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Major Breakthrough:
First Photos of Planets Around Other Stars
By
Jeanna Bryner
Senior Writer
02:00
am ET
Star.com
reports astronomers have taken what they say are the first-ever direct
images of planets outside of our solar system, including a visible-light
snapshot of a single-planet system and an infrared picture of a
multiple-planet system.
Earth-like worlds might also exist in the
three-planet system, but if so they are too
dim to photograph. The
other newfound planet orbits a star called
Fomalhaut, which is visible without the aid of a telescope. It is the
18th brightest star in the sky.
The massive worlds, each
much heftier than Jupiter (at least for the
three-planet system), could change how astronomers define the term
"planet," one planet-hunter said.
Breakthrough
technology
Until now,
scientists have inferred the presence of planets mainly by detecting an
unseen world's gravitational tug on its host star or waiting for the
planet to transit in front of its star and then detecting a dip in the
star's light. While these methods have helped to identify more than 300
extrasolar planets to date, astronomers have struggled to actually
directly image and see such inferred planets.
The four
photographed exoplanets are discussed in two research papers published
online today by the journal Science.
"Every extrasolar
planet detected so far has been a wobble on a graph. These are the first
pictures of an entire system," said Bruce Macintosh, an astrophysicist
from Lawrence Livermore National Laboratory in California, and part of
the team that photographed the multi-planet system in infrared light.
"We've been trying to image planets for eight years with no luck and now
we have pictures of three planets at once."
Astronomers
have claimed previously to have directly
imaged a planet, with at least two such objects, though not everybody
agreed the objects were planets. Instead, they may be dim, failed stars
known as brown dwarfs.
Multi-planet
snapshots
Macintosh, lead
researcher Christian Marois of the NRC Herzberg Institute of
Astrophysics in Canada, and colleagues used the Gemini North telescope
and W.M. Keck Observatory on Hawaii's Mauna Kea to obtain infrared
images. Infrared radiation represents heat and, along with everything
from radio waves to visible light and X-rays, is part of the
electromagnetic spectrum.
The trio of worlds
orbits a star named HR 8799, which is about 130 light-years away in the
constellation Pegasus and about 1.5 times as massive as the sun. The
planets are located at distances from their star of 24, 38 and 68
astronomical units (AU). (An astronomical unit equals the average
Earth-sun distance of 93 million miles, or about 150 million km.) Other
planet-finding techniques work out to only about 5 AU from a star.
The planet closest
to the star weighs in at 10 times the mass of Jupiter, followed by
another 10 Jupiter-mass planet and then, farther out, a world seven
times the heft of Jupiter.
By astronomical
standards, the planets are fresh out of the oven, forming about 60
million years ago. That means the orbs are still glowing from heat
leftover from their formation. Earth, by comparison, is about 4.5
billion years old.
The most distant planet orbits just inside a
disk of dusty debris, similar to that produced
by the icy objects of the solar system's Kuiper belt, which lies just
beyond the orbit of Neptune.
The setup of this
planetary system, along with its dusty belt, suggests it is a scaled-up
version of our solar system, Macintosh said. That means other planets
closer in to the host star could be waiting for discovery.
"I think there's a
very high probability that there are more planets in the system that we
can't detect yet," Macintosh said. "One of the things that distinguishes
this system from most of the extrasolar planets that are already known
is that HR 8799 has its giant planets in the outer parts — like our
solar system does — and so has 'room' for smaller terrestrial planets,
far beyond our current ability to see, in the inner parts."
Hubble's
discovery
University of
California, Berkeley, astronomer Paul Kalas led the team of astronomers
who took the visible-light snapshot of the single-planet system. The
exoplanet has been named Fomalhaut b, and is estimated to weigh no more
than three Jupiter masses.
The Hubble Space
Telescope's Advanced Camera for Surveys was used to make the image. The
camera is equipped with a coronagraph that blocks out the light of the
host star, allowing astronomers to view a much fainter planet.
"It's kind of like
if driving into the sun and suddenly you flip down your visor, you can
see the road easier," Kalas said during a telephone interview. In fact,
Fomalhaut b is 1 billion times fainter than its star. "It's not easy to
see. That kind of sensitivity has never been seen before," he added.
Fomalhaut b is about
25 light-years from Earth. Photos taken in 2004 and 2006 show the
planet's movement over a 21-month period and suggest the planet likely
orbits its star Fomalhaut every 872 years at a distance of 119
astronomical units (AU), or 11 billion miles (nearly 18 billion km).
That's about four times the distance between Neptune and the sun.
Kalas
suspected the planet's existence in 2004
(published in 2005) after Hubble images he had taken revealed a dusty
belt that had a sharp inner edge around Fomalhaut. The sculpted nature
of the ring suggested a planet in an elliptical orbit was shaping the
belt's inner edge. And it was.
"The gravity of
Fomalhaut b is the key reason that the vast dust belt surrounding
Fomalhaut is cleanly sculpted into a ring and offset from the star,"
Kalas said. "We predicted this in 2005, and now we have the direct
proof."
Kalas' team also
suspects that the planet could be surrounded by a ring system with the
dimensions of Jupiter's early rings, before the dust and debris
coalesced into the four Galilean moons.
What's a planet?
The successful image results could change
how planets are defined, said Sara Seager, an
astrophysicist at MIT who was not involved in the discoveries.
Until now, mass has
been one of the critical pieces of information that could place an
object into or out of the planet club. Objects that are too massive,
above about 13 Jupiter masses, are considered brown dwarfs. But now
formation could also be part of the formula. Both of the new planetary
systems revealed dusty disks and suggest the planets must have formed
similar to how planets in our solar system and elsewhere are thought to
have formed.
So, most astronomers
would call the four objects planets, although their masses are only
inferred from the luminosities seen in the images.
"Taken together,
these discoveries are going to change what we call a planet," Seager
told SPACE.com. "Until now people have been arguing about how big
can an object be and still be a planet."
Seager added,
referring to the multi-planet system, "People want to call the upper
mass 12 Jupiter masses. I think it's going to force us to reconsider
what a planet is, because even if they are more massive than what we
want to call a planet, they're in a disk." In addition, she said, nobody
has ever spotted three stars orbiting a host star, as would have to be
the case if you were to call the three planets something other than
planets.
Aiming for
Earth-like planets
These recent direct images reveal giant, gaseous
exoplanets in a new light for the first time, revealing not the effects
of the planets but the planets themselves. The next goal would be direct
images of an
Earth-like planet, the astronomers say.
"The discovery of
the HR 8799 system is a crucial step on the road to the ultimate
detection of another Earth," Macintosh said.
The problem is that
terrestrial (Earth-like) planets are orders of magnitude fainter than
the giant Jupiter-like worlds, and they are much closer in to their host
stars. That means the glare from the star would be overwhelming with
today's technology.
The pay-off could be
big, though, as such rocky planets could orbit within their habitable
zones (where temperatures would allow the existence of liquid water).
"There is plenty of
empty space between Fomalhaut b and the star for other planets to
happily reside in stable orbits," Kalas said. "We'll probably have to
wait for the James Webb Space Telescope to give us a clear view of the
region closer to the star where a planet could host liquid water on the
surface."
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