Viewed from above, our solar system’s planetary orbits around the sun resemble rings around a bulls-eye. Each planet, including Earth, keeps to a roughly circular path, always maintaining the same distance from the sun.

For decades, astronomers have wondered whether the solar system’s circular orbits might be a rarity in our universe. Now a new analysis suggests that such orbital regularity is instead the norm, at least for systems with planets as small as Earth.  

In a paper published in the Astrophysical Journal, researchers from MIT and Aarhus University in Denmark report that 74 exoplanets, located hundreds of light-years away, orbit their respective stars in circular patterns, much like the planets of our solar system.

These 74 exoplanets, which orbit 28 stars, are about the size of Earth, and their circular trajectories stand in stark contrast to those of more massive exoplanets, some of which come extremely close to their stars before hurtling far out in highly eccentric, elongated orbits.

“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.”

Ultimately, Van Eylen says 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. (In contrast, a planet with a more eccentric orbit might experience dramatic swings in climate as it orbited close in, then far out from its star.)

“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 says. “But what we find is, probably we don’t have to worry too much because circular cases are fairly common.”

Star-crossed numbers

In the past, researchers have calculated the orbital eccentricities of large, “gas giant” exoplanets using radial velocity — a technique that measures a star’s movement. As a planet orbits a star, its gravitational force will tug on the star, causing it to move in a pattern that reflects the planet’s orbit. However, the technique is most successful for larger planets, as they exert enough gravitational pull to influence their stars.

Researchers commonly find smaller planets by using a transit-detecting method, in which they study the light given off by a star, in search of dips in starlight that signify when a planet crosses, or “transits,” in front of that star, momentarily diminishing its light. Ordinarily, this method only illuminates a planet’s existence, not its orbit. But Van Eylen and his colleague Simon Albrecht, of Aarhus University, devised a way to glean orbital information from stellar transit data.

They first reasoned that if they knew the mass of a planet’s star, they could calculate how long a planet would take to orbit that star, if its orbit were circular. The mass of a star determines its gravitational pull, which in turn influences how fast a planet travels around the star.

By calculating a planet’s orbital velocity in a circular orbit, they could then estimate a transit’s duration — how long a planet would take to cross in front of a star. If the calculated transit matched an actual transit, the researchers reasoned that the planet’s orbit must be circular. If the transit were longer or shorter, the orbit must be more elongated, or eccentric.

Not so eccentric

To obtain actual transit data, the team looked through data collected over the past four years by NASA’s Kepler telescope — a space observatory that surveys a slice of the sky in search of habitable planets. The telescope has monitored the brightness of over 145,000 stars, only a fraction of which have been characterized in any detail.

The team chose to concentrate on 28 stars for which mass and radius have previously been measured, using asteroseismology — a technique that measures stellar pulsations, which reflect a star’s mass and radius.

These 28 stars host multiplanet systems — 74 exoplanets in all. The researchers obtained Kepler data for each exoplanet, looking not only for the occurrence of transits, but also their duration. Given the mass and radius of the host stars, the team calculated each planet’s transit duration if its orbit were circular, then compared the estimated transit durations with actual transit durations from Kepler data.

Across the board, Van Eylen and Albrecht found the calculated and actual transit durations matched, suggesting that all 74 exoplanets maintain circular, not eccentric, orbits.

“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.”

Van Eylen says the orbital results for these smaller planets may eventually help to explain why larger planets have more extreme orbits.

“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 says. “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.”

David Kipping, an astronomer at the Harvard-Smithsonian Center for Astrophysics, notes that Van Eylen’s sample of 74 exoplanets is a relatively small slice, considering the hundreds of thousands of stars in the sky.

“I think that the evidence for smaller planets having more circular orbits is presently tentative,” says Kipping, who was not involved in the research. “It prompts us to investigate this question in more detail and see whether this is indeed a universal trend, or a feature of the small sample considered.”

In regard to our own solar system, Kipping speculates that with a larger sample of planetary systems, “one might investigate eccentricity as a function of multiplicity, and see whether the solar system's eight planets are typical or not.”

This research was funded in part by the European Research Council.

Is this not too bold to present knowledge about planets in so distant star systems?Come on, it's hundreds of light years from Earth and all what you got is observations (from Earth and orbit mostly) and measuers of radiaton levels?How precise can that be?In so huge distances there must have been interferecnes (which are unmeasurable) which distorted the true value of measure.We still have, as mankind, too poor equipment to measure the far space.

If we look at the conditions of mutual movement of two celestial bodies, smaller body rotates around a larger, mathematically proved that a small movement of the body is done in an elliptical orbit, not a circular. If an object moves along a circular path, then there is the radial velocity of the body, there is no spin or any non-uniform motion, making it impossible to see the entire universe. If there is no eccentricity of the orbit, then the body rotates around its parent star at a constant rate, will be rotated on its axis once in its revolution, does not exist, then nor any other movement, and I Angular Momentum is a vector product of constant rotational speed and constant radius . If the eccentricity is zero, then the radius pericenter zero which leads to the fact that the parent star rotates around the center of mass, which is the very center of that star and hence plan that rotates around the center, which is not possible. Not something wrong along with your calculations !!

MIT is great. Awesome

Maximov is right. At such large distances , there can be all sorts of disturbances like gravitational lensing

Planetary astronomers have become extremely good at identifying bonafide planetary transit data from a large number of false positive identifications, many of which are caused by eclipsing binary stars or background stars. The small dip in stellar flux (light) that occurs when a planet transits across the face of its parent star, has a certain characteristic shape, known as a light curve. Other phenomena like gravitational lensing would not produce this kind of light curve, nor would gas or dust in the interstellar medium along our line of sight. Scientists searching for planets have extremely stringent checks and balances to ensure that when they announce the discovery of a planet, it is a very real object, orbiting a very real star. Whether the planetary system is 10 light years or 400 light years away is relatively unimportant.

Reprinted with permission of MIT News