Data from the Spitzer Space Telescope reveals that 55 Cancri e is very dark, and that its sun-facing side is blistering hot. Image: NASA/JPL-Caltech |
Scientists
on a planetary-heat-seeking mission have detected the first infrared light from
a super-Earth—in this case, a planet some 40 light-years away. And according to
their calculations, 55 Cancri e, a planet just over twice the size of Earth, is
throwing off some serious heat.
At
a toasty 3,700 F, the planet is hot enough to liquefy steel. And there’s not
much relief from the scorching heat: Researchers at Massachusetts Institute of
Technology (MIT) and other institutions say the planet may lack reflective
surfaces such as ice caps, instead absorbing most of the heat from its parent
star—much as Earth’s dark oceans trap heat from the sun.
Since
the planet’s discovery in 2004, scientists have unearthed a number of its
properties; the new findings, published in Astrophysical Journal Letters,
expand the physical profile of 55 Cancri e. The planet orbits the star 55
Cancri, part of the crablike constellation Cancer, which is bright enough to be
seen with the naked eye.
Using
telescopes on the ground and in space, scientists examine light patterns from a
star to determine the traits of any planets around it. Periodic dips in
starlight indicate that a planet has transited, or passed in front of, its
star. From this data, scientists have now calculated 55 Cancri e’s radius
(twice that of Earth’s) and the duration of its orbit (18 hrs, versus our
leisurely 365 days).
While
the stellar brilliance enables researchers to detect changes in starlight, it’s
much trickier to detect light of any wavelength—visible or infrared—from the
planet itself.
“This
planet is so close to the star that it’s very strongly irradiated,” says
co-author Brice-Olivier Demory, a postdoctoral researcher in MIT’s Department
of Earth, Atmospheric and Planetary Sciences. “It’s like in the movie ‘Avatar,’
where Pandora orbits the gas giant Polyphemus. Seeing Polyphemus from Pandora
gives an idea of how big the star would look from 55 Cancri e.”
Demory
says isolating the heat of the planet from the massive heat emitted from its
star would be like detecting the heat of one candle among an array of 10,000.
Super-hot super-Earth
Undaunted by such a task, Demory worked with Sara Seager, the Class of 1941
Professor of Physics and Planetary Science at MIT, and researchers from the MIT
Kavli Institute for Astrophysics and Space Research, the University of
Maryland, Washington’s Carnegie Institution, and the University of Liege in
Belgium to detect the planet’s thermal emissions.
The
group obtained observations from NASA’s Spitzer Space Telescope, which monitors
infrared radiation emitted by objects in the solar system and beyond. Demory
and his colleagues trained the telescope on 55 Cancri, observing the star
during a six-hour window during which the tiny exoplanet passed behind it—a
phenomenon known as an occultation.
Demory
measured the starlight before and after the planet’s occultation, discovering a
minute dip when the star completely eclipsed the planet. To make sure the dip
wasn’t merely a fluke, the team obtained three more sets of data for the same
orbital window, and analyzed all four datasets together.
“When
you stack all the data together, you see a beautiful dimming light that clearly
shows the light from the planet that disappears,” says co-author Michael
Gillon, principal investigator of the Spitzer telescope program. “This is the
first time that we see light from a planet that is that small.”
From
the planet’s infrared light, the researchers precisely calculated its temperature—a
scorching 2,360 K, or 3,700 F. With such high temperatures, Demory posits that
the planet is likely rather dark, having no reflective surfaces such as ice
caps, and probably absorbs most of the heat given off by its star.
The
planet’s temperature may also give researchers a clue about its atmosphere. 55
Cancri e orbits its star much like the moon circles Earth, always presenting
the same face. Demory suspects that much of 55 Cancri e’s heat remains on the “day side” of the planet, and that it would be difficult to circulate such high
temperatures to the planet’s dark side: In other words, it’s unlikely that the
super-hot planet harbors any strong winds.
Phil
Armitage, an associate professor of astrophysics at the University of Colorado,
says it’s extremely difficult for any instrument—including the Spitzer
telescope—to make a direct detection of an exoplanet. He sees the group’s
detection as “a great example of really pushing an instrument to its limits.”
He
adds that the planet’s infrared light will help identify more characteristics
from this particular super-Earth.
“Super-Earths
are fascinating objects because they don’t have any analogs in the solar
system,” Armitage says. “We don’t have a clear idea how they formed or even
what they’re made of. It’s a mystery that requires data beyond the planet’s
mass and radius to resolve.”
Going
forward, Demory hopes to obtain more data to map the planet’s infrared light as
it completes an orbit around its star. The results could illuminate the
different phases of the planet as it circles the star, similar to the waxing
and waning of Earth’s moon.
Seager
says that in addition to fleshing out the physical profile of 55 Cancri e, the
group’s techniques may be adopted to characterize other exoplanets in the
universe—even, perhaps, those as small as Earth.
“We’re
pushing toward smaller and smaller planets with techniques that are already
established,” Seager says. “Once you discover one, you want to find more. And
there are a lot of exoplanets out there.”
The
research is based on observations made with the Spitzer Space Telescope, which
is operated by the Jet Propulsion Laboratory (JPL) and the California Institute
of Technology under a contract with NASA. Support for this work was provided by
NASA through an award issued by JPL/Caltech.
