Discovery expands search for Earth-like planets: Newly spotted frozen world
orbits in a binary star system
Date: July 3, 2014
Source: Ohio State University
Summary: A newly discovered planet is expanding astronomers’ notions of where
Earth-like -- and even potentially habitable -- planets can form, and how to
find them. At twice the mass of Earth, the planet orbits one of the stars in the
binary system at almost exactly the same distance from which Earth orbits the
sun. However, because the planet's host star is much dimmer than the sun, the
planet is much colder than Earth -- a little colder, in fact, than Jupiter's icy
This artist's rendering shows a newly discovered planet (far right) orbiting
one star (right) of a binary star system. The discovery, made by a
collaboration of international research teams and led by researchers at The
Ohio State University, expands astronomers' notions of where to look for
planets in our galaxy.
Credit: Image by Cheongho Han, Chungbuk National University, Republic of
A newly discovered planet in a binary star system located 3,000 light-years from
Earth is expanding astronomers' notions of where Earth-like -- and even
potentially habitable -- planets can form, and how to find them.
At twice the mass of Earth, the planet orbits one of the stars in the binary
system at almost exactly the same distance from which Earth orbits the sun.
However, because the planet's host star is much dimmer than the sun, the planet
is much colder than Earth -- a little colder, in fact, than Jupiter's icy moon
Four international research teams, led by professor Andrew Gould of The Ohio
State University, published their discovery in the July 4 issue of the journal
The study provides the first evidence that terrestrial planets can form in
orbits similar to Earth's, even in a binary star system where the stars are not
very far apart. Although this planet itself is too cold to be habitable, the
same planet orbiting a sun-like star in such a binary system would be in the
so-called "habitable zone" -- the region where conditions might be right for
"This greatly expands the potential locations to discover habitable planets in
the future," said Scott Gaudi, professor of astronomy at Ohio State. "Half the
stars in the galaxy are in binary systems. We had no idea if Earth-like planets
in Earth-like orbits could even form in these systems. "
Very rarely, the gravity of a star focuses the light from a more distant star
and magnifies it like a lens. Even more rarely, the signature of a planet
appears within that magnified light signal. The technique astronomers use to
find such planets is called gravitational microlensing, and computer modeling of
these events is complicated enough when only one star and its planet are acting
as the lens, much less two stars.
Searching for planets within binary systems is tricky for most techniques,
because the light from the second star complicates the interpretation of the
data. "But in gravitational microlensing," Gould explained, "we don't even look
at the light from the star-planet system. We just observe how its gravity
affects light from a more distant, unrelated, star. This gives us a new tool to
search for planets in binary star systems."
When the astronomers succeeded in detecting this new planet, they were able to
document that it produced two separate signatures -- the primary one, which they
typically use to detect planets, and a secondary one that had previously been
only hypothesized to exist.
The first was a brief dimming of light, as the planet's gravity disrupted one of
the magnified images of the source star. But the second effect was an overall
distortion of the light signal.
"Even if we hadn't seen the initial signature of the planet, we could still have
detected it from the distortion alone," Gould said, pointing to a graph of the
light signal. "The effect is not obvious. You can't see it by eye, but the
signal is unmistakable in the computer modeling."
Gaudi explained the implications.
"Now we know that with gravitational microlensing, it's actually possible to
infer the existence of a planet -- and to know its mass, and its distance from a
star -- without directly detecting the dimming due to the planet," he said. "We
thought we could do that in principle, but now that we have empirical evidence,
we can use this method to find planets in the future."
The nature of these distortions is still somewhat of a mystery, he admitted.
"We don't have an intuitive understanding of why it works. We have some idea,
but at this point, I think it would be fair to say that it's at the frontier of
our theoretical work."
The planet, called OGLE-2013-BLG-0341LBb, first appeared as a "dip" in the line
tracing the brightness data taken by the OGLE (Optical Gravitational Lensing
Experiment) telescope on April 11, 2013. The planet briefly disrupted one of the
images formed by the star it orbits as the system crossed in front of a much
more distant star 20,000 light-years away in the constellation Sagittarius.
