An international team of astronomers from the United States and Chile has detected three giant planets in the HD 133131 system, a pair of stellar ‘twins’ that eclipse each other every 4,240 years.
Artist’s concept of giant planets orbiting in double-star system. Image credit: NASA / JPL-Caltech / T. Pyle.
HD 133131, also known as HIP 73674, is located approximately 163 light-years from Earth.
This binary system was discovered in 1972 by University of Chile astronomers Jurgen Stock and Herbert Wroblewski.
Scientists estimate its age at 9.5 billion years, compared to 4.6 billion years for our Sun.
The two components of this binary, HD 133131A and HD 133131B, are ‘twins’ of identical spectral type, G2V, which is the same as the Sun.
The stars are separated by only 360 AU (astronomical units), making them the most closely separated ‘twin’ pair with detected planets. The next-closest binary system that hosts planets is comprised of two stars that are about 1,000 AU apart.
This system is even more rare in that both ‘twins’ host planets.
HD 133131A hosts two moderately eccentric planets, labeled b and c. The planets are about 1.4 and 0.6 times the mass of Jupiter, respectively, and orbit their parent star at 1.44 and 4.79 AU.
HD 133131B hosts one moderately eccentric planet, labeled b. The planet has 2.5 times the mass of Jupiter and orbits its star at a distance of 6.4 AU.
The HD 133131 system is also unusual because both components are ‘metal poor,’ meaning that most of their mass is hydrogen and helium, as opposed to other elements like iron or oxygen.
Most stars that host giant planets are ‘metal rich.’ Only six other metal-poor binary systems with exoplanets have ever been found, making this discovery especially intriguing.
This is an illustration of the binary system HD 133131. Image credit: Timothy Rodigas.
Adding to the intrigue, the authors used very precise analysis to reveal that the stars are not actually identical ‘twins’ as previously thought, but have slightly different chemical compositions, making them more like the stellar equivalent of fraternal twins.
This could indicate that one star swallowed some baby planets early in its life, changing its composition slightly.
Alternatively, the gravitational forces of the detected giant planets that remained may have had a strong effect on fully-formed small planets, flinging them in towards the star or out into space.
“The probability of finding a system with all these components was extremely small, so these results will serve as an important benchmark for understanding planet formation, especially in binary systems,” said lead author Dr. Johanna Teske, of the Carnegie Institution for Science in Washington.
The discovery is the first exoplanet detection made based solely on data from the Planet Finder Spectrograph, a high precision optical spectrograph operating with the 6.5-m Magellan II Telescope at Las Campanas Observatory in Chile.
“We are trying to figure out if giant planets like Jupiter often have long and, or eccentric orbits,” Dr. Teske explained.
“If this is the case, it would be an important clue to figuring out the process by which our Solar System formed, and might help us understand where habitable planets are likely to be found.”
The team’s findings have been accepted for publication in the Astronomical Journal (arXiv.org preprint).
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Johanna K. Teske et al. 2016. The Magellan PFS Planet Search Program: Radial Velocity and Stellar Abundance Analyses of the 360 AU, Metal-Poor Binary ‘Twins’ HD 133131A & B. Astronomical Journal, accepted for publication; arXiv: 1608.06216v2
