A terrestrial planet candidate in a temperate orbit around
Proxima Centauri
Guillem Anglada-Escud´e
1∗
, Pedro J. Amado
2
, John Barnes
3
,
Zaira M. Berdi˜nas
2
, R. Paul Butler
4
, Gavin A. L. Coleman
1
,
Ignacio de la Cueva
5
, Stefan Dreizler
6
, Michael Endl
7
,
Benjamin Giesers
6
, Sandra V. Jeffers
6
, James S. Jenkins
8
,
Hugh R. A. Jones
9
, Marcin Kiraga
10
, Martin K¨urster
11
,
Mar´ıa J. L´opez-Gonz´alez
2
, Christopher J. Marvin
6
, Nicol´as Morales
2
,
Julien Morin
12
, Richard P. Nelson
1
, Jos´e L. Ortiz
2
,
Aviv Ofir
13
, Sijme-Jan Paardekooper
1
, Ansgar Reiners
6
,
Eloy Rodr´ıguez
2
, Cristina Rodr´ıguez-L´opez
2
, Luis F. Sarmiento
6
,
John P. Strachan
1
, Yiannis Tsapras
14
, Mikko Tuomi
9
,
Mathias Zechmeister
6
.
July 13, 2016
1
School of Physics and Astronomy, Queen Mary University of London, 327 Mile End Road, London
E1 4NS, UK
2
Instituto de Astrofsica de Andaluca - CSIC, Glorieta de la Astronoma S/N, E-18008 Granada, Spain
3
Department of Physical Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
4
Carnegie Institution of Washington, Department of Terrestrial Magnetism 5241 Broad Branch Rd.
NW, Washington, DC 20015, USA
5
Astroimagen, Ibiza, Spain
6
Institut f¨ur Astrophysik, Georg-August-Universit¨at G¨ottingen Friedrich-Hund-Platz 1, 37077 G¨ottingen,
Germany
7
The University of Texas at Austin and Department of Astronomy and McDonald Observatory 2515
Speedway, C1400, Austin, TX 78712, USA
8
Departamento de Astronoma, Universidad de Chile Camino El Observatorio 1515, Las Condes,
Santiago, Chile
9
Centre for Astrophysics Research, Science & Technology Research Institute, University of Hert-
fordshire, Hatfield AL10 9AB, UK
10
Warsaw University Observatory, Aleje Ujazdowskie 4, Warszawa, Poland
11
Max-Planck-Institut f¨ur Astronomie K¨onigstuhl 17, 69117 Heidelberg, Germany
12
Laboratoire Univers et Particules de Montpellier, Universit de Montpellier, Pl. Eug`ne Bataillon -
CC 72, 34095 Montpellier C´edex 05, France
13
Department of Earth and Planetary Sciences, Weizmann Institute of Science, 234 Herzl Street,
1
Rehovot 76100, Israel
14
Astronomisches Rechen-Institut, M¨onchhofstrasse 12-14 69120 Heidelberg Germany
∗
Corresponding author E-mail:
g.anglada@qmul.ac.uk
Authors listed in alphabetical order after corresponding author.
At a distance of 1.295 parsecs,
1
the red-dwarf Proxima Centauri (
α Centauri C, GL 551,
HIP 70890, or simply Proxima) is the Sun’s closest stellar neighbour and one of the best studied
low-mass stars. It has an effective temperature of only ∼
3050 K, a luminosity of ∼
0.1 per
cent solar, a measured radius of 0.14 R
⊙
2
and a mass of about 12 per cent the mass of the
Sun. Although Proxima is considered a moderately active star, its rotation period is ∼
83
days,
3
and its quiescent activity levels and X-ray luminosity
4
are comparable to the Sun’s. New
observations reveal the presence of a small planet orbiting Proxima with a minimum mass of
1.3 Earth masses and an orbital period of ∼
11.2 days. Its orbital semi-major axis is ∼
0.05 AU,
with an equilibrium temperature in the range where water could be liquid on its surface.
5
The results presented here consist of the analysis of previously obtained Doppler measurements
(pre-2016 data), and the confirmation of a signal in a specifically designed follow-up campaign in
2016. The Doppler data comes from two precision radial velocity instruments, both at the European
Southern Observatory (ESO): the High Accuracy Radial velocity Planet Searcher (HARPS) and the
Ultraviolet and Visual Echelle Spectrograph (UVES). HARPS is a high-resolution stabilized echelle
spectrometer installed at the ESO 3.6m telescope (La Silla observatory, Chile), and is calibrated in
wavelength using hollow cathode lamps (Th Ar). HARPS has demonstrated radial velocity measure-
ments at ∼1 ms
−1
precision over time-scales of years,
6
including on low-mass stars.
7
All HARPS
spectra were extracted and calibrated with the standard ESO Data Reduction Software, and radial
velocities were measured using a least-squares template matching technique.
7
HARPS data is sepa-
rated into two datasets. The first set includes all data obtained before 2016 by several programmes
(HARPS pre-2016). The second HARPS set comes from the more recent Pale Red Dot campaign
(PRD hereafter), which was designed to eliminate period ambiguities using new HARPS observa-
tions and quasi-simultaneous photometry. The HARPS PRD observations consisted of obtaining one
spectrum almost every night between Jan 19th and March 31st 2016. The UVES observations used
the Iodine cell technique
8
and were obtained in the framework of the UVES survey for terrestrial
planets around M-dwarfs between 2000 and 2008. The spectra were extracted using the standard
procedures of the UVES survey,
9
and new radial velocities were re-obtained using up-to-date Io-
dine reduction codes.
10
Since systematic calibration errors produce correlations among observations
within each night,
11
we consolidated Doppler measurements through nightly averages to present a
simpler and more conservative signal search. This led to 72 UVES, 90 HARPS pre-2016 and 54
HARPS PRD epochs. The PRD photometric observations were obtained using the Astrograph for
the South Hemisphere II telescope (ASH2 hereafter,
12
SII and H
α
narrowband filters) and the Las
Cumbres Observatory Global Telescope network (LCOGT.net,
13
Johnson B and V bands), over the
same time interval and similar sampling as the HARPS PRD observations. Further details about
2