Kepler-78b circles its star every eight and a half hours. (David A. Aguilar/Harvard-Smithsonian Center for Astrophysics)

Astronomers have found the first roughly Earth-sized planet outside our solar system that seems to be made of the same stuff as Earth: rock and iron.

While it's been speculated that other small planets found circling other stars are rocky, Kepler-78b is the first one confirmed to have a density almost identical to that of Earth, suggesting a similar composition, reported two studies published this week in the journal Nature. One was led by Andrew Howard at the University of Hawaii and the other by Francesco Pepe at the University of Geneva.

Kepler-78b is about double Earth`s mass and located about 400 light years from Earth in the constellation Cygnus. It was discovered earlier this year using NASA’s planet-hunting Kepler spacecraft, launched in 2009. The spacecraft has since gone out of service after its positioning system broke.

While it has some key similarities to Earth, one thing makes the two planets starkly different from one another — the distance between Kepler 78b and its star is just a hundredth of the distance between the Earth and the sun.

Hotter than 2,000 C

Because of that, Howard`s team estimates the temperature on the day side of the planet, facing the star, is between 2,000 and 2,800 C, making it uninhabitable.

“Any gaseous atmosphere around Kepler-78b would probably have been lost long ago to photoevaporation by the intense starlight,” said the paper co-authored by Howard, assistant astronomer from the University of Hawaii at Manoa’s Institute for Astronomy.

The planet’s orbit, which takes it around its star just once every 8½ hours, has astronomers scratching their heads about how it ended up so close to its star, which used to be much bigger and would once have engulfed the orbit of Kepler-78b.

“It [Kepler 78-b] couldn't have formed in place because you can't form a planet inside a star,” said Dimitar Sasselov, an astronomer at the Harvard-Smithsonian Center for Astrophysics who co-authored Pepe`s paper.


Measuring mass is tricky

While scientists have already confirmed the discovery of more than 150 planets by Kepler, including many that are roughly Earth-sized, it has been more difficult to confirm that some of them are rocky like Earth. Determining a planet`s composition requires astronomer`s to calculate its density, which requires them to know its mass and radius.

Measuring a planet`s mass is not easy. But the two research teams did it using a technique called ultra-high-precision Doppler spectroscopy.

Using an instrument on the 10-metre Keck telescope on Mauna Kea in Hawaii, Howard`s team observed subtle changes in the colour of starlight caused by the wobbling of the star. The wobbling is caused by the orbiting planet`s gravity. The data was used estimate the planet`s mass at 1.7 times that of Earth and its density as 5.3 to 5.6 grams per cubic centimetre.

University of Geneva astronomer Pepe and his colleagues did a similar calculation by using data from an instrument on the European Southern Observatory`s 3.6-metre telescope in La Silla, Chile, and came up with similar numbers — they estimated Kepler-78b has 1.9 times the mass of Earth and a density of 5.6 grams per cubic centimetres.

The Earth`s density is 5.5 grams per cubic centimetre, and a similar density implies a similar planetary composition.