been reported for other M-dwarf stars.
18
The time-series of indices based on chromospheric emis-
sion lines (e.g. H
α
) do not show evidence of periodic variability, even after removing data points
likely affected by flares. We also investigated possible correlations of the Doppler measurements
with activity indices by including linear correlation terms in the Bayesian model of the Doppler
data. While some indices do show hints of correlation in some campaigns, including them in the
model produces lower probabilities due to overparameterization. Flares have very little effect on
our Doppler velocities, as has already been suggested by previous observations of Proxima.
19
More
details are provided in the methods section and as Extended Data Figures. Since the analysis of
the activity data failed to identify any stellar activity feature likely to generate a spurious Doppler
signal at 11.2 days, we conclude that the variability in the data is best explained by the presence of a
planet (Proxima b, hereafter) orbiting the star. All available photometric light curves were searched
for evidence of transits, but no obvious transit-like features were detectable in our light curves. We
used Optimal Box-Least-Squares codes
20
to search for candidate signals in data from the All Sky
Automatic Survey.
3
No significant transit signal was found down to a depth of about 5% either. The
preferred orbital solution and the putative properties of the planet and transits are given in Table 1.
The Doppler semi-amplitude of Proxima b (∼
1.4 ms
−1
) is not particularly small compared
to other reported planet candidates.
6
The uneven and sparse sampling combined with longer-term
variability of the star seem to be the reasons why the signal could not be unambiguously confirmed
with pre-2016 rather than the amount of data accumulated. The corresponding minimum planet
mass is ∼
1.3 M
⊕
. With a semi-major axis of ∼0.05 AU, it lies squarely in the center of the
classical habitable zone for Proxima.
5
As mentioned earlier, the presence of another super-Earth
mass planet cannot yet be ruled out at longer orbital periods and Doppler semi-amplitudes
<3 ms
−1
.
By numerical integration of some putative orbits, we verified that the presence of such an additional
planet would not compromise the orbital stability of Proxima b.
Habitability of planets like Proxima b -in the sense of sustaining an atmosphere and liquid water
on its surface- is a matter of intense debate. The most common arguments against habitability are
tidal locking, strong stellar magnetic field, strong flares, and high UV & X-ray fluxes; but none of
these have been proven definitive. Tidal locking does not preclude a stable atmosphere via global
atmospheric circulation and heat redistribution.
22
The average global magnetic flux density of Prox-
ima is 600±150 Gauss,
23
which is quite large compared to the Sun’s value of 1 G. However, several
studies have shown that planetary magnetic fields in tidally locked planets can be strong enough to
prevent atmospheric erosion by stellar magnetic fields
24
and flares,.
25
Because of its close-in orbit,
Proxima b suffers X-ray fluxes ∼400 times that of Earth’s, but studies of similar systems indicate that
atmospheric losses can be relatively small.
26
Further characterization of such planets can also inform
us about the origin and evolution of terrestrial planets. For example, forming Proxima b from in-situ
disk material is implausible because disk models for small stars would contain less than 1
M
Earth
of solids within the central AU. Instead, either 1) the planet migrated in via type I migration,
27
2)
planetary embryos migrated in and coalesced at the current planet’s orbit, or 3) pebbles/small plan-
etesimals migrated via aerodynamic drag
28
and later coagulated into a larger body. While migrated
planets and embryos originating beyond the ice-line would be volatile rich, pebble migration would
5
-6.00
-4.00
-2.00
0.00
2.00
4.00
6.00
RV [m s
-1
]
0.95
1.00
1.05
1.10
1.15
Normalized flux
0.95
1.00
1.05
1.10
1.15
Normalized flux
0.95
1.00
1.05
1.10
1.15
Normalized flux
0.95
1.00
1.05
1.10
1.15
Normalized flux
9.6
9.8
10.0
10.2
10.4
m
2
[km
2
s
-2
]
2457400
2457420
2457440
2457460
2457480
Julian Date [days]
0.80
1.20
1.60
2.00
2.40
m
3
[km
3
s
-3
]
a
b
c
d
e
f
g
ASH2 SII
HARPS PRD RV
ASH2 H-
α
LCOGT V
LCOGT B
HARPS PRD m
2
HARPS PRD m
3
Figure 3: Time-series obtained during the Pale Red Dot campaign. HARPS-PRD radial velocity
measurements (panel a), quasi-simultaneous photometry from ASH2 (panels b and c) and LCOGT
(panels d and e) and central moments of the mean line profiles (panels f and g). The solid lines
show the best fits. A dashed line indicates a signal that is not sufficiently significant. Excluded mea-
surements likely affected activity events (e.g. flares) are marked with grey arrows. The photometric
time-series and
m
2
all show evidence of the same ∼80 day modulation. Error bars correspond to
formal 1-
σ uncertainties.
produce much drier worlds. In this sense, a warm terrestrial planet around Proxima offers unique
follow-up opportunities to attempt further characterization via transits -on going searches-, via direct
imaging and high-resolution spectroscopy in the next decades,
29
and –maybe– robotic exploration
in the coming centuries.
30
References
[1] van Leeuwen, F. Validation of the new Hipparcos reduction. Astron. Astrophys. 474, 653–664
(2007).
[2] Boyajian, T. S.
et al. Stellar Diameters and Temperatures. II. Main-sequence K- and M-stars.
Astrophys. J. 757, 112 (2012).
6