The VLBA radio telescope reconstructs for the first time the movements of an exoplanet with a double star

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[EN VIDÉO] Exoplanets
What are exoplanets, where are they found and why are they so interesting? Video answer!

There are several methods for detecting exoplanetsthe best known are those of transit and gears radial. The first gives the orbital period of the’exoplanet and an estimate of its radius. But it has several drawbacks, not all exoplanets make transits, the method does not give the mass of the exoplanet and it essentially only indicates the presence of an exoplanet for each transit.

The second method also gives the orbital period and it has the advantage of being applied in the absence of transit, which increases the number of detectable candidates. But, except for the occurrence also of a transit, it only gives a bound on the mass of an exoplanet. On the other hand, the curve of the radial velocities of the host star directly shows the presence of several exoplanets.

There is a reference frame, that of the center of mass, where a planet and a star revolve around it as shown in this animation. © ESA Science & Technology

An ancient technique for detecting celestial bodies

There is a method which gives more precise mass estimates than that of the radial velocities and it is the astrometric method which consists in detecting and measuring either Doppler shifts — indirectly manifesting movements of oscillation of a star in response to the gravitational attraction of its procession of exoplanets — but indeed to measure and directly detect the movements of the star around the center of mass of the system ofstars on the celestial vault.

This is the astrometric method, already used to detect the existence of the planet Neptune by its disturbances on the planet Uranusbut that the astronomers have applied to the case of exoplanets already in 1943 when the astronomer Kaj Strand, working at the Sproul observatory of Swarthmore College (United States), announced that its astrometric measurements had revealed the presence of a planet in orbit around the star 61 Cygni.

The fact that a star revolves around its center of mass with an exoplanet around it introduces oscillatory movements on the celestial vault for an observer on Earth. The measurement of these movements makes it possible to go back to the characteristics of the celestial bodies in orbit. © ESA Science & Technology

In 1960 and 1963, still in the same observatory, astronomers Sarah Lippincott and Peter Van de Kamp again let it be known that they thought they had detected two exoplanets by astrometry. One, about ten times the mass of Jupiter would be in orbit around the star Lalande 21185 and the other around Barnard’s star.

None of these detections have since been confirmed (although there does appear to be an exoplanet around Barnard’s star), not even after the first detection of an exoplanet around a main sequence star in 1995 by the Swiss Michel Mayor and Didier Queloz.

But today a team of radio astronomers reported via a publication in Astronomical Journal an open-access version of which can be found at arXiv that, not only did she think she had astrometrically revealed an exoplanet, but she also succeeded in extracting from astrometric measurements the first complete representation of the movements of an exoplanet with those of stars from binary system to which it is linked. This is the first 3D reconstruction of such a triple system, a reconstruction that cannot be obtained with other methods of detecting and studying exoplanets according to one of the authors of the article, Salvador Curiel, of the National Autonomous University of Mexico (Unam).

The performance, which concerns a Jupiter-sized exoplanet orbiting one of the stars of the GJ 896AB Binary System located about 20 light years from Sun in the Milky Waywas made possible thanks to the radio telescopes distributed over the surface of the Earth and which constitute the Very Long Baseline Array (VLBA).

This animation illustrates the orbital motions of a pair of stars and a planet orbiting one of the stars. In this case, they are red dwarf stars, the most common type in our Milky Way Galaxy. The larger one, around which the planet orbits, has about 44% the mass of our Sun, while the smaller one has about 17% the mass of the Sun. They are separated by about the distance of Neptune from the Sun and revolve around each other once every 229 years. The two stars are called GJ 896AB. © Sophia Dagnello, NRAO/AUI/NSF

A key to understanding the formation of planets

These antennas, which make it possible to make opening summary and to have the equivalent of a giant radio telescope several thousand kilometers in diameter, observed GJ 896AB at times, from 2006 to 2011, then in 2020. The data collected was supplemented by measurements made in the visible range of 1941 to 2017. The laws of celestial mechanics have finally been applied to extract from this data the 3D movements of the stars and the planet and not only those, apparent, on the celestial vault.

Oscillations of the stars around the System’s center of mass provided a bonus estimate of the exoplanet’s mass, about twice that of Jupiter, as well as that of its orbital period, about 284 days. It was also possible to deduce that the exoplanet had an inclined orbital plane of 148 degrees compared to that of the stars of the binary, which implies that the exoplanet rotates in the opposite direction to that of the stars around it. on the other, at a distance comparable to that of Venus under the sun.

According to one of the researchers involved in these discoveries, Gisela Ortiz-LeonUnam and the Institute Max Planck of radio astronomy: This is the first time that such a dynamic structure has been observed on a planet associated with a compact binary system that presumably formed in the same protoplanetary disk. “.

His colleague Joel Sanchez-Bermudez, also working at Unam, specifies: Further detailed studies of this and similar systems can help us gain important insights into how planets form in binary systems. There are several theories for the formation mechanism, and more data may possibly point to which one is most likely. In particular, current models indicate that such a large planet is highly unlikely as a companion to such a small star, so perhaps these models need to be revised. “.

Also in a press release from National Radio Astronomy Observatorythe radio astronomer Amy Mioduszewski adds in conclusion: We can do a lot more work like this with the upcoming Next Generation VLA (ngVLA). With it, we may be able to find planets as small as Earth “.

The main methods of detecting exoplanets. © CEA Research

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