1
Harvard Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA.
2
CNRS (Centre National de
la Recherche Scientifique), IPAG (Institut de Planétologieet d’Astrophysique de Grenoble), F-38000 Grenoble, France.
3
Université
Grenoble Alpes, IPAG, F-38000 Grenoble, France.
4
Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, 1290
Versoix, Switzerland.
5
University of Colorado, 391 UCB, 2000 Colorado Avenue, Boulder, Colorado 80305, USA.
6
Massachusetts
Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02138, USA.
7
Perth Exoplanet Survey Telescope,
Perth, Western Australia, Australia.
8
Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326,
13388 Marseille, France.
9
Instituto de Astrofísica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain.
10
Instituto de Astrofísica e
Ciéncias do Espaço, Universidade do Porto, CAUP (Centro de Astrofísica da Universidade do Porto), Rua das Estrelas, 4150-762
Porto, Portugal.
11
Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo, Alegre, 4169-
007 Porto, Portugal.
12
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125,
USA.
!
A temperate rocky super-Earth
transiting a nearby cool star
Jason A. Dittmann
1
, Jonathan M. Irwin
1
, David Charbonneau
1
, Xavier Bonfils
2,3
, Nicola
Astudillo-Defru
4
,Raphaëlle D. Haywood
1
, Zachory K. Berta-Thompson
5
, Elisabeth R.
Newton
6
, Joseph E. Rodriguez
1
, Jennifer G. Winters
1
, Thiam-Guan Tan
7
, Jose-Manuel
Almenara
2,3,4
, François Bouchy
8
, Xavier Delfosse
2,3
, Thierry Forveille
2,3
, Christophe
Lovis
4
, Felipe Murgas
2,3,9
, Francesco Pepe
4
, Nuno C. Santos
10,11
, Stephane Udry
4
, Anaël
Wünsche
2,3
, Gilbert A. Esquerdo
1
,David W. Latham
1
& Courtney D. Dressing
12
!
M dwarf stars, which have masses less than 60 per cent that of the Sun, make up 75
per cent of the population of the stars in the Galaxy
1
. The atmospheres of orbiting
Earth-sized planets are observationally accessible via transmission spectroscopy
when the planets pass in front of these stars
2,3
. Statistical results suggest that the
nearest transiting Earth-sized planet in the liquid-water, habitable zone of an M
dwarf star is probably around 10.5 parsecs away
4
. A temperate planet has been
discovered orbiting Proxima Centauri, the closest M dwarf
5
, but it probably does
not transit and its true mass is unknown. Seven Earth-sized planets transit the very
low-mass star TRAPPIST-1, which is 12 parsecs away
6,7
, but their masses and,
particularly, their densities are poorly constrained. Here we report observations of
LHS 1140b, a planet with a radius of 1.4 Earth radii transiting a small, cool star
(LHS 1140) 12 parsecs away. We measure the mass of the planet to be 6.6 times that
of Earth, consistent with a rocky bulk composition. LHS 1140b receives an
insolation of 0.46 times that of Earth, placing it within the liquid-water, habitable
zone
8
. With 90 per cent confidence, we place an upper limit on the orbital
eccentricity of 0.29. The circular orbit is unlikely to be the result of tides and
therefore was probably present at formation. Given its large surface gravity and
cool insolation, the planet may have retained its atmosphere despite the greater
luminosity (compared to the present-day) of its host star in its youth
9,10
. Because
LHS 1140 is nearby, telescopes currently under construction might be able to search
for specific atmospheric gases in the future
2,3
.
MEarth
11,12
consists of two arrays of eight 40-cm-aperture telescopes, one in the
Northern Hemisphere at the Fred Lawrence Whipple Observatory (FLWO) in Arizona,
USA, and the other in the Southern Hemisphere at Cerro Tololo Inter-American
Observatory, Chile. This survey monitors small stars (less than 33% the size of the Sun)
that are estimated to lie within 100 light years of the Sun for transiting extrasolar planets.
Since January 2014, these telescopes have gathered data nearly every clear night,
monitoring the brightnesses of these stars for signs of slight dimming, which would be
indicative of a planet transiting in front of the star. MEarth-South monitors these stars at a
cadence of approximately 30 min, but is capable of performing high-cadence
observations in real time if a possible planetary transit is detected to be in progress
13
. We
used the MEarth-South telescope array to monitor the brightness of the star LHS 1140,
beginning in 2014.
