Detection of super-Earth transit puts ground-based telescopes in the hunt

Artist's conception shows the Earth (left) compared to the super-Earth 55 Cancri e (right), whose transit across the front of its host star was detected from the ground (Image: NASA/JPL)

When you're hunting for exoplanets many light years away, the complications posed by the Earth's atmosphere can make the search incredibly difficult for ground-based telescopes. That's why space-based telescopes, such as Hubble, Spitzer and Kepler, are generally employed for the job. But now for the first time, astronomers have detected the transit of a super-Earth in front of a nearby Sun-like star, which could see ground-based telescopes more widely used in categorizing the growing number of exoplanets expected to be discovered in the next few years.

The feat was achieved using the 2.5-m (8.2-ft) Nordic Optical Telescope situated on the island of La Palma in the Canary Islands. Although it is only a moderate-sized facility by today's standards, the observatory is equipped with state-of-the-art instruments that allowed it to detect exoplanet 55 Cancri e as it crossed in front of its host star, 55 Cancri, which is located 40 light years from Earth and is just visible with the naked eye. Although the planet's transit had been observed before, these had all been achieved with space-borne telescopes.

The planet was initially identified in 2004 using the radial velocity method, whereby the movements of a star in response to the gravitational pull of a planet affect the star's normal color signature. The planet was later confirmed by transit observations captured by the MOST and Spitzer space telescopes. Now its transit has been observed from a ground-based telescope, which has to deal with the extra complication of looking through the Earth's turbulent atmosphere.

With the planet only blocking a small fraction of the starlight and dimming the star's brightness by 1/2,000th – or 0.05% – for nearly two hours, it is the shallowest transit of an exoplanet observed from the ground. It is also only the second super-Earth to be detected with a ground-based telescope, the first being GJ 1214b, which circles a red dwarf, in 2009. But 55 Cancri e is the first super-Earth circling a G-type main-sequence star like our Sun that has been detected using a ground-based telescope.

"Our observations show that we can detect the transits of small planets around Sun-like stars using ground-based telescopes," says Ernst de Mooij of Queen's University Belfast in the United Kingdom, lead author of the study. "This is especially important because upcoming space missions such as TESS and PLATO should find many small planets around bright stars and we will want to follow up the discoveries with ground-based instruments."

NASA's TESS (Transiting Exoplanet Survey Satellite) mission is scheduled to launch in 2017 with the primary goal of detecting small planets with bright host stars in the solar neighborhood to allow characterization of the planets and their atmospheres. PLATO (Planetary Transits and Oscillations of stars) is a space observatory originally proposed for the ESA's Cosmic Vision 2015-2025 Programme, and given the nod in February 2014. It will be tasked with characterizing rocky exoplanets of all sizes.

"We expect these surveys to find so many nearby, terrestrial worlds that space telescopes simply won't be able to follow up on all of them," says co-author Mercedes Lopez-Morales of the Harvard-Smithsonian Center for Astrophysics (CfA). "Future ground-based instrumentation will be key, and this study shows it can be done."

The team's paper will be published in The Astrophysical Journal Letters.

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