Alien
worlds, once hidden from knowledge, are now being discovered in droves,
stunning astronomers with their unique features and sheer numbers. The
discoveries are so common that more and more don't even get reported outside
scientific circles.
Take the
announcement at the end of May of a massive planet, dubbed TrES-3,
that zips around its star in an amazingly rapid 31 hours, giving the planet a
1.3-day year. Astronomers issued a press release, but you might not have heard
about it because the discovery was so overshadowed by other planet announcements
and barely received news coverage.
"It's
pretty routine now," said Alan Boss, a planet formation theorist at the
Carnegie Institution of Washington. "Most planets that are found are not deemed
worthy of a press release because they are sort of becoming 'one more planet.'"
The total
is now more than 200 extrasolar
planets confirmed. And this is the tip of the iceberg in planet finds. Astronomers
have more tools than ever, and technology is so advanced that planet discovery has become almost
mundane.
The
regularity of planet finds, luckily, is buffered by the wild variety in the
discoveries themselves, including the following contrasts: nascent worlds of
just a million years versus those that are billions of years old; hot gas
giants and icy Neptune-like orbs; planets that whip around their parent
stars with cosmic speed and others that seem to creep at a slug's pace; and
planets orbiting
double-stars, red-dwarf stars and even so-called failed stars.
Transit
technique
Astronomers
spotted TrES-3 as part of the Trans-atlantic Exoplanet Survey while looking for
transiting planets, or those that pass directly in front of their home star
with respect to Earth. It was detected with a network of telescopes in Arizona, California, and the Canary Islands. When TrES-3 coasted in front of its home star,
the telescopes picked up a slight dimming of the star's light, by about 2.5
percent. The scientists used the dimming to estimate the planet's mass, size
and other properties.
It is
located 800 light-years away in the constellation Hercules about 10 degrees
west of Vega, one of the brightest stars in the summer skies of the northern
hemisphere.
"It is also
a very massive planet—about twice the mass of the solar system's biggest
planet, Jupiter—and is one of the planets with the shortest known periods,"
said a co-discoverer of TrES-3 Georgi Mandushev of the Lowell Observatory in Arizona.
The giant
orb orbits so close to its parent star, about 50 times closer than Earth is to the Sun, the astronomers
estimate its temperature soars to about 1,500 degrees Kelvin.
Stellar
wobbles
While the
"transit method" provides astronomers with the best indirect information about
an exoplanet, so far only about 20 transiting planets have been spotted.
That's why
the most successful (based on the number of planet finds) teams have relied on
the so-called wobble
method, or radial-velocity technique.
"The radial-velocity
teams are the most successful," Boss told SPACE.com. "They are a victim
of their own success. They are able to get more and more telescope time,
because they can prove to the assignment committees that give out the time that
'if you give us so many more nights we can probably find you so many more
planets,'" Boss said.
He added,
"The key bottleneck for finding more planets is simply more time on a
telescope."
The firsts
and superlatives
In addition
to finding new worlds, the burgeoning field has achieved many firsts.
In 2001, a
team led by David Charbonneau of the Harvard-Smithsonian Center for
Astrophysics used the Hubble Space Telescope to detect for the first time the atmosphere of an extrasolar
hot Jupiter called HD 209458b.
Another hot
Jupiter, Upsilon
Andromeda b, revealed for the first time an exoplanet with a temperature
variation across its surface: One side has temperatures rivaling those found
deep in a volcano while the other face could plunge below freezing.
Superlatives
abound as well, with discoveries gaining fame as the windiest, tiniest, most
massive and fastest orbiter.
- Shortest
orbital period in catalog: HD 41004 B b completes a full orbit in 1.328
days.
- Longest
orbit: HD 154345 b takes 13,100 days to orbit its parent star.
- Lightest
planet: Gliese
581 C weighs just five Earth masses.
Planet
organizer
In an
effort to keep track of the rapidly increasing list of exoplanets, a group of
astronomers published a catalog of nearby exoplanets within 652 light-years of
Earth in a 2006 issue of the Astrophysical Journal, though they realize
updates will be a must on a routine basis.
"Without
question, the catalog presented here will become out of date before it is
printed," the researchers say in the published report of the catalog.
But with
such a huge sample of relatively nearby planets, theorists now have the chance
to test out their theories in the "real world."
"This whole
business of extrasolar planets has been a real boon for theorists because so
far they had only one planetary system to study—and that was ours," Mandushev
said in a telephone interview.
For
instance, when does an object stop being a planet and become a star, a
threshold that theory places at 10 to 15 Jupiter masses and beyond which an
object can ignite hydrogen fusion to power a stellar glow?
The real
goal
The
ultimate goal, say many planet hunters, is to find Earth-like planets, or those
with similar masses, orbits and rocky compositions to Earth. And beyond finding
the physical Earth-like attributes would be to find life. So far no "Earths"
have been identified, though observatories are coming online with the
sensitivity to detect small objects that orbit far from their host stars, as
our planet does.
"The hunt
is still on for rocky, Earth-like planets," said Jason Wright, an astronomer at
the University of California, Berkeley, who was part of the team compiling the
exoplanet catalog.
And
astronomers have identified the first Earth-like planet that could support
liquid water and harbor life. The "super Earth," Gliese
581 C, weighs about five Earth masses and is either a rocky planet or one
covered entirely by oceans, astronomers speculate.
Multi-planet
systems are also a goal. So far about 25 multi-planet systems have been
identified with two such systems supporting four planets.
"We haven't
found a clone of the solar system yet," Boss said. "But that's only ruling out
maybe 10 percent of the stars. The other 90 percent could have exact solar
system analogs and we wouldn't know it because we haven't been able to take
data for long enough to actually find their planetary systems."
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