Astronomers using the Kepler Space Telescope have observed the gravitational tugging of one previously hidden planet on another known planet orbiting the sun-like star KOI-872. This technique led to Neptune’s discovery in the year 1846, via its gravitational influence on Uranus. This is the first time the technique has been successfully used to identify a planet outside our solar system. The paper describing these results was published in the journal Science on May 10, 2012.
KOI-872 hosts a known planet, KOI-872b, that is 80% the size of our solar system’s largest planet, Jupiter. Unlike Jupiter, which takes 12 earthly years to orbit our sun, KOI-872b orbits its sun in only 34 days. Dr. David Nesvorny of the Southwest Research Institute (SWRI) in Boulder, Colorado and colleagues discovered the new planet after noticing that KOI-872b repeatedly sped up and slowed down in its orbit. Perturbations like these can only happen if another planet is orbiting close enough to exert some gravitational influence. By carefully measuring the magnitude of the tugs on KOI-872b, the researchers deduced that a second planet with 30% more mass than Saturn could explain the observations. The previously hidden planet, KOI-872c, orbits its sun every 57 days.
The star is located in the direction of the northern constellation Cygnus the Swan, which doesn’t rise in May until a little before midnight.
While investigating the source of the perturbations on KOI-872b, the team also uncovered evidence for a third planet, provisionally named KOI-872.03, nearly twice the size of Earth and orbiting every 6.8 days. It is unlikely that any of these planets are suitable for life. Given their mass and size, KOI-872b and KOI-872c are probably gas giants like Jupiter and Saturn and therefore lack solid surfaces. KOI-872.03, while most likely a rocky world like our own, is much too close to its sun to be habitable. Sitting just 3 million kilometers from its parent star (in contrast to about 150 million kilometers for Earth’s distance from the sun), the scorched surface of KOI-872.03 sits at a not-so-balmy 1200 degrees Celsius – hot enough to melt gold.
The planets were found using data from the Kepler Space Telescope. Kepler, launched in early 2009, is monitoring 145,000 stars in Cygnus for evidence of extrasolar planets. Kepler does this by looking for transits: periodic dips in starlight caused by a planet passing in front of the star. By carefully timing the frequency of the dips and measuring how much starlight is blocked, astronomers can calculate the orbital period and size of the occulting planet. Researchers were led to KOI-872c when they noticed that the dips in starlight caused by KOI-872b sometimes came up to two hours late or two hours early.
On June 5, you will be able to watch a much closer transit when Venus marches across the face of the sun!
So astronomers have found a hidden world via its gravitationally tugging on a neighboring planet. The last time this technique was used to discover a planet was in 1846, when Urbain le Verrier observed deviations in the orbit of Uranus which hinted at the existence of an eighth planet in the outer solar system. Using le Verrier’s calculations, Johanne Galle at the Berlin Observatory found Neptune – just one degree from where le Verrier predicted it would be. With the Kepler telescope, astronomers can now use the same trick to find new planets orbiting other stars in our galaxy!
Bottom line: Dr. David Nesvorny of the Southwest Research Institute (SWRI) in Boulder, Colorado and colleagues have discovered a new planet, KOI-872c, by observing its gravitationally influence on a neighboring world. This is the first time this technique has been used on planets in another solar system and adds yet another proven method to the ways astronomers discover and characterize extrasolar planets. These astronomers published this result in the journal Science on May 10, 2012.
Chris Crockett got his Ph.D. in astronomy from UCLA in 2011 and worked at Lowell Observatory and the U.S. Naval Observatory. He then realized he enjoyed talking about astronomy a lot more than actually doing it. After being awarded a Mass Media Fellowship in 2013 by the American Association for the Advancement of Science, he spent a summer writing for Scientific American, then went on to become the staff astronomy writer at Science News from 2014 - 2017. These days, he freelances, focusing on stories about astronomy, planetary science, and physics. His work has appeared in Science News, Scientific American, Smithsonian Magazine, Knowable, Sky & Telescope, and the American Physical Society's online magazine Physics.