Three exoplanets discovered in the system of the red dwarf GJ 357

The Gl 357 system (Image NASA's Goddard Space Flight Center/Chris Smith)
The Gl 357 system (Image NASA’s Goddard Space Flight Center/Chris Smith)

An article published in the journal “Astronomy & Astrophysics” reports the discovery of three exoplanets in the system of the red dwarf GJ 357. A team of researchers led by Rafael Luque of the Canary Islands Astrophysics Institute identified the innermost exoplanet, named GJ 357 b, thanks to the observations conducted by NASA’s TESS space telescope while the other two, named GJ 357 c and GJ 357 d, were discovered using the radial velocity method thanks to data collected over twenty years of observations of various ground-based telescopes. The three exoplanets could all be rocky and the outermost is within ​​its star system’s habitable zone.

At the end of the first year of scientific activity of the TESS space telescope, launched on April 18, 2018, not only the exoplanet candidates but also the ones confirmed thanks to follow-up observations are frequently increasing. Many new discoveries concern exoplanets that orbit red dwarfs, which in the past were neglected but are the most abundant. GJ 357 has a mass that is about one third of the Sun’s and its study triggered a series of discoveries obtained by putting together data collected from various telescopes in various periods.

The first discovery took place in the normal ways of the TESS space telescope, which detects the slight drops in brightness of the stars when one or more planets pass between them and the Earth. This also happened observing the star GJ 357 and at that point the researchers sought confirmation of the existence of the candidate discovered finding also traces of two more exoplanets farther away from their star with the radial velocity method. Rafael Luque stated that somehow those planets were hidden in the measurements made at a number of observatories over many years and that it took TESS to point out an interesting star where they could discover them.

The exoplanet GJ 357 b discovered by TESS was defined by the researchers as a hot Earth because it has a size about 22% higher than the Earth’s but it’s so close to its star that its year lasts only 3.9 Earth days. For this reason the estimated temperature on its surface without taking into account a possible atmosphere is 254° Celsius (490° Fahrenheit). It will take follow-up observations to understand if it has an atmosphere and in that case its composition. This is one of the best candidates found since it’s the third exoplanet closest to the Earth discovered with the transit method.

The data about the other two exoplanets are more vague because they weren’t directly observed but some characteristics were deduced. The radial velocity method allows to estimate the masses of the exoplanets because it’s based on their gravitational influence but without direct observations it says nothing about their size or the possible presence of an atmosphere.

The exoplanet GJ 357 c has a mass at least 3.4 times the Earth’s – so it could be a super-Earth – and a year of the duration of 9.1 Earth days. The estimated temperature on its surface without taking into account a possible atmosphere is 127° Celsius (260° Fahrenheit). Given that no transits of GJ 357 c were detected, the researchers concluded that its orbit is slightly tilted relative to its neighbor’s.

The exoplanet GJ 357d has a mass at least 6.1 times the Earth’s so it could be a super-Earth but also a mini-Neptune. If it were a rocky planet its size could be twice as the Earth’s. However, it’s the most interesting exoplanet because its year lasts 55.7 Earth days and that puts it at the outer borders of ​​its star system’s habitable zone.

Because of its position, the exoplanet GJ 357 d could be a super-Mars with an estimated temperature on its surface without taking into account a possible atmosphere of -53° Celsius (-64° Fahrenheit). When Mars was young and still had a magnetic field capable of shielding its atmosphere, the conditions on its surface were much more similar to those on Earth. This means that there are chances that GJ 357 d is habitable but only in the presence of a set of characteristics since it should be a rocky planet with a magnetic field and an atmosphere similar to the Earth’s. The TESS space telescope hasn’t detected any transit but in this case it’s simply possible that its system was observed during a period in which it didn’t pass in front of its star.

For the moment there are some hypotheses regarding the exoplanet GJ 357 d but astronomers must hope that it passes in front of its star during a period in which a follow-up observation is carried out to really understand its characteristics, starting from checking whether it’s a super-Earth or a mini-Neptune.

This research not only confirms the excellent results that the TESS space telescope is bringing but the usefulness of many other observations that also include long surveys carried out with ground-based telescopes. The available data can be useful even after years when a new study allows to obtain key information to be combined with existing ones. In the case of the GJ 357 system, this means moving to a new phase of follow-up studies.

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