Of all the planets in the solar system, Earth is distinctly hospitable to life because of its distance from the sun.

As our planet follows its orbit, it gets sufficiently close to the sun as to take advantage of solar energy, while staying just distant enough so the temperature on Earth is low enough to support life. The circular orbit that it follows is also crucial to maintaining a relatively stable climate.

Now, a new study published in the Astrophysical Journal suggests that planets roughly the same size as Earth follow a similar circular orbit — and this could be significant in the hunt for life outside the solar system.

Researchers from Aarhus University in Denmark and MIT looked at 74 exoplanets that are hundreds of light years away, and found that each of these planets orbited the center of its own solar system in a circular path. The scientists used NASA's Kepler space telescope to discern the orbital periods of these planets.

The planets were were found to be about the same size as Earth, and unlike other planetary objects that tend to orbit extremely elliptical paths – getting dangerously close to their host star – these smaller planets retain a constant distance from their host star while orbiting their circular trajectory.

"We want to understand why some exoplanets have extremely eccentric orbits, while in other cases, such as the solar system, planets orbit mostly circularly," said Vincent Van Eylen, from MIT's department of physics. "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."

Van Eylen said this finding is good news in the hunt for life elsewhere, because in order for a planet to be habitable, it would likely be about the same size as Earth — small, and made up of rock, rather than gas.

Small planets that follow a circular orbit are also more hospitable to life, because they retain a stable climate throughout the year. Planets with eccentric orbits, by contrast, may go through dramatic climate swings.

"Our findings are helpful in identifying which planets are habitable because the location of the habitable zone depends on eccentricity, and to determine occurrence rates inferred for these planets because planets on circular orbits are less likely to transit," the researchers wrote. 

NASA Goddard Space Flight Center | Flickr