A newly-discovered, Earth-like planet has left astronomers mystified because they don’t understand how it can possibly exist. The planet, labelled Kepler-78b, is rocky like our own world, is of similar size and orbits a star that is much like the Sun. But it is far too hot for life. 

An artist’s impression of planet Kepler-78b close to its home star. Credit: TNG/Avet Harutyunyan

That is because Kepler-78b zips round its own sun so closely, at a distance of less than 1.6 million km (one million miles), that the surface rocks will be made of molten lava. Experts say it is doomed to be destroyed.

The mystery is that they cannot explain how it got to be so close to the star which must have been bigger in its early days. It cannot have formed in its present orbit, where a “year” lasts just eight and a half hours, as it would have been inside the star.

However, if the planet formed further away and migrated closer in, they would expect it to have been swallowed up by the star already. The astronomers do know, however, that the planet is living on borrowed time and will be ripped apart by the star’s gravity relatively soon on an astronomical timescale.

It was first detected 400 light-years away in the constellation of Cygnus by NASA’s now-defunct Kepler space telescope. Using powerful telescopes in the Canary Islands and Hawaii, European and American teams of astronomers discovered that Kepler-78b is only 20 per cent larger than the Earth with a diameter of 14,800 km (9,200 miles) and weighs nearly twice as much. This suggests that it has an Earth-like composition of iron and rock.

Italy’s Telescopio Nazionale Galileo which made the observations from La Palma. Credit: TNG/Avet Harutyunyan

“This planet is a complete mystery,” says astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics (CfA). “We don’t know how it formed or how it got to where it is today. What we do know is that it’s not going to last forever.”

Colleague Dimitar Sasselov said: “Kepler-78b is going to end up in the star very soon, astronomically speaking. It couldn’t have formed in place because you can’t form a planet inside a star. It couldn’t have formed further out and migrated inward, because it would have migrated all the way into the star. This planet is an enigma.”

Dr Ken Rice, of the University of Edinburgh said: “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.”

Two teams studied Kepler-78b using a new, high-precision spectrograph known as HARPS-North, on Italy’s Telescopio Nazionale Galileo at the Roque de los Muchachos Observatory on La Palma, and a similar instrument called the HIRES spectrograph at the Keck Observatory, Hawaii. They were pleased to find their results agreed with each other.

Dr Chris Watson, of Queen’s University Belfast said: “This result showcases the tremendous progress in this field, both in terms of advancing technology and developing innovative techniques. Just 5 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.”

Their results are published this week in the journal Nature.