Astronomers have measured the passing of a super-Earth in front of a
bright, nearby Sun-like star using a ground-based telescope for the
first time. The transit of the exoplanet 55 Cancri e is the shallowest
detected from the ground yet. Since detecting a transit is the first
step in analyzing a planet's atmosphere, this success bodes well for
characterizing the many small planets that upcoming space missions are
expected to discover in the next few years.
The international research team used the 2.5-meter Nordic Optical
Telescope on the island of La Palma, Spain, a moderate-sized facility by
today's standards but equipped with state-of-the-art instruments, to
make the detection. Previous observations of this planet transit had to
rely on space-borne telescopes.
The host star, 55 Cancri, is located just 40 light-years away from us
and is visible to the naked eye. During its transit, the planet crosses
55 Cancri and blocks a tiny fraction of the starlight, dimming the star
by 1/2000th (or 0.05%) for almost two hours. This shows that the planet
is about twice the size of Earth, or 16,000 miles in diameter.
"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.
He continues, "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."
TESS is a NASA mission scheduled for launch in 2017, while PLATO is
to be launched in 2024 by the European Space Agency; both will search
for transiting terrestrial planets around nearby bright stars.
"With this result we are also closing in on the detection of the
atmospheres of small planets with ground-based telescopes," says
co-author Mercedes Lopez-Morales of the Harvard-Smithsonian Center for
Astrophysics (CfA). "We are slowly paving the way toward the detection
of bio-signatures in Earth-like planets around nearby stars."
"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," says study co-author Ray Jayawardhana
of York Univerity in Canada. "Remote sensing across tens of light-years
isn't easy, but it can be done with the right technique and a bit of
ingenuity."
The planet 55 Cancri e is about twice as big and eight times as
massive as Earth. With a period of 18 hours, it is the innermost of five
planets in the system. Because of its proximity to the host star, the
planet's dayside temperature reaches over 3100° Fahrenheit (1700°
Celsius), hot enough to melt metal, with conditions far from hospitable
to life. Initially identified a decade ago through radial velocity
measurements, it was later confirmed through transit observations with
the MOST and Spitzer space telescopes.
Until now, the transits of only one other super-Earth, GJ 1214b
circling a red dwarf, had been observed with ground-based telescopes.
The Earth's roiling air makes such observations extremely difficult. But
the team's success with 55 Cancri e raises the prospects of
characterizing dozens of super-Earths likely to be revealed by upcoming
surveys.
"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.
Future ground-based instrumentation will be key, and this study shows it
can be done," adds Lopez-Morales.
The research team also includes Raine Karjalainen and Marie Hrudkova
of the Isaac Newton Group of Telescopes. Their findings appear in a
paper to be published in The Astrophysical Journal Letters.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.
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