Circular orbits common in small planets

Circular orbits in Earth-sized exoplanets are common throughout the Universe, according to new research that uses a unique method to determine orbital patterns hundreds of lightyears away.

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A diagram illustrating transit detection, by which scientists use the dip in brightness and the size of the star to determine the size or radius of a planet.
Credit: NASA Ames

Circular orbits are likely the norm for small, Earth-sized exoplanets, a new study has suggested, signifying good news in the search for other habitable planets like our own.

An analysis of 74 small exoplanets orbiting 28 stars hundreds of lightyears away shows that, unlike larger gas giants, Earth-sized exoplanets do generally stick to circular orbital patterns around their stars.

The findings of the study may have answered a question astronomers have been pondering for decades; whether our Solar System’s circular orbits are common throughout the rest of the Universe.

The study was carried out by researchers from the Massachusetts Institute of Technology (MIT) and Aarhus University in Denmark.

“Twenty years ago, we only knew about our solar system, and everything was circular and so everyone expected circular orbits everywhere,” says Vincent Van Eylen, a visiting graduate student in MIT’s Department of Physics. “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.”

In order for a planet to be habitable, it must be about the size of Earth and made of solid rock, rather than gas. But just as important is the shape of its orbit. If a planet exhibits a circular orbit around its star, then it will support a stable climate all year round. However, if the planet exhibits a more irregular, eccentric orbit, it will experience dramatic shifts in climate as it moves close to, then far away from its star.

Astronomers are able to calculate the orbital structure of larger, gas giant exoplanets by measuring the ‘tug’ exerted on the central star as the orbit passes close by. But this technique – known as radical velocity – does not work for smaller exoplanets, as they are not big enough to exert a noticeable influence.

In order to locate smaller planets, researchers study the light given off by a star, looking for dips in intensity that signify a planet is crossing or transiting in front of that star. This is known as transit detecting. Van Eylen and his colleague, Simon Albrecht of Aarhus University, have devised a way of using transit detecting to learn more about smaller planets’ orbital patterns.

The process involves using the mass and radius of a planet’s star to calculate how long a planet would take to orbit that star, providing the orbit is circular. They are then able to able to estimate how long it would take for a planet to cross in front of a star and, if the actual transit matches their calculations, determine whether or not the planet’s orbit is circular.

The team used data collected over four years by NASA’s Kepler telescope, examining the brightness of over 145,000 stars. They collated this data for 74 exoplanets and then calculated each planet’s transit duration, comparing it with their estimated transit durations.

“We found that most of them matched pretty closely, which means they’re pretty close to being circular,” Van Eylen says. “We are very certain that if very high eccentricities were common, we would’ve seen that, which we don’t.

“We want to understand why some exoplanets have extremely eccentric orbits, while in other cases, such as the solar system, planets orbit mostly circularly. This is one of the first times we’ve reliably measured the eccentricities of small planets, and it’s exciting to see they are different from the giant planets, but similar to the solar system.”


Front image: Artist’s depiction of Kepler-444, a system home to five small planets that were detected by the dimming that occurs when they transit the disk of their parent star
Image credit: NASA/JPL-Caltech/AMES/Univ. of Birmingham
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