A rendering of the silhouette of 55 Cancri e transiting its parent star, compared to the Earth and Jupiter transiting our sun. Image: Jason Rowe, NASA/Ames; Jaymie Matthews, UBC |
A
group of astronomers led by a Massachusetts Institute of Technology (MIT)
professor has spotted an exoplanetary eclipse of a star only 40 light years
away—right around the corner, astronomically speaking—revealing a “super-Earth.”
The
far-out planet, named 55 Cancri e,
is twice as big as Earth and nearly nine times more massive. It is most likely
composed of rocky material, similar to Earth, supplemented with light elements
such as water and hydrogen gas. Scientists estimate the planet’s surface is
much hotter than ours: close to 2,700 C.
Exoplanets
have captivated astronomers in recent years as interest in finding life on
other Earth-like planets has intensified.
But
Josh Winn, the Class of 1942 Career Development Assistant Professor of Physics
at MIT, says exobiologists should probably not flock to 55 Cancri e looking for signs of life: The temperatures are just
too high to sustain living organisms. But he suspects the exoplanet will
attract the telescopes of many astronomers, mainly for reasons of visibility: 55 Cancri e is relatively close to
Earth compared to other known exoplanets, and, as a result, the star around
which the planet orbits appears roughly 100 times brighter than any other star
with an eclipsing planet.
“Everything
we do in astronomy is starving for more light,” Winn says. “The more light a
star gives you, the more chances you have of learning something interesting …
and everyone’s been waiting for a system like this that you can study in great
detail.”
An 18-hour year
Winn and his colleagues collected starlight data continuously for two weeks
from Canada’s
Microvariability and Oscillations of Stars space telescope (MOST). They
directed the satellite scope toward 55
Cancri e based on a tip from doctoral student Rebekah Dawson of the Harvard-Smithsonian Center for Astrophysics. Last year, Dawson published a
mathematical analysis of existing data on 55 Cancri e, and found it took the planet 18 hours to orbit its
star.
Her
results suggested 55 Cancri e
was much closer to its star than previously thought, and Winn immediately saw
an opportunity to catch sight of an eclipse.
“If
[a planet] is just hugging the star, there’s a greater chance of an eclipse,
versus if the planet is really far out, in which case you have to be luckier to
see it right in front of the star,” he says.
An
eclipse has the potential to unlock many mysteries about an exoplanet. For
example, astronomers can identify a planet’s diameter, mass, composition, and
atmospheric conditions by measuring the differences in light as a planet passes
in front of, or “transits,” its star. However, only a handful of rocky
exoplanets have been known to transit, and every one of them eclipses a faint
star.
‘A firefly across a searchlight’
For two weeks, Winn and his colleagues tracked the brightness of 55 Cancri e’s super-bright star,
discovering tiny dips in the data that occurred every 18 hours, a finding that
confirmed Dawson’s original theory by suggesting the occurrence of an
exoplanetary eclipse.
The
results of the study have been accepted for publication in The Astrophysical
Journal Letters. Winn hopes the study will prompt astronomers to explore 55 Cancri e with their own tools and
telescopes.
Dawson’s findings prompted
another group at MIT to investigate the rocky exoplanet. Sara Seager, the Ellen
Swallow Richards Professor of Extrasolar Planets at MIT, and Brice-Olivier
Demory, a postdoc in the Department of Earth, Atmospheric, and Planetary
Sciences, detected a
transit of 55 Cancri e
using NASA’s Warm Spitzer, an infrared space telescope. From the spectral data
they collected, the group calculated the planet’s dimensions, confirming Winn’s
calculations.
Demory
and Seager plan to commandeer the telescope again next year to catch the
planet, this time behind its star. By measuring the difference between the
light given off from the planet in front of and behind its star, the group
could determine exactly how much light the planet itself gives off, which could
in turn give researchers clues about the planet’s atmospheric composition.
“It’s
still going to be hard to learn everything about this planet,” Winn says. “But
at least we have what might be the best system in the sky to study it.”
