New discovery: small planets have circular orbits

June 17, 2015 - 05:25

Scientists develop new method to measure planetary orbits. It can provide important clues on how the planets formed and whether they have the potential to host life.

In our own solar system, planets orbit around the sun in nearly circular orbits. But what about the planetary orbits in other solar systems? Scientists have developed a new method to measure them. (Illustration: NASA)

You probably know that planets orbit stars – just like The Earth revolves around its own star, the Sun.

However, we know surprisingly little about the shape of other planetary orbits – for example whether they are circular, elliptical, very elongated or only slightly elongated.

Now, scientists have developed a new method to measure the orbital shape of these planets.

"For the first time, we’ve been able to measure the trajectory of small planets located outside our own solar system and we can see that many of the small planets have nearly circular paths," says Vincent Van Eylen, a PhD student at Stellar Astrophysics Centre at Aarhus University.

Circular orbits

In the new study, the scientists measured the orbital courses of 74 smaller exoplanets – i.e. planets orbiting stars other than our sun.

They discovered that most of them had circular orbits.

"Up until now we’ve only been able to measure the orbit of large planets and measurements show that most large planets have very elongated and elliptical orbits. Now, we have shown that this is different for small exoplanets. The small planetary orbits that we have measured are basically circular," says Van Eylen.

'Eccentricity' - what is that?

Eccentricity is a measure of how elongated a circle is. A perfectly round circle has an eccentricity of zero. The higher the eccentricity, the more elongated and stretched the circle becomes.

Van Eylen points out that studying the eccentricity of planetary orbits can tell us whether the planet has conditions suitable for life to exist.

"One of the factors that may influence whether a planet can support life is whether or not the orbit is circular because this affects the planet’s climate. If the planet's orbit is highly eccentric, the climate will be unstable, making it less likely that the planet sustains life," says Van Eylen.

He explains that Earth, along with all other planets in our solar system, follows a roughly circular orbit around the Sun and therefore has a very low eccentricity.

Probably no life on the 74 planets

The scientists behind the new study estimate that the types of orbits they have measured here offer the best conditions for life. So, could there be life on any of these 74 exoplanets? The scientists emphasise, that the answer is most probably no.

None of the planets are located at an appropriate distance to their star, which is needed for water to exist in a liquid form. Life as we know it, could therefore not exist.

But the hope is that in the future, the newly developed method to measure the shape of planetary orbits might help scientists looking for life on other planets.

The vertical axis shows how the planetary eccentricity, while the horizontal axis indicates the planetary weight (mass). The red spots are planets of our own Solar System; blue spots indicate the 74 exoplanets measured in the new study. Grey and black spots indicate exoplanets, as measured previously. (Illustration: University of Aarhus)

"In the future, this method can also be used in the search for habitable planets. We now know that the method works, so we just have to wait for more data," says Professor Simon Albrecht from Aarhus University in a press release.

According to planetary scientist at Harvard University, Lars Buchhave, knowledge of the eccentricity of planetary orbits also gives us an important glimpse of a planet’s history.

"It is important to know how circular or eccentric an exoplanet’s orbit is, because it can tell us something about how the planetary system was formed," says Buchhave.

How to measure a planet’s orbit

How did the scientists measure the shape of planets’ orbit?

Van Eylen explains that they used data from the famous Kepler satellite which has been studying the stars for several years and has found hundreds of these exoplanets.

Planets do not light up in space in the same way that stars do so Kepler cannot observe them directly. Instead, it discovers planets by the 'shadows' created when they cross in front of their own star, causing a dip in the light emitted from the star.

Comparing how long this dip in the star’s brightness occurs for with existing knowledge of the individual stars, scientists could calculate the shape of their planetary orbit and use it to calculate the eccentricity.

"If the field is completely circular then we know exactly how long it will take for the planet to move past its star. By measuring how fast the planet actually moves we can figure out how far it is from being a perfect circle, "says Van Eylen.

Expanding our knowledge beyond the major planets

The development of the new method required knowledge of astroseismology, “a method that the people in Aarhus are amongst the best in the world at performing," says Buchhave.

The new method differs from previous methods used to measure the shape of planetary orbits, which were only able to measure the orbits of larger planets – often as big or even bigger than Jupiter, explains Van Eylen.

The new study was recently published in the scientific journal Astrophysical Journal.


Read the Danish version of this article on

Translated by
Catherine Jex

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