article image

Exoplanet with weird orbit found by citizen scientists

Elizabeth Howell, News Writer
Nov 6, 2014, 18:04 UTC

Sen—Whizzing 2,300 light-years from Earth is a planet that will not stick to a consistent orbit around its star. While this made it extremely difficult to find, astronomers said, they were able to track it down with the help of a volunteer force of amateur scientists.

The planet is called PH3c and is about four times the mass of Earth, which places it in a category of planets often called "super-Earths" because they are fairly close to our own. The atmosphere of PH3c is very different from our own, however, being mostly made up of hydrogen and helium. This means its composition more resembles a gas or ice planet such as Jupiter or Neptune.

But it was PH3c's orbit that caught astronomers' attention. One common method of seeking exoplanets is by examining the light coming from stars and seeing if they dim at regular times. A small dimming could indicate that a planet is passing across the face of the star.


An artist's conception of exoplanet PH1, found with assistance of volunteers from Planet Hunters in 2012. Image credit: Haven Giguere/Yale

This pattern is possible to pick up using a computer, but a more irregular orbit is difficult. PH3c's orbit showed extreme pertubation due to the gravitational effects of other planets in its system, which were nudging it constantly.

"On Earth, these effects are very small, only on the scale of one second or so," stated Joseph Schmitt, a Yale University graduate student who led the research. "PH3c’s orbital period changed by 10.5 hours in just 10 orbits."

While hard to spot by computer, the orbital changes came to light after being scrutinized by volunteers who work with the Planet Hunters project. The Zooniverse-run web portal, which is co-ordinated with Yale and the University of Oxford, invites ordinary people to probe data from the Kepler space telescope to assist professional astronomers in their work.


The Planet Hunters program uses data from the Kepler space telescope, a NASA observatory that has observed thousands of exoplanets since 2009. Credit: NASA/Ames/JPL-Caltech

Once the middle planet was suspected, astronomers also obtained data on two other planets in the system. Outer planet PH3d is a bit bigger and more massive than Saturn, while inner planet PH3b may be rocky, like Earth.

"Finding the middle planet was key to confirming the others and allowing us to find their masses," Schmitt said. "The outer planet’s orbital period also changes slightly, by about 10 minutes. You need to see both planets’ changing orbital periods in order to find out the masses of the planets. One planet doesn’t give enough information."

Planet Hunters has helped find about 60 planet candidates since 2010 and is now engaged in a search to see if certain types of planets form around certain types of stars.