VLBA produces first full 3-D view of binary star-planet system

By precisely tracing a small, almost imperceptible, wobble in a nearby star's motion through space, astronomers have discovered a Jupiter-like planet orbiting that star, which is one of a binary pair. Their work, using the National Science Foundation's Very Long Baseline Array (VLBA), produced the first-ever determination of the complete, 3-dimensional structure of the orbits of a binary pair of stars and a planet orbiting one of them. This achievement, the astronomers said, can provide valuable new insights on the process of planet formation. 

Though more than 5,000 extrasolar planets have been discovered so far, only three have been discovered using the technique -- called astrometry -- that produced this discovery. However, the feat of determining the 3-D architecture of a binary-star system that includes a planet "cannot be achieved with other exoplanet discovery methods," said Salvador Curiel, of the National Autonomous University of Mexico (UNAM).

"Since most stars are in binary or multiple systems, being able to understand systems such as this one will help us understand planet formation in general," Curiel said.

The two stars, which together are called GJ 896AB, are about 20 light-years from Earth -- close neighbors by astronomical standards. 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 percent of the mass of our Sun, while the smaller is about 17 percent as massive as the Sun. They are separated by about the distance of Neptune from the Sun, and orbit each other once every 229 years.

For their study of GJ 896AB, the astronomers combined data from optical observations of the system made between 1941 and 2017 with data from VLBA observations between 2006 and 2011. They then made new VLBA observations in 2020. The continent-wide VLBA's supersharp resolution -- ability to see fine detail -- produced extremely precise measurements of the stars' positions over time. The astronomers performed extensive analysis of the data that revealed the stars' orbital motions as well as their common motion through space.

Detailed tracing of the larger star's motion showed a slight wobble that revealed the existence of the planet. The wobble is caused by the planet's gravitational effect on the star. The star and planet orbit a location between them that represents their common center of mass. When that location, called the barycenter, is sufficiently far from the star, the star's motion around it can be detectable.

The astronomers calculated that the planet has about twice the mass of Jupiter and orbits the star every 284 days. Its distance from the star is slightly less than Venus' distance from the Sun. The planet's orbit is inclined roughly 148 degrees from the orbits of the two stars. 

"This means that the planet moves around the main star in the opposite direction to that of the secondary star around the main star," said Gisela Ortiz-León, of UNAM and the Max Planck Institute for Radioastronomy. "This is the first time that such dynamical structure has been observed in a planet associated with a compact binary system that presumably was formed in the same protoplanetary disk", she added.

"Additional detailed studies of this and similar systems can help us gain important insights into how planets are formed in binary systems. There are alternate theories for the formation mechanism, and more data can possibly indicate which is most likely," said Joel Sanchez-Bermudez, of UNAM. "In particular, current models indicate that such a large planet is very unlikely as a companion to such a small star, so maybe those models need to be adjusted," he added. 

The astrometric technique will be a valuable tool for characterizing more planetary systems, the astronomers said. "We can do much more work like this with the planned Next Generation VLA (ngVLA)," said Amy Mioduszewski, of the National Radio Astronomy Observatory. "With it, we may be able to find planets as small as the Earth."

The astronomers are reporting their findings in the 1 September issue of the Astronomical Journal.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.