The discovery of an alien world with a strange inclined orbit has puzzled astronomers

An exoplanet’s orbit around a star in a binary star system has been depicted in three dimensions for the first time. The planet orbits its star at a different angle to the orbital plane of the two stars, and this offset may provide clues to how planets form in binary systems.

Exoplanet GJ 896Ab (note the lowercase b for planet) was discovered in the binary system GJ 896AB (note the uppercase A and B for two stars), which is located 20.3 light-years from Earth. Astronomers have tracked the motion of a binary star system in space using archival optical observations dating from 1941 to 2017, with additional data spanning 2006 to 2011 collected by the Very Long Baseline Array (VLBA), which is a network of 10 radio telescopes. . strung across the United States. The researchers also made new observations with the VLBA in 2020.

Astronomers led by Salvador Curiel of the National Autonomous University of Mexico (UNAM) have found that as the star GJ 896A moves through space, it wobbles along its path. This wobble is caused by an orbiting planet with a mass 2.3 times that of Jupiter, which completes one rotation every 284.4 Earth days. Between the planet and its star is a common center of mass known as the barycenter; the star’s wobble is the result of its motion around this common center of mass.

Related: 10 Amazing Exoplanet Discoveries

Artist’s impression of a gas giant planet orbiting GJ 896A, with its companion star B in the background. (Image credit: Sofia Daniello, NRAO/AUI/NSF)

This method of detecting the movement of stars in space and any deviations in this movement is called astrometry. Astrometry is the only means by which the orbits of a planetary system with more than one star can be interpreted in three dimensions, as astronomers visually detect the oscillation and orientation of the orbits.

Interestingly, the plane in which the planet rotates is offset by 148 degrees relative to the plane of the orbit of the two stars around each other.

“This means that the planet is moving around the main star in the opposite direction of the secondary star around the main star,” says Gisela Ortiz-Leon, an astronomer at UNAM and the Institute of Radio Astronomy. Max Planck in Germany and member of the research team. project, the message says. (will open in a new tab).

Less than 4% of known exoplanets are in binary star systems. This small fraction is partly because planets are more difficult to detect in binary systems, and also because models suggest that the existence of a companion star could clip and destabilize the planet-forming disk.

“These models may need to be adjusted,” said Joel Sanchez-Bermudez of UNAM.

The planet’s orbit is at an angle of 148 degrees to the orbital plane of the two stars. (Image credit: Sofia Daniello, NRAO/AUI/NSF)

One problem with forming such a system is that it takes the gas giants 5 to 10 million years to accrete all of their gas from the planet-forming disk around them. However, current models suggest that such a disk in a binary system exists for less than 1 million years before the companion star’s gravitational tides tear the disk apart.

In addition, in the case of GJ 896AB, two of the stars are red dwarfs, which makes the existence of a giant planet in the system even more surprising. Scientists believe that red dwarfs lack the necessary raw materials to form giant planets, but the presence of a gas giant in this binary system suggests that planets may form differently when two stars are present.

“Additional detailed studies of this and similar systems could help us gain important insights into how planets form in binary systems,” Sanchez-Bermudez said.

Currently, there are two competing models for the formation of binary systems and their planets. One is called disk fragmentation, in which there is initially a single disk that forms stars and planets, which becomes gravitationally unstable and breaks up into two separate disks that form two stars and any planets around them.

Another model is called turbulent fragmentation. In this explanation, turbulence in the original cloud of gas results in two or more dense concentrations of material that independently collapse to form stars and any accompanying planets.

These models should now account for the GJ 896Ab offset by 148 degrees. What better reproduces the strange system may indicate how binary systems are formed.

The study was published September 1 in The Astronomical Journal.

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