kepler 78b

Earth-sized ‘lava world’ discovered

A doomed “lava world” with a similar mass and density to that of Earth has been discovered orbiting a star 400 light-years away.

Observations suggest the planet, named Kepler 78b, is composed mostly of rock and iron, much like our own planet.

But its extremely close proximity to its host star - a hundredth of the distance between the Earth and the Sun - remains something of a puzzle.

Discovery of Earth-like planet Kepler-78b raises hopes of more

Kepler-78b is 400 light years away, and has a rocky interior and an iron core – although its surface is 2,000C hotter

Scientists have found the most similar planet to Earth so far discovered, which is only a little bigger and seems to be made of very similar materials.

Describing the results in Nature, two independent teams of scientists found the planet has a diameter of 9,200 miles (around 1.2 times that of Earth), and contains a rocky interior and an iron core. That’s where similarities to our home planet end, as Kepler-78b zips around its star every eight and a half hours at a distance of 1m miles, and the surface temperature on this planet is probably at least 2,000C higher than the hottest day on Earth.

The scientists used the transit method to determine the orbit and size of the planet Kepler-78b – where they watched the light given off by the parent star, Kepler-78, and noted how often and how much the light dims every time the planet passed in front of it. Calculating the planet’s mass involved watching the motion of the star itself and watching how theits tiny gravitational pull of the planet made the star wobble.

The total mass of Kepler-78b, which is 400 light years from Earth, was calculated as around 1.7 times that of Earth and its density is around 5.3g per cubic centimetre (slightly lower than Earth’s 5.5g per cubic centimetre).

“This is a very important result, and goes to the question of how common planets like our own are in the galaxy,” said Bill Chaplin, an astrophysicist at the University of Birmingham. “Getting a good estimate of the size of the planet and a good estimate of the mass – is still very rare.”

Subhanjoy Mohanty, an astrophysicist at Imperial College London, said the discovery was an important step towards finding true analogues of Earth, since the few previously known planets with similar compositions were significantly larger.

Recent findings have shown that planets such as Kepler-78b seemed to be very common, he said. “The majority of sun-like stars appear to have super-Earths (planets with a few times the mass and size of the Earth), often multiple ones, in very small orbits (orbital periods less than 100 days, compared with one year for the Earth). This is very interesting, because our solar system does not have such an architecture: only one planet, Mercury, orbits in such a short period, and it is much smaller than the Earth.

"In other words, the current statistics, with which the new discovery is consistent, suggest that our solar system may be anomalous, perhaps in the initial conditions in the disc of gas and dust around the young sun out of which our planetary system formed. So observations like the one presented are very important for understanding both what appears to be a standard planet formation pathway, as well as investigating how conditions in our own system may have differed.”

Kepler-78b does not have much of a future, as according to the astronomers who found it, its sun will continue to drag it closer and eventually, in about 3bn years, the planet will be ripped apart and disappear.

Source: The Guardian