A team of researchers has confirmed the first planet detected using the astrometry technique by the European Space Agency's (ESA) Gaia spacecraft. The planet, designated Gaia-4b, orbits a star with a mass of 64% that of the Sun and has a mass 12 times that of Jupiter. The planet's orbital period is 570 days.
The discovery was made using data from the NEID spectrograph, a high-precision radial velocity spectrograph designed to measure the wobble of stars caused by the gravitational pull of orbiting planets. NEID is funded by the NASA/NSF Exoplanet Exploration Program and is installed on the 3.5-meter WIYN Telescope at the National Science Foundation's Kitt Peak National Observatory, a program of NOIRLab.
Gaia uses a different technique to detect the movement of a star, called astrometry. This technique measures the movement of a star as it is tugged by the gravity of an orbiting planet, observing how the star moves in comparison to the background or surrounding stars. Gaia's precise measurements have allowed it to identify a list of stars that appear to be moving as if they are being pulled by an exoplanet, known as "Astrometric Objects of Interest" (Gaia-ASOIs).
However, the movement of these stars is not necessarily caused by a planet. The 'star' could be a pair of stars that are too close together for Gaia to recognize as separate objects. The small positional changes that could be caused by a planet might actually be the result of the near-perfect cancellation of the larger positional changes of the stars.
To rule out these binary stars and uncover true planets, follow-up observations with spectroscopy are needed. The NEID instrument, along with two other spectrographs, the Habitable-zone Planet Finder (HPF) located on the 10-meter Hobby Eberly Telescope at McDonald Observatory in Texas, and the FIES spectrograph located on the 2.6-meter Nordic Optical Telescope in La Palma, Canary Islands, were used to conduct these follow-up observations.
The research team observed 28 stellar systems with candidate planets identified by Gaia. Of these, 21 were found to be false positives, actually binary star systems. One system was confirmed to be a star hosting a brown dwarf, an object with a mass intermediate between planets and stars. However, one system was confirmed to be a star hosting a giant planet, Gaia-4b.
Gaia-4b is not only the first planet detected by Gaia using the astrometry technique, with a fully independently determined orbital solution, but it is also one of the most massive planets known to orbit a low-mass star.
The discovery of Gaia-4b marks a significant milestone in exoplanet research. It demonstrates the power of the astrometry technique and the importance of ground-based observations like those made by NEID to confirm planetary candidates in the era of Gaia's planet detections.