Circular orbits common amongst Earth-sized exoplanets

Researchers from MIT and Aarhus University in Denmark have solved a decades-long riddle, demonstrating for the first time that the circular orbits maintained by Earth and the other planets in the Milky Way are not unique to our solar system’s worlds.

As the authors report in the latest edition of The Astrophysical Journal, they discovered a total of 74 exoplanets, some located hundreds of light years from Earth, that orbit their respective stars in essentially circular patterns, maintaining essentially the same distance at all times.

Astronomers had long wondered if these types of orbits were considered rare for planets located in other parts of the universe, and the new paper reveals that this type of orbital regularity is the norm, at least when it comes to planets that are roughly the same size as Earth.

Why is orbital circularity so important?

These 74 exoplanets, which orbit a total of 28 different stars, have far different orbits than massive exoplanets, many of which travel around their stars in eccentric orbits. They come extremely close to their host stars at one point before moving farther away as they continue their journey.

Vincent Van Eylen, a visiting graduate student in MIT’s Department of Physics, told redOrbit via email that the findings “will help astronomers in understanding the process of planetary formation,” which can result in either in these types of circular orbits or in the “highly eccentric orbits… commonly observed for exoplanets as massive as Jupiter.”

“It’s exciting that our results show orbits which are close to circular, in contrast with previous exoplanet measurements for massive planets,” Van Eylen said. “The similarity with the almost circular orbits for the planets in the solar system is striking, although we don’t fully understand yet why this is the case,” he added, noting that it could be related either to the small size of the planets or the number of planets contained within a system.

In addition to the learning more about planetary formation, the researchers said that they wanted to understand the circularity of exoplanet orbits because of the influence that eccentricity has on a planet’s climate. If most Earth-like planets had highly eccentric orbits, their climate would vary greatly over the course of a year as it moved closer to and further away from its star.

Good news in the search for life on other worlds

According to Van Eylen, the discovery that circular orbits appear to be the norm for smaller exoplanets is good news when it comes to the search for extraterrestrial life. While little is known about most exoplanets, the two factors that are most important are that these planets are small enough to be rocky, and are located in their star’s habitable zone (and thus are capable of supporting liquid water).

“Up until now, we knew almost nothing about the orbits of such planets,” he told redOrbit via email. “Now our evidence suggests that such planets may well have circular orbits, which means their climates are likely to be more stable. If they were eccentric like some of the massive exoplanets we know, their climates would be very variable when the planets move closer and further away from the star during one planet year.”

Van Eylen said that he emphasized the word “suggests” because, while they have found circular orbits for Earth-sized planets, none of those worlds are far enough away from their stars to have liquid water. With access to better data, he and his colleagues hope to learn more about such planets in the future – and maybe even discover some additional surprises.



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