According to a team of astronomers at Aarhus University in Denmark, Earth-sized terrestrial planets orbit their host stars in circular patterns, much like the planets of our Solar System.
This is an artist’s impression of an exoplanet. Image credit: Ron Miller.
Solar System planets move on almost circular orbits. In contrast, many extrasolar gas giants travel on highly elliptical orbits, whereas the shape of the orbits of smaller, more terrestrial, exoplanets has remained largely elusive.
Now, a new study using data from NASA’s Kepler Space Telescope shows that trajectories of small, terrestrial planets stand in stark contrast to those of more massive exoplanets.
The authors of the study – Aarhus University astronomers Dr Simon Albrecht and Vincent Van Eylen – analyzed photometric data for 74 Earth-sized planets around 28 bright stars, and found that all of these planets maintain circular, not eccentric, orbits.
“The orbital results for these smaller planets (Kepler-10b and -c, Kepler-23b, -c and -d, Kepler-25b and -c, and others) may eventually help to explain why larger planets have more extreme orbits,” said Van Eylen, lead author of the paper accepted for publication in the Astrophysical Journal (arXiv.org preprint).
“Twenty years ago, we only knew about our Solar System, and everything was circular and so everyone expected circular orbits everywhere,” he said.
“Then we started finding giant exoplanets, and we found suddenly a whole range of eccentricities, so there was an open question about whether this would also hold for smaller planets. We find that for small planets, circular is probably the norm.”
Ultimately, that’s good news in the search for life elsewhere. Among other requirements, for a planet to be habitable, it would have to be about the size of Earth – small and compact enough to be made of rock, not gas.
If a small planet also maintained a circular orbit, it would be even more hospitable to life, as it would support a stable climate year-round.
Van Eylen added: “if eccentric orbits are common for habitable planets, that would be quite a worry for life, because they would have such a large range of climate properties. But what we find is, probably we don’t have to worry too much because circular cases are fairly common.”
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Vincent Van Eylen & Simon Albrecht. 2015. Eccentricity from transit photometry: small planets in Kepler multi-planet systems have low eccentricities. ApJ, accepted for publication; arXiv: 1505.02814
