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Astronomers confirm discovery of unseen exoplanet

Charles Black, Founder and CEO of Sen
Dec 24, 2013, 8:00 UTC

Sen—A team of European astronomers have confirmed the presence of an unseen but predicted exoplanet in a distant solar system designated Kepler-88.

The newly confirmed planet, Kepler-88c, had not been seen crossing in front of its parent star but its existence had been predicted because of the gravitational perturbation it caused on Kepler-88b, a planet which the Kepler space telescope had previously observed transiting the star.

The team used the SOPHIE (Spectrograph for Observation of Phenomena of Stellar Interiors and Exoplanets) spectrograph at the Observatoire de Haute-Provence to confirm the presence of Kepler-88c. 

Over 3,500 planet candidates emerged from Kepler's constant gaze at over 150,000 stars, looking for dips in light that could indicate the presence of a transiting planet. So far nearly 200 of the planet candidates have been confirmed as planets after further investigation. However, a planet may be orbiting one of the stars in Kepler's field of view but have an orbital plane that is misaligned with the line of sight from Earth, meaning it could not be seen transiting from Kepler's viewpoint. Kepler-88c is one such example, requiring alternative detection techniques to discover its existence.

In analysing Kepler's data therefore astronomers may find solar systems with multiple planets orbiting the same star even though only one of them can be seen transiting.

A key sign that a star hosts more than one planet is where the predicted orbital period of a transiting planet is perturbed. This is because planets that share the same host star gravitationally interact with each other, causing perturbations in the predicted transit times of the planets, known as transit timing variations (TTV).

Transit timing variations can be used as a diagnostic of gravitational interactions between planets in a multi-planet system.

Predicting the existence of a planet from the orbits of those around was in fact used in our own Solar System. Neptune was discovered in 1846 after mathematical calculations demonstrated that anomalies in Uranus' orbit must be due to an unseen planet.

The TTV technique is sensitive enough to detect planets with the mass of Earth, and can therefore be used to discover the existence of non-transiting planets that affect the orbital motion of transiting planets.

In the case of Kepler-88, research led by David Nesvorny of the Southwest Research Institute predicted that Kepler-88 had two planets with the orbtial period of the outer planet being two times longer than the transiting inner planet. In our Solar System a similar configuration exists with Earth and Mars, with Mars taking almost two Earth years to orbit the Sun. 

The new research was able to independently confirm the existence of Kepler-88c using the SOPHIE spectrograph. The researchers were able to measure the mass of the planet which agreed perfectly with the mass predicted by analysing TTV. 

"This is the first time that the mass of an unseen exoplanet inferred based on transit timing variation is independently confirmed by another technique," explained Susana Barros, a researcher at the Laboratoire d'Astrophysique de Marseille and lead author of the paper. 

Independently confirming the mass predicted by TTV supports the validity of TTV as a technique for detecting unseen planets. 

Professor Magali Deleuil, leader of exoplanet research at the Laboratoire d'Astrophysique de Marseille said: "This independent confirmation is a very important contribution to the statistical analyzes of the Kepler multiple planet systems. It helps to better understand the dynamical interactions and the formation of planetary systems. This also permits to anticipate the future exploration of exoplanetary systems from space as would do the PLATO mission."

Kepler was launched in 2009 to look for planets around other stars. It did this by monitoring more than 150,000 stars to search for dips in light that could indicate the presence of a transiting planet. The telescope's extended mission, which had been set to last until 2016, was stopped earlier this year after a second reaction wheel failed. Kepler needed three of its four reaction wheels to work to ensure it maintained a constant field of view. However, Kepler collected so much data during its operational years that scientists are likely to continue to make discoveries as they analyse the backlog of data.

Although not currently gathering data, a repurposed Kepler mission, dubbed K2, to revive the telescope has been proposed.

The research paper is published in Astronomy & Astrophysics.