[meteorite-list] Major Breakthrough: First Photos of Planets Around Other Stars

[Sterling K. Webb]

http://www.space.com/scienceastronomy/081113-hubble-exoplanet.html

Major Breakthrough: First Photos of Planets Around Other Stars 
By Jeanna Bryner -- 13 November 2008


Astronomers have taken what they say are the first-ever 
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 a
stronomical 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."