There may be small, rocky planets in the nearby Alpha Centauri triple-star system that have been overlooked, according to a new study published in the Astronomical Journal (arXiv.org preprint).
This artist’s conception shows a hypothetical rocky exoplanet in the Alpha Centauri system. Image credit: Michael S. Helfenbein.
Alpha Centauri, also known as Rigil Kentaurus, Rigil Kent and Gliese 559, is the closest stellar system to Earth.
This triple system is made up of the bright binary star formed by Alpha Centauri A and B, plus the faint red dwarf star Alpha Centauri C.
The two brighter components are roughly 4.35 light-years away from us. Alpha Centauri C, better known as Proxima Centauri, is slightly closer, at 4.23 light-years.
Compared to our Sun, Alpha Centauri A is of the same stellar type G2, but slightly bigger. Alpha Centauri B, a K1-type star, is slightly smaller and less bright.
Alpha Centauri A and B orbit a common center of gravity once every 80 years, with a minimum distance of about 11 times the distance between the Earth and the Sun.
Last year, the discovery of Proxima b, an Earth-like planet orbiting Proxima Centauri, set off a new wave of scientific and public interest in this triple-star system.
“Because Alpha Centauri is so close, it is our first stop outside our Solar System,” said Yale University’s Professor Debra Fischer, co-author of the study.
“There’s almost certain to be small, rocky planets around Alpha Centauri A and B.”
The findings are based on data coming in from a new wave of more advanced spectrographic instruments at observatories located in Chile: (i) CTIO High Resolution Spectrograph (CHIRON), a spectrograph built by Professor Fischer and colleagues; (ii) High Accuracy Radial Velocity Planet Searcher (HARPS), built by a team from Switzerland; and (iii) Ultraviolet and Visual Echelle Spectrograph (UVES), part of ESO’s Very Large Telescope Array.
“The precision of our instruments hasn’t been good enough, until now,” Professor Fischer said.
The astronomers analyzed more than a decade of radial-velocity measurements for Proxima Centauri and Alpha Centauri A and B from these instruments.
“The Universe has told us the most common types of planets are small planets, and our study shows these are exactly the ones that are most likely to be orbiting Alpha Centauri A and B,” Professor Fischer said.
The team determined that for Alpha Centauri A, there might still be orbiting planets that are smaller than 50 Earth masses.
For Alpha Centauri B there might be orbiting planets than are smaller than 8 Earth masses.
And for Proxima Centauri, there might be orbiting planets that are less than one-half of Earth’s mass.
In addition, the study eliminated the possibility of a number of larger planets.
“This takes away the possibility of Jupiter-sized planets causing asteroids that might hit or change the orbits of smaller, Earth-like planets,” said study lead author Lily Zhao, a graduate student at Yale University.
“This is a very green study in that it recycles existing data to draw new conclusions.”
“By using the data in a different way, we are able to rule out large planets that could endanger small, habitable worlds and narrow down the search area for future investigations.”
“This new information will help astronomers prioritize their efforts to detect additional planets in the system,” the study authors said.
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Lily Zhao et al. 2017. Planet Detectability in the Alpha Centauri System. AJ 155, 24; doi: 10.3847/1538-3881/aa9bea
