The year 2017 has certainly been turning out to be a busy one for the discovery of extrasolar planets orbiting nearby stars. This should not be too surprising as the efforts of an expanding number of surveys using a variety of increasingly sensitive techniques are finally bearing fruit after years and even decades of work. The latest announcement was made on April 19, 2017 along with the publication the following day of a paper in the respected peer-reviewed science journal, Nature, with Jason Dittmann (Harvard-Smithsonian Center for Astrophysics) as the lead author. An international team of astronomers using precision photometry and radial velocity measurements have detected a super Earth-size exoplanet orbiting the nearby red dwarf called LHS 1140. This interesting new find, known as LHS 1140b, also appears to orbit inside its parent star’s habitable zone. So, what are the prospects that this newly discovered world is indeed habitable?
LHS 1140 is a V magnitude 14.2 red dwarf located in the constellation of Cetus – The Sea Monster. The star’s LHS designation comes from the Luyten Half Second catalog whose definitive edition was published in 1979 by Dutch-American astronomer Willem Jacob Luyten (1899-1994) who had spent his long career observing dim, fast moving stars. Coincidentally, a potentially habitable exoplanet has been recently discovered orbiting another nearby red dwarf he found called “Luyten’s Star” (see “Habitable Planet Reality Check: The Nearby GJ 273 or Luyten’s Star”). It was the 0.666 arc second per year proper motion that had brought LHS 1140 to Luyten’s attention and earned it an entry in the Luyten Half Second catalog (i.e. star’s with a proper motion greater than a half second per year).
The first trigonometric parallax for LHS 1140 was published only this past January 2017 as part of the ongoing effort by RECONS (REsearch Consortium on Nearby Stars). This group has been performing long term photometric and astrometric surveys of suspected nearby red dwarfs over the last two decades as interest in this class of stars has expanded. In RECONS’ most recent work using the CTIO/SMARTS (Cerro Tololo Inter-American Observatory/Small and Moderate Aperture Research Telescope System) 0.9-meter telescope in Chile, the trigonometric parallaxes of 151 red dwarf stars were determined for the first time and published in a paper with Jennifer Winter (Harvard-Smithsonian Center for Astrophysics) as the lead author. Their absolute trigonometric parallax measurement for LHS 1140 indicates a distance of 40.7±1.4 light years – about 27% closer than an earlier distance estimate based on the star’s photometry.
This new distance determination combined with existing photometry from the 2MASS survey allows the star’s other properties to be estimated using empirical relationships derived from well characterized red dwarfs. LHS 1140 is estimated to have a mass of 0.146±0.019 times that of the Sun, a radius of 0.186±0.013 times and a luminosity of 0.002891±0.00021 times. These are fairly typical values for a small M dwarf star, although the star has yet to have a spectral classification published. With a rotation period of 130 days determined by long term photometric monitoring, LHS 1140 is estimated to have an age in excess of five billion years.
Searching for Exoplanets
Starting in 2014, the brightness of LHS 1140 has been monitored on a regular basis by the MEarth Project. MEarth is run by the Fred Lawrence Whipple Observatory located on Mt. Hopkins, Arizona and funded by the National Science Foundation with the aim of monitoring the brightness of nearby red dwarf stars in the hopes of detecting the transits of any orbiting exoplanets. The project has two facilities each equipped with eight automated 0.4-meter telescopes fitted with CCD imagers. MEarth-North is located at Mt. Hopkins while MEarth-South is at the CTIO facility in Chile. As it was taking data at its normal 30-minute cadence, MEarth-South’s telescope 1 detected a possible transit of LHS 1140 on September 15, 2014 and started taking data at a higher cadence to characterize the event better.
As MEarth continued to collect data over the following months in search of additional transit events, the TRES spectrograph on the 1.5-meter Tillinghast reflector at the Whipple Observatory was used to obtain initial reconnaissance spectra of LHS 1140. No evidence was found for a stellar companion, real or apparent, that could contaminate the measurements or produce a false positive. At this stage, more precise radial velocity measurements began to be made by the European team of astronomers operating the HARPS (High Accuracy Radial velocity Planet Search) spectrograph attached to the European Southern Observatory’s 3.6-meter telescope in La Silla, Chile. HARPS, which has been used to survey nearby red dwarfs over the last couple of decades looking for the reflex motion caused by orbiting exoplanets, gathered a total of 144 precision radial velocity measurements between November 23, 2015 and December 13, 2016 with a typical accuracy of ±4.1 meters per second for each measurement.
