Astronomers Detect a New Super-Earth Orbiting a Red Dwarf Star

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, recognized 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 star harbors 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 star 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.

Reference: “A super-Earth on a close-in orbit around the M1V star GJ 740: A HADES and CARMENES collaboration” by B. Toledo-Padrón, A. Suárez Mascareño, J. I. González Hernández, R. Rebolo,, M. Pinamonti, M. Perger, G. Scandariato, M. Damasso, A. Sozzetti, J. Maldonado, S. Desidera, I. Ribas, G. Micela, L. Affer, E. González-Alvarez, G. Leto, I. Pagano, R. Zanmar Sánchez, P. Giacobbe, E. Herrero, J. C. Morales, P. J. Amado, J. A. Caballero, A. Quirrenbach, A. Reiners and M. Zechmeister, 7 April 2021, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202040099

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 program 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.

Merging neutron stars

The option to measure the gravitational waves of two merging neutron stars has offered the chance to answer some of the fundamental questions about the structure of matter. At the extremely high temperatures and densities in the merger scientists conjecture a phase-transition where neutrons dissolve into their constituents: quarks and gluons. In the current issue of Physical Review Letters, two international research groups report on their calculations of what the signature of such a phase transition in a gravitational wave would look like. Quarks, the smallest building-blocks of matter, never appear alone in nature. They are always tightly bound inside the protons and neutrons. However, neutron stars, weighing as much as the Sun, but being just the size of a city like Frankfurt, possess a core so dense that a transition from neutron matter to quark matter may occur. Physicists refer to this process as a phase transition, similar to the liquid-vapor transition in water. In particular, such a phase transition is in principle possible when merging neutron stars form a very massive meta-stable object with densities exceeding that of atomic nuclei and with temperatures 10,000 times higher than in the Sun's core.The measurement of gravitational waves emitted by merging neutron stars could serve as a messenger of possible phase transitions in outer space. The phase transition should leave a characteristic signature in the gravitational-wave signal.

The research groups from Frankfurt, Darmstadt and Ohio as well as from Darmstadt and Wroclaw (used modern supercomputers to calculate what this signature could look like. For this purpose, they used different theoretical models of the phase transition.

In case a phase transition takes place more after the actual merger, small amounts of quarks will gradually appear throughout the merged object.

"With aid of the Einstein equations, we were able to show for the first time that this subtle change in the structure will produce a deviation in the gravitational-wave signal until the newly formed massive neutron star collapses under its own weight to form a black hole," explains Luciano Rezzolla, who is a professor for theoretical astrophysics at the Goethe University.

In the computer models of Dr. Andreas Bauswein from GSI Helmholtzzentrum fur Schwerionenforschung in Darmstadt a phase transition already happens directly after the merger - a core of quark matter forms in the interior of the central object. "We succeeded to show that in this case there will be a distinct shift in the frequency of the gravitational wave signal," says Bauswein.

"Thus, we identified a measurable criterion for a phase transition in gravitational waves of neutron star mergers in the future."

Not all of the details of the gravitational-wave signal are measurable with current detectors yet. However, they will become observable both with the next generation of detectors, as well as with a merger event relatively close to us.

A complementary approach to answer the questions about quark matter is offered by two experiments: By colliding heavy ions at the existing HADES setup at GSI and at the future CBM detector at the Facility for Antiproton and Ion Research (FAIR), which is currently under construction at GSI, compressed nuclear matter will be produced.

In the collisions, it might be possible to create temperatures and densities that are similar to those in a neutron-star merger. Both methods give new insights into the occurrence of phase transitions in nuclear matter and thus into its fundamental properties.