The distance to LHS 1140 has been measured through trigonometric parallax to be 12.47
± 0.42 parsecs (ref. 14). Combined with the 2MASS K
s
magnitude
15
and empirically
determined stellar relationships
16,17
, we estimate the stellar mass to be 14.6% that of the
Sun and the stellar radius to be 18.6% that of the Sun. We estimate the metal content of
the star to be approximately half that of the Sun ([Fe/H] = −0.24 ± 0.10; 1σ error), and we
measure the rotational period of the star to be 131 days from our long-term photometric
monitoring (see Methods).
On 15 September 2014 UT, MEarth-South identified a potential transit in progress
around LHS 1140, and automatically commenced high-cadence follow-up observations
(see Extended Data Fig. 1). Using a machine-learning approach (see Methods), we
selected this star for further follow-up observations.
We gathered two high-resolution (resolution R = 44,000) reconnaissance spectra with the
Tillinghast Reflector Echelle Spectrograph (TRES) on the 1.5-m Tillinghast reflector
located at the Fred Lawrence Whipple Observatory (FLWO) on Mt Hopkins, Arizona,
USA. From these spectra, we ruled out contamination from additional stars and large
systemic accelerations, and concluded that this system was probably not a stellar binary
or false positive (see Methods). We subsequently obtained 144 precise radial velocity
measurements with the High Accuracy Radial Velocity Planet Searcher (HARPS)
spectrograph
18
from 23 November 2015 to 13 December 2016 UT.
On 19 June 2016 UT, MEarth-South detected an additional transit of LHS 1140b through
the MEarth trigger; when combined with the radial velocities and our initial trigger, we
identified three potential orbital periods. On the basis of one of these, the 24.738-day
period, we back-predicted a third, low-signal-to-noise transit from 23 December 2014 UT
(see Extended Data Fig. 1). With this ephemeris, we predicted a transit on 01 September
2016 UT, the egress of which was observed by the Perth Exoplanet Survey Telescope
(PEST; see Methods). On 25 September 2016 and 20 October 2016 UT we obtained two
complete transit observations with four of the eight MEarth-South telescopes.
Figure 1 | Photometric transit and radial-velocity measurements of LHS 1140b. a, Phase-folded transit
observations from all transits (purple), with our transit model over-plotted as a red line. These data were
binned in 160 3-min bins. Here we have corrected each individual light curve from each telescope with a
zero-point offset, and with a linear correction for the air mass of the observation. For both full-transit
observations we also apply a correction that is linear in time so that the flux level of LHS 1140 is equal
before and after the transit. b, 144 measurements of the line-of-sight (radial) velocity of LHS 1140 taken
with the HARPS spectrograph (purple points, duplicate observations are shown in the shaded regions,
error bars are 1σ). A value of zero corresponds to a radial velocity equal to that of the host star. We have
removed variability due to stellar activity and plot only the radial-velocity perturbations induced by the
planet, and have phase-folded the radial velocities to the orbital period. Our best-fitting Keplerian orbit is
shown as the solid purple line.
We initially fitted our photometric transit and radial-velocity measurements
simultaneously (see Methods). This fit serves as input to a comprehensive radial-velocity
analysis that takes into account not only the reflex motion of the planet, but also the
intrinsic variations of the host star via Gaussian process regression
19,20
(see Methods).
We find that LHS 1140b has a mass 6.65 ± 1.82 times that of Earth and a radius 1.43 ±
0.10 times that of Earth, and orbits around LHS 1140 with a period of 24.73712 ±
0.00025 days (all error bars are 1σ) and an eccentricity that is constrained to be less than
0.29 (at 90% confidence; see Methods). In Fig. 1, we show the phased and binned transit
light curve for LHS 1140b and the phased radial-velocity curve; in Extended Data Fig. 1
we show each individual transit. Our phased radial-velocity curve shows the radial
velocity from the influence of LHS 1140b only; the stellar contribution has been
removed. In Table 1, we show the system parameters for LHS 1140 and LHS 1140b, with
68% confidence intervals on these parameters.
A simple structural model surrounded by a magnesium silicate mantle can explain the
observed mass and radius (see Fig. 2). Although our best fitting values imply a much
higher iron core-mass fraction than that of Earth (0.7 compared to 0.3), our uncertainties
on the mass and the radius can only rule out Earth- like compositions at 2σ confidence.