Finally on June 19, 2016, MEarth-South’s telescopes 1 and 6 detected another transit event for LHS 1140. By combining the MEarth and HARPS data that were then available, the team was able to identify three possible orbital periods that would fit the data. Looking back at the MEarth archive, it turned out that a partial event had been observed on December 23, 2014 but had gone unrecognized. While bad weather at various sites enlisted to help make follow up observations of LHS 1140 hampered efforts to spot subsequent possible transit events, the egress of one transit on September 1, 2016 was successfully observed by PEST (Perth Exoplanet Survey Telescope) in Australia – a home observatory equipped with a 0.3-meter Meade LX-200 SCT telescope operated by amateur astronomer, Thian-Guan Tan (who is also one of the two dozen coauthors of Dittmann et al.). On September 25, 2016 and again on October 20, MEarth was able to observe two more transits using four of their telescopes operating at a cadence of one brightness measurement every 46 seconds.
An analysis of the combined (as well as partially overlapping) photometric and radial velocity data sets allowed the effects of the star itself on the data to be characterized and the signature of a transiting exoplanet orbiting LHS 1140 to be analyzed. It was found by Dittmann et al. that LHS 1140b, as the exoplanet is now designated, was in an orbit with a period of 24.73 days. It was this relatively long period combined with the operational limitations of the MEarth system (and a bit of bad luck with weather, etc.) which hampered efforts to observe additional transits resulting in the 21-month wait for observing a second transit. Based on the star’s properties, the semimajor axis of the orbit of LHS 1140b is 0.0875±0.0041 AU and the amount of energy it receives (the effective stellar flux or Seff) is 0.46 times that of the Earth – comfortably inside most definitions of the habitable zone (HZ).
Based on the depth of the transit events, the radius of LHS 1140b was determined by Dittmann et al. to be 1.43±0.10 times that of the Earth (or RE). And since a 5.34±1.1 meter per second variation in the radial velocity was clearly seen in the HARPS data while the transit data allowed the inclination of the orbit with respect to the plane of the sky to be measured as 89.912°±0.071°, the mass of LHS 1140b was calculated to be 6.65±1.82 times that of the Earth (or ME). While this find needs to be independently confirmed, the detection is quite robust with a low false alarm probability considering that two independent methods were employed in its detection. No other exoplanets have been detected so far in the data sets by Dittmann et al. but the presence of smaller planets can not be excluded (although a dedicated analysis would be required to set any meaningful detection limits from either the radial velocity or photometric data).
A thorough assessment of the habitability of any extrasolar planet would require a lot of detailed data on the properties of that planet, its atmosphere, its spin state, the evolution of its volatile content and so on. Unfortunately, at this very early stage, the only information typically available to scientists about extrasolar planets are basic orbit parameters, a rough measure of its size and/or mass and some important properties of its sun. Combined with theoretical extrapolations of the factors that have kept the Earth habitable over billions of years (not to mention why our neighbors are not habitable today), the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “potentially habitable”. And by “habitable”, I mean in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface – one of the presumed prerequisites for the development of life as we know it. While there may be other worlds that might possess environments that could support life, these would not be Earth-like habitable worlds of the sort being considered here.
The first step in assessing the potential habitability of the planet found orbiting LHS 1140 is to determine what sort of world it is: is it a rocky planet like the Earth or is it a volatile-rich mini-Neptune possessing a deep, hot atmosphere dominated by hydrogen with little prospects of being habitable in a conventional sense. While we usually have insufficient information to make an estimation of the bulk composition of an exoplanet, fortunately that is not the case with LHS 1140b. Combining the 1.4 RE radius found from the transit data with the 6.6. ME mass found from the radial velocity measurements, the mean density of LHS 1140b is calculated to be 12.5±3.4 times that of water. While the uncertainty in the mean density is rather large at the moment, it is still good enough to exclude the possibility that LHS 1140b is a volatile-rich mini-Neptune as well as other possible low-density planet models. Instead, it would seem to be a rocky planet without a substantial atmosphere but probably with a higher iron fraction that the Earth – about 0.7 for LHS 1140b versus 0.3 for the Earth.
Another important criterion which can be used to determine if a planet is potentially habitable is the amount of energy it receives from its parent star known as the effective stellar flux or Seff. According to the work by Kopparapu et al. on the limits of the habitable zone (HZ) based on detailed climate and geophysical modeling, the inner limit of the HZ is conservatively defined by the runaway greenhouse limit where a planet’s temperature would soar even with no CO2 present in its atmosphere resulting in the loss of all of its water in a geologically brief time in the process. For a planet with a mass of 5 ME orbiting LHS 1140 which has an effective surface temperature of 3131±100 K, this happens at an Seff value of 0.99, according to Kopparapu et al., which corresponds to a orbital semimajor axis of 0.055 AU.
