Hot, metallic Earth-sized planet discovered

Scientists have discovered a hot, metallic, Earth-sized planet orbiting a dwarf star located 260 million light-years away. Named K2-229b, the planet is almost 20 per cent larger than Earth but has a mass which is over two-and-a-half times greater. It reaches a day side temperature of over 2000 degrees Celsius. It is located very close to its host star (0.012 AU, around a hundredth of the distance between the Earth and the Sun), which itself is a medium-sized active K dwarf in the Virgo Constellation. K2-229b orbits this star every fourteen hours.

Using the K2 telescope, researchers from Aix-Marseille Universite in France and University of Warwick in the UK employed the Doppler spectroscopy technique - also known as the 'wobble method' - to discover and characterise this faraway planet. The astronomers knew the planet was there due to dips in the light from its host star as it orbited, periodically blocking starlight.

They then calculated the size, position and mass of K2-229b by measuring the radial velocity of the star, and finding out how much the starlight 'wobbles' during orbit, due to the gravitational tug from the planet, which changes depending on the planet's size. "Mercury stands out from the other Solar System terrestrial planets, showing a very high fraction of iron and implying it formed in a different way," said David Armstrong from the University of Warwick. "We were surprised to see an exoplanet with the same high density, showing that Mercury-like planets are perhaps not as rare as we thought," Armstrong said.

"Interestingly K2-229b is also the innermost planet in a system of at least three planets, though all three orbit much closer to their star than Mercury," he said. The dense, metallic nature of K2-229b has numerous potential origins, and one hypothesis is that its atmosphere might have been eroded by intense stellar wind and flares, as the planet is so close to its star.

Another possibility is that K2-229b was formed after a huge impact between two giant astronomical bodies in space billions of years ago - much like the theory that the Moon was formed after Earth collided with a body the size of Mars. Discovering details about far-flung planets across the universe gives us more clues as to how planets in our own solar system formed, researchers said.

As K2-229b is similar to Mercury, knowing more about the former can potentially reveal more about the latter, they said.