VLBA's first comprehensive 3D image of a binary star-planet system

Credit: Sophia Dagnello, NRAO/AUI/NSF.

Astronomers found a Jupiter-like planet circling a nearby star, one of a binary pair, by accurately following a minuscule, nearly undetectable wobble in its journey across space. Using the National Science Foundation's Very Long Baseline Array (VLBA), they were able to determine the whole 3-dimensional structure of the orbits of a binary pair of stars and a planet circling one of them for the first time. According to scientists, this feat can bring vital new insights into the process of planet creation.

According to Salvador Curiel of the National Autonomous University of Mexico (UNAM), while more than 5,000 extrasolar planets have been detected thus far, only three have been discovered utilizing the technique known as astrometry, which yielded this finding. However, alternative exoplanet detection methods are incapable of detecting the 3D architecture of a binary-star system including a planet. Because most stars are binary or many systems, understanding systems like this one will help us understand planet formation in general.

The two stars, known as GJ 896AB, are around 20 light-years apart from Earth, making them close neighbors by astronomical standards. They are red dwarf stars, which are the most frequent kind in our galaxy. The bigger one, around which the planet revolves, has around 44 percent of the mass of our Sun, while the smaller one has approximately 17 percent of the mass of the Sun. They are roughly the size of Neptune from the Sun and circle each other once every 229 years.

In this artist's conception, a small star (orange) is orbited by a Jupiter-like planet (blue), and by a more-distant companion star (red). Credit: Sophia Dagnello, NRAO/AUI/NSF

The researchers merged data from optical observations of GJ 896AB taken between 1941 and 2017 with data from VLBA observations conducted between 2006 and 2011. In 2020, they conducted new VLBA observations. The super-sharp resolution of the continent-wide VLBA's capacity to perceive small detail provided extraordinarily exact measurements of the stars' locations throughout time. The researchers analyzed the data thoroughly, revealing the stars' orbital movements as well as their shared motion across space.

A detailed analysis of the bigger star's velocity revealed a small wobble, revealing the planet's existence. The wobble is generated by the gravitational pull of the planet on the star. The star and planet revolve around a point in space that represents their joint center of mass. When that place, known as the barycenter, is sufficiently distant from the star, the motion of the star around it may be detected.

The planet has almost twice the mass of Jupiter and circles the star every 284 days, according to the researchers. It is only slightly closer to the star than Venus is to the Sun. The planet's orbit is inclined by around 148 degrees relative to the orbits of the two stars.

According to Gisela Ortiz-León of UNAM and the Max Planck Institute for Radioastronomy, this suggests that the planet orbits the primary star in the opposite direction as the secondary star orbits the main star. This is the first time such a dynamical structure has been detected on a planet associated with a compact binary system created in the same protoplanetary disk.

Additional extensive investigations of this and related systems, according to Joel Sanchez-Bermudez of UNAM, can help us acquire crucial insights into how planets develop in binary systems. There are several possibilities about the mechanism of creation, and more evidence may reveal which is more plausible. Current models, in particular, imply that such a big planet as a partner to such a tiny star is extremely unusual, thus those models may need to be revised.

According to scientists, the astrometric approach will be a useful tool for describing other planetary systems.

The National Radio Astronomy Observatory's Amy Mioduszewski stated, "We can do much more work like this with the planned Next Generation VLA" (ngVLA). We may be able to detect planets as tiny as the Earth with it.

The researchers' results will be published in the September 1 edition of The Astronomical Journal.

Journal Information: Salvador Curiel et al, 3D Orbital Architecture of a Dwarf Binary System and Its Planetary Companion, The Astronomical Journal (2022). DOI: 10.3847/1538-3881/ac7c66

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