The outer limit of the HZ is conservatively defined as corresponding to the maximum greenhouse limit of a CO2-rich atmosphere where the addition of more of this greenhouse gas would not increase a planet’s surface temperature any further. For a star like LHS 1140, this conservative outer limit for the HZ has an Seff of 0.24 corresponding to a orbital semimajor axis of 0.112 AU. With an Seff of 0.46, LHS 1140b is comfortably in the middle of this conservatively defined HZ even with the current uncertainties in its properties
Although the close proximity of the exoplanet to its parent star likely means that LHS 1140b is a slow or synchronous rotator, two decades of increasingly sophisticated climate modelling has shown that this is not the impediment to habitability that it was once thought to be. In fact, being located well into the HZ implies that LHS 1140b will likely have a fairly substantial CO2-rich atmosphere as a natural consequence of the carbonate-silicate cycle which regulates planetary temperatures on geologically active worlds. This atmosphere, combined with any oceans, would help transport heat from the perpetually daylit side to the night side to create temperate conditions globally. More detailed climate models specifically for this exoplanet, which will surely be published in the months to come, should provide us with more information on the potential range of climates of LHS 1140b.
The initial impression is that LHS 1140b has fairly good prospects of being potentially habitable since it is a rocky planet which orbits inside of the HZ of its parent star. However, this generally good assessment comes with the usual caveats about the potential habitability of any exoplanet orbiting a red dwarf. There have been a fair number of predictions made over the decades about various proposed mechanisms that could desiccate such exoplanets, strip them of their atmospheres, irradiate their surfaces from flares and so on – processes which could compromise the habitability of such worlds. There have been yet other predictions published in the peer-reviewed literature which suggest these might not be major impediments to potential habitability especially for larger exoplanets (which are expected to form secondary atmospheres later) and exoplanets in larger orbits (which are farther away from damaging flares and other effects of stellar activity).
The current observations of LHS 1140b can not exclude the possibility that LHS 1140b has been stripped of all of its volatiles leaving it a cold, desert planet devoid of water or even a substantial atmosphere. There are currently observations being made with NASA’s Hubble Space Telescope to determine the current UV flux of LHS 1140 – data that can help to assess the potential habitability of LHS 1140b today. However, the low density of the recently discovered TRAPPIST-1f implies that it has held onto its volatiles despite close orbit around ultracool dwarf (see “Habitable Planet Reality Check: The Seven Planets of TRAPPIST-1”). Likewise, the detection of water vapor in the very hot atmosphere of GJ 1132b (a super-Earth size exoplanet discovered in 2015 using MEarth-South) also suggests that exoplanets orbiting red dwarf stars can retain their water and other volatiles over time raising hopes that LHS 1140b has done so as well. Only more observations of this class of exoplanets to characterize them and their atmospheres will provide the data needed to confirm or refute these sometimes contradictory predictions being made. Fortunately, LHS 1140b is an ideal target for exactly these sorts of studies.
Based on what little we known, the claim that LHS 1140b is potentially habitable appears to have some merit. This new find has a density consistent with a rocky composition and it seems to orbit comfortably inside of the conservatively defined habitable zone (HZ) of its parent star. But like so many potentially habitable exoplanets that have been found orbiting red dwarfs, this assessment assumes that some of the proposed processes which can compromise the habitability of any exoplanets orbiting red dwarfs will not adversely affect this world. With this in mind, this exoplanet would seem to rank with other promising potentially habitable planets like Kepler 186f, Kepler 442b and other recent finds (see “Top Five Known Potentially Habitable Planets”).
Fortunately, LHS 1140b offers astronomers an ideal target for addressing the outstanding questions surrounding the habitability of planets orbiting red dwarfs. Additional photometric and radial velocity measurements, along with a more detailed characterization of the parent star, are guaranteed to improve our knowledge of the properties of LHS 1140b and open the possibility of finding additional exoplanets orbiting the host star in the process. And future spectroscopic observations with telescopes like NASA’s James Web Space Telescope should allow the atmosphere of this exoplanet to be probed so that its properties can be studied. As two of the coauthors of Dittmann et al. and members of the HARPS team, Xavier Delfosse and Xavier Bonfils (CNES/IPAQ), stated in an ESO press release on this discovery:
“The LHS 1140 system might prove to be an even more important target for the future characterization of planets in the habitable zone than Proxima b or TRAPPIST-1. This has been a remarkable year for exoplanet discoveries!”.
This reviewer agrees completely!
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Here is a brief video from ESO showing a hypothetic trip from Earth to LHS 1140.
For a complete collection of articles about our other neighboring star systems and the searches for exoplanets orbiting them, see Drew Ex Machina’s page on Nearby Stars.
Jason A. Dittmann et al., “A temperate rocky super-Earth transiting a nearby cool star”, Nature, Vol 544., No. 7650, pp. 333-336, April 20, 2017 [Preprint]
R.K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013
Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014
Jennifer G. Winters et al., “The Solar Neighborhood XXXVIII. Results from the CTIO/SMARTS 0.9m: Trigonometric Parallaxes for 151 Nearby M Dwarf Systems”, The Astronomical Journal, Vol. 153, No. 1, Article id. 14, January 2017
Newly Discovered Exoplanet May be Best Candidate in Search for Signs of Life, ESO Science Release eso1712, April 19, 2017 [Press Release]