The planets in our solar system hold a relatively circular orbit, but scientists have remained unsure if exoplanets behave the same way. Massive exoplanets have been known to exhibit irregular orbital patterns, but the research on Earth-sized planets' orbits was not as clear.
A new exploration of 74 exoplanets revealed most that are Earth-sized or smaller also move in circular patterns, the Massachusetts Institute of Technology (MIT) reported.
"Twenty years ago, we only knew about our solar system, and everything was circular and so everyone expected circular orbits everywhere," said 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."
The findings could influence our search for intelligent life in the universe. A planet that hosted life would most likely need to be Earth-sized and rocky, but a regular circular orbital pattern would make it even more hospitable.
"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," Van Eylen said. "But what we find is, probably we don't have to worry too much because circular cases are fairly common."
In the past, the orbits of larger "gas giant" exoplanets have been determined using radial velocity, which measures a star's movement. The gravitational force of these large planets is enough to influence the movement of the star during orbit, but smaller planets do not exert enough "tug" for this technique to be viable. Smaller planets are usually spotted through a transit-detecting method in which researchers look for dips in light caused by the object passing in front of its host star, the team used this phenomenon to also look at the planets' orbits.
The researchers used the mass and radius of a planet's star to calculate how long it would take to make a full revolution around it in the case of a circular orbit; they were then able to determine how long it would take to pass in front of the star on the visible side given these circumstances. If the calculations matched the actual amount of time it took for the planet to pass in front of the star, it suggested the planet had a circular orbit.
In order to gain this information, the researchers looked at observations from NASA's Kepler telescope, which has monitored the brightness of over 145,000 stars. They chose to focus on 28 stars for which mass and radius had already been measured. The calculations suggest all 74 planets looked at in the study orbit in relatively circular patterns.
"We found that most of them matched pretty closely, which means they're pretty close to being circular," Van Eylen said. "We are very certain that if very high eccentricities were common, we would've seen that, which we don't."
The researchers hope these new insights into the behavior of Earth-sized and smaller exoplanets will eventually help explain why larger gas giants' have more eccentric orbital patterns.
"We want to understand why some exoplanets have extremely eccentric orbits, while in other cases, such as the solar system, planets orbit mostly circularly," Van Eylen said. "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."
The findings were published in a recent edition of the Astrophysical Journal.