We conclude that LHS 1140b is a rocky planet without a substantial gas envelope.
b)a)
Figure 2 | Masses, radii, distances, insolation and stellar size of known transiting planets. a, The mass
and radius of LHS 1140b indicate a terrestrial composition. Other planets with measured masses and radii
are shown, with darker points indicating smaller density uncertainties. The red points correspond to GJ
1214b (top) and GJ 1132b (below LHS 1140b). Error bars, 1σ. Mass–radius curves for two-layer rocky
planets with 0%, 25% and 50% of their mass in iron cores are shown as solid lines. b–d, Planetary radius
versus insolation, stellar radius and distance, respectively. Planets with dynamical mass determinations
are shown in black; those without are shown in grey. The red data correspond to GJ 1214b and GJ 1132b,
as in a, and the darker red circles to the TRAPPIST-1 planets; these are the nearby planets around small
stars that are most accessible to characterization by the James Webb Space Telescope (JWST), as indicated
in c and d. The shaded region in b is the M-dwarf habitable zone
8
. We note that this habitable zone is only
appropriate for planets orbiting M dwarfs, and most of the planets in this diagram orbit much larger stars.
The area of each circle is proportional to the transit depth and hence observational accessibility. LHS
1140b has a lower insolation than Earth, and orbits a small star 12 parsecs from the Sun, making it a
temperate, rocky planet that may be accessible to atmospheric characterization.
We searched for additional planets both in the MEarth-South light curve and as periodic
signals in the HARPS residuals. We did not find any compelling signals in these data.
After subtracting the radial- velocity signal due to stellar rotation and LHS 1140b, the
Lomb–Scargle periodogram of the residuals shows a series of broad peaks at periods
greater than 60 days, possibly associated with stellar activity. At shorter periods, the
highest peaks are the result of the window function of our radial-velocity observations
(see Methods). We do not find any notable periodic signals or additional triggers that
would be suggestive of another transiting planet, although planets smaller than LHS
1140b could elude detection.
Compact, coplanar, multi-planet systems are common around M dwarfs
22,23
, and all
coplanar objects at periods of less than the orbital period of LHS 1140b would also
transit, although their size may be too small to have been detected. The surface density of
the protoplanetary disk in which LHS 1140b formed may dictate the properties of any
additional planets in this system. A high-surface-density disk may shorten the timescale
for planet formation to be before the dissipation of the gaseous disk component, allowing
otherwise terrestrial planets to accrete large envelopes of hydrogen and helium
24
. LHS
1 10
Planet Mass
(Earth masses)
0.5
1.0
1.5
2.0
2.5
3.0
Planet Radius (Earth radii)
LHS1140b
a)
Venus
Earth
Uranus
Neptune
10
-1
10
0
10
1
10
2
10
3
10
4
Insolation
(relative to Earth)
LHS1140b
b)
0.2 0.4 0.6 0.8 1.0
Stellar Radius
(solar radii)
LHS1140b
c)
10 100 1000
Distance
(parsecs)
LHS1140b
d)
1140b does not have such a gaseous component, so it might have formed from a low-
surface-density disk, consistent with the low metallicity of the star, although in this
scenario we would expect additional rocky worlds. Further radial-velocity and
photometric monitoring of LHS 1140 is warranted.
A recent study
8
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, and
we estimate its age to exceed 5 Gyr (see Methods). In its youth, LHS 1140 was more
luminous, and a larger fraction of its spectrum was released at ultraviolet wavelengths.
During this period, the atmosphere of LHS 1140b was therefore subjected to increased
irradiance and greater levels of ionizing radiation, and LHS 1140b probably did not enter
the liquid-water, habitable zone until approximately 40 Myr after the formation of the
star
9
. This amount of time may have been sufficient for the atmosphere to have
experienced a runaway greenhouse, with water being dissociated in the upper atmosphere
and the hydrogen permanently lost to atmospheric escape
9
. If so, then the planet’s
atmosphere would be dominated by abiotic O
2
, N
2
and CO
2
. However, recent work has
suggested that super-Earths can have an extended magma-ocean phase
10
, in which case
the timescale over which LHS 1140b outgassed its secondary atmosphere may have
exceeded the time for the star to reach its current luminosity. In this scenario, volatiles
such as H
2
O would have remained in the mantle of the planet until after the host star
dimmed and its fractional ultraviolet emission decreased. Inferences of the history of the
atmosphere would be strengthened with better observational constraints on the emission
of M dwarfs at young ages, and with more detailed models of the initial composition of
the atmosphere from outgassing and the delivery of volatiles through late-stage cometary
impacts. Observations of the current ultraviolet emission of LHS 1140 by the Hubble
Space Telescope will be able to be used to assess the current high-energy flux that is
infringing upon LHS 1140b, and will be helpful in determining the current habitability of
LHS 1140b and constraining any ongoing atmospheric escape from the planet.
