Exoplanet’s tight star orbit baffles scientists

A newly discovered exoplanet, Kepler-78b with Earth-like density, baffles scientists with its star orbit, which is one of the tightest known orbits.

According to astronomers, including scientists from the University of Warwick, Kepler-78b is a planet that ‘should not exist’ as it orbits its star every eight and a half hours at a distance of less than one million miles.

According to current theories of planet formation, it could not have moved there as it would have been pulled into the star by gravitational forces, nor could it have formed so close to its star.

Kepler-78b, with an Earth-like density, mass and size, orbits a Sun-like star called Kepler 78 and is currently the smallest exoplanet for which both the mass and radius are known accurately.

Hence, the close proximity to its star poses a challenge to theorists.

The star, Kepler 78, was once larger than it is now. Theoretically, the current orbit of Kepler-78b would have previously been inside the star.

However, a planet cannot form inside a star and it could not have been formed further away and moved to where it is today as it would have been pulled into the star, given its relatively small size and low density.

Professor Don Pollacco of the University of Warwick said: “The detection of Kepler-78b is a tremendous achievement for the HARPS-N spectrograph.

“Not only is it one of the few confirmed planets with an Earth-like radius and density, but the accuracy of the measurements are close to the best ever achieved from the ground.”

The HARPS-North (High Accuracy Radial velocity Planet Searcher for the Northern hemisphere) spectrograph is a high-precision radial-velocity instrument.

Scientists regard it a miracle that Kepler-78b still exists today. Theorists accordingly predict that the planet will disappear within three billion years as gravitational forces draw Kepler-78b into its star.

Dr Chris Watson of Queen’s University Belfast commented: “This result showcases the tremendous progress… in terms of advancing technology and developing innovative techniques.

“Just five years ago this work would have been impossible, and as we probe deeper and deeper what we are finding is that science fact is weirder than science fiction – the planet Kepler-78b certainly fits this bill.”

Kepler-78b is also the first known Earth-sized exoplanet with an Earth-like density.

It is about 20 percent larger than the Earth with a diameter of 9,200 miles, and weighs almost twice as much.

It has a density similar to Earth’s, which suggests an Earth-like composition of iron and rock.

Kepler-78b does, however, differ from Earth in its short orbital period and its high temperature.

Dr Ken Rice of the University of Edinburgh added: “Although this planet is clearly too hot to support life, it is still very exciting to now be discovering planets that are not only similar in mass to the Earth, but also similar in composition.”