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

There are several methods for detecting exoplanets, the best known are those of transit and radial velocities. 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 basically 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 accurate mass estimates than that of radial velocities and that is the astrometric method which consists of detecting and no longer measuring Doppler shifts — indirectly manifesting oscillation movements of a star in response to the gravitational attraction of its procession of exoplanets — but indeed to directly measure and detect the movements of the star around the center of mass of the system of stars 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 Uranus, but which astronomers 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…

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