From above a planet about twice the size of Jupiter, this artist's conception shows the star that planet is orbiting and that star's binary companion in the distance. Credit: Sophia Dagnello, NRAO/AUI/NSF. Hi-Res File
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.
Two Stars and a Planet
Artist's animation illustrates the orbital motions of a binary star pair and a planet orbiting one of the stars.
Credit: Sophia Dagnello, NRAO/AUI/NSF.
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.
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.