"Before the dip, this was just another microlensing event," Gould said. It was
one of approximately 2,000 discovered every year by OGLE, with its new
large-format camera that monitors 100 million stars many times per night
searching for such events.
"It's really the new OGLE-IV survey that made this discovery possible," he
added. "They got a half dozen measurements of that dip and really nailed it."
From the form of the dip, whose "wings" were traced out in MOA (Microlensing
Observations in Astrophysics) data, they could see that the source was headed
directly toward the central star.
Then, for two weeks, astronomers watched the magnified light continue to
brighten from telescopes in Chile, New Zealand, Israel and Australia. The teams
included OGLE, MOA, MicroFUN (the Microlensing Follow Up Network), and the Wise
Even then, they still didn't know that the planet's host star had another
companion -- a second star locked into orbit with it. But because they were
already paying close attention to the signal, the astronomers noticed when the
binary companion unexpectedly caused a huge eruption of light called a caustic
By the time they realized that the lens was not one star, but two, they had
captured a considerable amount of data -- and made a surprising discovery: the
Weeks after all signs of the planet had faded, the light from the binary-lens
caustic crossing became distorted, as if there were a kind of echo of the
original planet signal.
Intensive computer analysis by professor Cheongho Han at Chungbuk National
University in Korea revealed that the distortion contained information about the
planet -- its mass, separation from its star, and orientation -- and that
information matched perfectly with what astronomers saw during their direct
observation of the dip due to the planet. So the same information can be
captured from the distortion alone.
This detailed analysis showed that the planet is twice the mass of Earth, and
orbits its star from an Earth-like distance, around 90 million miles. But its
star is 400 times dimmer than our sun, so the planet is very cold -- around 60
Kelvin (-352 degrees Fahrenheit or -213 Celsius), which makes it a little colder
than Jupiter's moon Europa. The second star in the star system is only as far
from the first star as Saturn is from our sun. But this binary companion, too,
is very dim.
Still, binary star systems composed of dim stars like these are the most common
type of star system in our galaxy, the astronomers said. So this discovery
suggests that there may be many more terrestrial planets out there -- some
possibly warmer, and possibly harboring life.
Three other planets have been discovered in binary systems that have similar
separations, but using a different technique. This is the first one close to
Earth-like size that follows an Earth-like orbit, and its discovery within a
binary system by gravitational microlensing was by chance.
"Normally, once we see that we have a binary, we stop observing. The only reason
we took such intensive observations of this binary is that we already knew there
was a planet," Gould said. "In the future we'll change our strategy."
In particular, Gould singled out the work of amateur astronomer and frequent
collaborator Ian Porritt of Palmerston North, New Zealand, who watched for gaps
in the clouds on the night of April 24 to get the first few critical
measurements of the jump in the light signal that revealed that the planet was
in a binary system. Six other amateurs from New Zealand and Australia
contributed as well.
Other project collaborators hailed from Ohio State, Warsaw University
Observatory, Chungbuk National University, Harvard-Smithsonian Center for
Astrophysics, University of Cambridge, Universidad de Concepción, Auckland
Observatory, Auckland University of Technology, University of Canterbury, Texas
A&M University, Korea Astronomy and Space Science Institute, Solar-Terrestrial
Environment Laboratory, University of Notre Dame, Massey University, University
of Auckland, National Astronomical Observatory of Japan, Osaka University,
Nagano National College of Technology, Tokyo Metropolitan College of
Aeronautics, Victoria University, Mt. John University Observatory, Kyoto Sangyo
University, Tel-Aviv University and the University of British Columbia.
Funding came from the National Science Foundation, NASA (including a NASA Sagan
Fellowship), European Research Council, Polish Ministry of Science and Higher
Education, National Research Foundation of Korea, U.S.-Israel Binational Science
Foundation, Japan Society for the Promotion of Science, Marsden Fund from the
Royal Society of New Zealand and the Israeli Centers of Research Excellence.