Re: Worlds with Double Sunsets Common.


"Jeffrey Turner" <jturner@xxxxxxxxxxxx> wrote in message news:130r27mb9i4l139@xxxxxxxxxxxxxxxxxxxxx
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Worlds with Double Sunsets Common
http://news.yahoo.com/s/space/20070329/sc_space/worldswithdoublesunsetscommon

Ker Than
Staff Writer
SPACE.com Thu Mar 29, 1:15 PM ET

Astronomers might not have to look in a galaxy far, far away after all
to find a world with double sunsets like Luke Skywalker's home planet
Tatooine.

Astronomers using NASA's Spitzer Space Telescope have found that
twin-star systems are just as likely to be surrounded by dusty debris
disks as ones with only a single star. Debris disks are made up of
asteroid-sized rock chunks and other material that could be leftovers
of planets that have formed in the system.

Astronomers have theorized that planets could form with little trouble
in two-star systems, called binaries, despite the more complex
gravitational tugging. The new study provides strong observational
evidence to support that idea.

'There appears to be no bias against having planetary system formation
in binary systems,' said study leader David Trilling of the University
of Arizona. 'There could be countless planets out there with two or
more suns.'

The majority of bright stars like our Sun have at least one stellar
companion, so the new finding bolsters the idea, long predicted by
theory, that the universe abounds with worlds that have two Suns.

A planetary nursery

Trilling and his team looked for disks in 69 binary systems between 50
and 200 light-years away from Earth. All the stars are more massive
and younger than our middle-aged Sun. The researchers found that about
40 percent of the binary systems they looked at had disks. This
frequency is a bit higher than that for a comparable sample of single
stars and suggests planets are at least as common around binary stars
as they are around single stars.

Deepak Rhagavan, an astronomer at Georgia State University who was not
involved in the study, says the new findings are exciting because they
are the first evidence of a planetary nursery in a multiple star
system. 'Until now, we knew planets existed [in multiple star
systems], but I think this is the first time that we've gotten a
comprehensive study that looks at the debris disk where planets are
born,' Rhagavan said.

Last year, Rhagavan's team reported that many star systems known to
harbor planets actually contained two, and in some cases, even three,
stars.

Alan Boss, a planet formation theorist at Carnegie Institution of
Washington, says the finding is encouraging news for planet hunters.
'It's pretty reassuring,' said Boss, who also was not involved in the
study. 'This really goes in the direction of making planets more
frequent than they would be otherwise.'

Tight binaries

Surprisingly, most of the debris disks found in the new survey were
around so-called tight binary systems, where the stars are separated
by 500 AU or less. One AU is equal to the distance between the Earth
and the Sun.

I think there must be a misprint here, 500 AU is hardly "tight". Maybe 0.1
AU or less?

I don't think it is a misprint.

What's 45 billion miles between friends?

Scientists know of about 50 planets that have two Suns, but all of
them belong to 'wide' binary systems, where the stars are separated by
about 1,000 AU.

I wonder how far the planets are from the CoG.

My impression is that the planets are nowhere near the system CoG. They are
in close orbits around one of the two stars. Closer than 1 AU. The other star,
for them, is more of the nature of a distant self-luminous planet.

'The fact that they've found some positive evidence of planet-forming
disks being around close binaries is really a new step,' Boss said.

Some scientists had previously argued that planet formation would be
stifled in tight binary systems because of the large gravitational
interactions between the stars.

'The idea was that the extra star would stir up the stuff in the
planet forming disk so much that you would never form a planet,'
Trilling told SPACE.com.

Trilling said his team's results might mean that planet formation
favors tight binaries over single stars. However, it could also be
that tight binaries are just dustier, and thus easier to spot. Further
observations will be required to determine which of these explanations
is correct.

A human gazing at a double sunset on a world with two Suns like
Skywalker's Tatooine might not find the scene so alien after all,
Trilling said. 'It would be kind of like what you see on Earth, but
with an extra Sun following in the sky,' he said. 'Maybe it's a little
hotter during the day.'

Not at those distances, I suspect. Or does he mean hotter than on
Neptune?

Assuming Trilling is talking about a planet in orbit around one star at
1 AU, with another star out at 500 AU I doubt that the extra heating would
be noticed. If the second star is a twin of the sun, I calculate that it
would be roughly as bright as the full moon (though without having a
perceptible disk).
http://en.wikipedia.org/wiki/Apparent_magnitude

.

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