Astronomers at Queen's University, Belfast measure Earth-sized planet outside our solar system
Kepler-78b is just one million miles from its Sun-like star and its rocks are molten lava
Astronomers at Queen’s University Belfast have, for the first time, weighed and measured an Earth-sized planet outside our own solar system.
Details of the breakthrough are being published in Nature, one of the world’s most prestigious science journals.
The planet, known as Kepler-78b, is the smallest exoplanet (planet outside our solar system) for which an accurate radius and mass is known, and its existence has baffled scientists.
It is just 20 per cent bigger than Earth and, like Earth, has an iron core and a rocky interior.
But that’s where the similarities with our planet end.
Kepler-78b is just one million miles from its Sun-like star (Kepler 78) – which is extremely close in astronomy terms.
This means it endures intense heat, its rocks are molten lava, and it takes just eight-and-a-half hours to orbit its star, compared to Earth’s 365-day orbit.
Dr Chris Watson from Queen’s Astrophysics Research Centre says its existence and extremely tight orbit has baffled astronomers.
He said: “Kepler-78b is a scorching lava world that, put simply, shouldn’t exist. Its close proximity to its star, and how it got there, is still a mystery.
"What we do know is that it won’t exist forever. Gravitational tides will slowly disrupt Kepler-78b, drawing it closer to its star and eventually ripping it apart.”
The planet was first identified by NASA's Kepler space mission, which has monitored 150,000 stars for brightness variations caused by planets crossing the face of their parent star.
It is this variation in brightness that allowed astronomers to calculate the radius of Kepler-78b. Determining its mass and composition was much more difficult – a process in which Queen’s University's Dr Watson played a key role.
“If an alien civilisation were looking at our solar-system and were to catch Jupiter transiting our Sun they would see a one per cent dip in light," he said.
"When the Kepler spacecraft looked at the Kepler-78 star, it saw something much smaller - a tiny 0.02 per cent dip in starlight caused by a planet just 20 per cent bigger than our Earth.
"The problem is, transits only give the radius of a planet, but in order to confirm what it is made of, we need to weigh it - a much more complex task. In order to do this, a team of astronomers from the UK, Europe and the USA measured the 'wobble' of the star as it is forced to orbit its common centre of gravity with the planet.
"Using the HARPS-N (High Accuracy Radial Velocity Planet Searcher for the Northern hemisphere) instrument, based on the huge Telescopio Nazionale Galileo telescope on La Palma in the Canary Islands, we found that the orbiting Kepler-78b caused its parent star to move at a speed comparable to walking pace.
"From this we calculated the planet’s mass. This, coupled with its radius, allowed us to determine that Kepler-78b is composed of rock and iron – much like our Earth. But unlike Earth, the planet is much too hot to support life.”
“This discovery is a fine example of the great diversity of the exoplanets that we are uncovering, thanks to the tremendous progress in astronomy technology and techniques.
"Nature seems to like conjuring up planets in the least expected places. Just five years ago this work would have been impossible. As we probe deeper and deeper we are finding that science fact is stranger than science fiction. Kepler-78b certainly fits that bill.”
Dr Watson was one of three UK astronomers who worked on the HARPS-N project along with scientists from the University of Geneva, St Andrew’s University and the Harvard-Smithsonian Center for Astrophysics.
Queen's University's work on the project was funded by a science exploitation grant from the Science and Technology Facilities Council (STFC). Details of the discovery will be published in Nature alongside research from a separate group of US-based scientists, which confirms the findings of Dr Watson’s team.