1.727±0.032 REarth. The mean densities of planets b and c are 7.5±1.0 g/cm3 and 4.7±1.1 g/cm3, respectively, both consistent with the Earth's ratio of iron to magnesium silicate. The orbital eccentricities of LHS 1140 b and c are consistent with circular orbits and constrained to be below 0.06 and 0.31, respectively, with 90% confidence. Because the orbits of the two planets are co-planar and because we know from previous analyses of Kepler data that compact systems of small planets orbiting M dwarfs are commonplace, a search for more transiting planets in the LHS 1140 system could be fruitful. LHS 1140 c is one of the few known nearby terrestrial planets whose atmosphere could be studied with the upcoming James Webb Space Telescope.

At first glance it's not clear how this compares with the earlier study that showed a planet at 92 days, but they do see peaks in their periodogram in this period regime but note that they are likely due to stellar activity.

Well,at least we have another transiting planet around this star. Inclinations of planets b,c give big hope we can find another planets transiting in this system.Stellar activity is always problem if 92 days signal is due to planet we have big chance to find its transit.Larger radius for planet b places it in the radius gap valley despite 1.73 earth radius planet has rocky composition.Planet c composition can contain up to 6% of water, but proximity to the host star rules out large amount of water if not all water at all so ratio Fe/Mg is surely lower than earth 32/68,even lower than Trappist 1 planets

Planet b with mass 6.98 ± 0.89 and radius 1.73 ± 0.03, assuming it has only <<1 wt% water, likely suggests the planet is composed of ~20~25 wt% core and ~75~80 wt% mantle. Zeng et al (2016) two-layered mass-radius relation, which accurately reproduces Earth and Venus core mass fraction, predicts the core mass fraction of planet b to be 0.23.
Since planet b and c formed in the same protoplanetary disk, it is reasonable to assume the refractory materials ratio, Fe/Mg and Mg/Si, should be widely similar. If the core to mantle ratio of planet c is also 0.25~0.33 like its neighbor, it likely contains ~10 wt% of water, equivalent to ~800 of Earth oceans. With this amount of water in mind, the planet c is unlikely to be desiccated within at least several billion years even it receives intense stellar radiation.
It will be interesting to show how the inner planet accreted more water than the outer planet, since snowline should be far from the star during accretion period, possibly by inward migration?
Rocky planets with radius greater 1.6-1.7 always exist extremely close to the host stars, because their super-Earth masse usually accrete massive hydrogen atmosphere and they were originally mini-Neptune, but later intense radiation blew the atmosphere away leaving only the rocky core. Planet b with such high mass and temperate radiation is still rocky, implying a currently less studied or unknown planet formation mechanism.

Spinelli et al. "The high-energy radiation environment of the habitable-zone super-Earth LHS 1140b"
https://arxiv.org/abs/1906.08783

From the abstract:

Quote :

LHS 1140 is in the 25th percentile of least variable M4-M5 dwarfs of the GALEX sample. Analyzing the UV flux experienced by the HZ planet LHS 1140b, we find that outside the atmosphere it receives a NUV flux <2% with respect to that of the present-day Earth, while the FUV/NUV ratio is ~100-200 times higher. This represents a lower limit to the true FUV/NUV ratio since the GALEX FUV band does not include Lyman-alpha, which dominates the FUV output of low-mass stars. This is a warning for future searches for biomarkers, which must take into account this high ratio. The relatively low level and stability of UV flux experienced by LHS 1140b should be favorable for its present-day habitability.

Attempt to detect the atmosphere.

Diamond-Lowe et al. "Simultaneous Optical Transmission Spectroscopy of a Terrestrial, Habitable-Zone Exoplanet with Two Ground-Based Multi-Object Spectrographs"
https://arxiv.org/abs/1909.09104

Non-detection but illustrates the difficulty of doing this, particularly on rocky planets without the large scale-height of a hydrogen atmosphere.

ESPRESSO treatment. A possible third planet.

Planetary system LHS 1140 revisited with ESPRESSO and TESS
https://arxiv.org/abs/2010.06928

Quote :

LHS 1140 is an M dwarf known to host two known transiting planets at orbital periods of 3.77 and 24.7 days. The external planet (LHS 1140 b) is a rocky super-Earth that is located in the middle of the habitable zone of this low-mass star, placing this system at the forefront of the habitable exoplanet exploration. We further characterize this system by improving the physical and orbital properties and search for additional planetary-mass components in the system, also exploring the possibility of co-orbitals. We collected 113 new radial velocity observations with ESPRESSO over a 1.5-year time span with an average photon-noise precision of 1.07 m/s. We determine new masses with a precision of 6% for LHS 1140 b (6.48±0.46 M⊕) and 9% for LHS 1140 c (mc=1.78±0.17 M⊕), reducing by half the previously published uncertainties. Although both planets have Earth-like bulk compositions, the internal structure analysis suggests that LHS 1140 b might be iron-enriched. In both cases, the water content is compatible to a maximum fraction of 10-12% in mass, which is equivalent to a deep ocean layer of 779±650 km for the habitable-zone planet LHS 1140 b. Our results also provide evidence for a new planet candidate in the system (md=4.8±1.1 M⊕) on a ~78.9-day orbital period, which is detected through three independent methods. The analysis also allows us to discard other planets above 0.5 M⊕ for periods shorter than 10 days and above 2 M⊕ for periods up to one year. Finally, our analysis discards co-orbital planets of LHS 1140 b down to 1 M⊕. Indications for a possible co-orbital signal in LHS 1140 c are detected in both radial velocity and photometric data, however. The new characterization of the system make it a key target for atmospheric studies of rocky worlds at different stellar irradiations

Possible signs of water?

Edwards et al. "Hubble WFC3 Spectroscopy of the Habitable-zone Super-Earth LHS 1140 b"
https://arxiv.org/abs/2011.08815

The authors are fairly cautious about the result, hopefully JWST will provide more clarity.