Sen—Astronomers have made the first ever observation from a ground-based telescope of a super-Earth passing in front of a star similar to the Sun.

They used a moderate-sized instrument at the international observatory on La Palma, in the Canary Islands, to measure the tiny fade in light caused by the transit.

The exoplanet, known as 55 Cancri e, was already known to exist. It orbits a star dimly visible to the naked eye called 55 Cancri, and was discovered from Texas in 2004, by detecting its effect on the star.

But its nature was not properly understood until a few years later after it was studied by the MOST Space Telescope (it stands for Microvariability and Oscillation of Stars) which was designed and built in Canada, and NASA’s Spitzer space telescope.

They showed it to be much smaller than the many gas giants known to orbit other stars. Its mass is about eight times that of our own planet and its diameter is twice the Earth’s, making it the first so-called super-Earth to be discovered.

While transits of giant “hot Jupiters” have been monitored from the ground in the past, 55 Cancri e is very much smaller. Only one other super-Earth has been seen in transit from the surface of our planet, and that was orbiting a smaller and fainter red dwarf star.

The fade of 55 Cancri e, which is the shallowest seen with a ground-based observatory, was recorded by a team using the 2.5-meter Nordic Optical Telescope on La Palma. It watched the starlight dim by just 1/2000th, or 0.05 per cent, over two hours.

The planet 55 Cancri e is the innermost of five planets known to orbit the star, zipping around it in a year that is only 18 hours long. The planetary system is right on our cosmic doorstep, lying just 40 light-years away.

The discovery team say their success is important because it suggests astronomers will be able to characterise the many smaller planets that are expected to be found around nearby stars by future space missions.

Dr Ernst de Mooij, of Queen’s University Belfast, UK, is lead author of a study by the team that will appear in the Astrophysical Journal Letters. He said in a statement: “Our observations show that we can detect the transits of small planets around Sun-like stars using ground-based telescopes.

“This is especially important because upcoming space missions such as TESS and PLATO should find many small planets around bright stars.”

TESS is a NASA mission due for launch in 2017, while PLATO is scheduled by the European Space Agency to fly in 2024. Both will search for terrestrial planets transiting nearby bright stars.

The team that meaured 55 Cancri e’s transit now plans to look for signs of water vapour in the planet’s atmosphere.

Co-author Professor Ray Jayawardhana, of Canada’s York University, said: “It’s remarkable what we can do by pushing the limits of existing telescopes and instruments, despite the complications posed by the Earth’s own turbulent atmosphere.

“Observations like these are paving the way as we strive towards searching for signs of life on alien planets from afar. Remote sensing across tens of light-years isn’t easy, but it can be done with the right technique and a bit of ingenuity.”

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