In recent years there has been an exhaustive study of red dwarf stars to
find exoplanets in orbit around them. These stars have effective surface
temperatures between 2400 and 3700 K (over 2000 degrees cooler than the
Sun), and masses between 0.08 and 0.45 solar masses. In this context, a team
of researchers led by Borja Toledo Padrón, a Severo Ochoa-La Caixa doctoral
student at the Instituto de Astrofísica de Canarias (IAC), specializing in
the search for planets around this type of stars, has discovered a
super-Earth orbiting the star GJ 740, a red dwarf star situated some 36
light years from the Earth.
The planet orbits its star with a period of 2.4 days and its mass is around
3 times the mass of the Earth. Because the star is so close to the Sun, and
the planet so close to the star, this new super-Earth could be the object of
future researches with very large diameter telescopes towards the end of
this decade. The results of the study were recently published in the journal
Astronomy & Astrophysics.
“This is the planet with the second shortest orbital period around this type
of star. The mass and the period suggest a rocky planet, with a radius of
around 1.4 Earth radii, which could be confirmed in future observations with
the TESS satellite”, explains Borja Toledo Padrón, the first author of the
article. The data also indicate the presence of a second planet with an
orbital period of 9 years, and a mass comparable to that of Saturn (close to
100 Earth masses), although its radial velocity signal could be due to the
magnetic cycle of the star (similar to that of the Sun), so that more data
are needed to confirm that the signal is really due to a planet.
The Kepler mission, recognised at one of the most successful in detecting
exoplanets using the transit method (which is the search for small
variations in the brightness of a star caused by the transit between it and
ourselves of planets orbiting around it), has discovered a total of 156 new
planets around cool stars. From its data it has been estimated that this
type of stars harbours an average of 2.5 planets with orbital periods of
less than 200 days. “The search for new exoplanets around cool stars is
driven by the smaller difference between the planet’s mass and the star’s
mass compared with stars in warmer spectral classes (which facilitates the
detection of the planets’ signals), as well as the large number of this type
of stars in our Galaxy”, comments Borja Toledo Padrón.
Cool stars are also an ideal target for the search for planets via the
radial velocity method. This method is based on the detection of small
variations in the velocity of a star due to the gravitational attraction of
a planet in orbit around it, using spectroscopic observations. Since the
discovery in 1998 of the first radial velocity signal of an exoplanet around
a cool star, until now, a total of 116 exoplanets has been discovered around
this class of stars using the radial velocity method. “The main difficulty
of this method is related to the intense magnetic activity of this type of
stars, which can produce spectroscopic signals very similar to those due to
an exoplanet”, says Jonay I. González Hernández, an IAC researcher who is a
co-author of this article.
The study is part of the project HADES (HArps-n red Dwarf Exoplanet Survey),
in which the IAC is collaborating with the Institut de Ciències de l’Espai
(IEEC-CSIC) of Catalonia, and the Italian programme GAPS (Global
Architecture of Planetary Systems), whose objective is the detection and
characterization of exoplanets round cool stars, in which are being used
HARPS-N, on the Telescopio Nazionale Galileo (TNG) at the Roque de los
Muchachos Observatory (Garafía, La Palma). This detection was possible due
to a six year observing campaign with HARPS-N, complemented with
measurements with the CARMENES spectrograph on the 3.5m telescope at the
Calar Alto Observatory (Almería) and HARPS, on the 3.6m telescope at the La
Silla Observatory (Chile), as well as photometric support from the ASAP and
EXORAP surveys. Also participating in this work are IAC researchers
Alejandro Suárez Mascareño, and Rafael Rebolo.
Reference:
B. Toledo-Padrón, A. Suárez Mascareño, J. I. González Hernández, R. Rebolo,
et al. “A super-Earth on a close-in orbit around the M1V star GJ 740”.
Astronomy & Astrophysics, April 7th, 2021. DOI:
10.1051/0004-6361/202040099
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