Astronomers Say Moons Like Ours Are Uncommon

[Dr. Spitzer]

Check out Spitzer's latest find:

 

The next time you take a moonlit stroll, or admire a full, bright-white moon looming in the night sky, you might count yourself lucky. New observations from NASA's Spitzer Space Telescope suggest that moons like Earth's -- that formed out of tremendous collisions -- are uncommon in the universe, arising at most in only 5 to 10 percent of planetary systems.

 

"When a moon forms from a violent collision, dust should be blasted everywhere," said Nadya Gorlova of the University of Florida, Gainesville, lead author of a new study appearing Nov. 20 in the Astrophysical Journal. "If there were lots of moons forming, we would have seen dust around lots of stars -- but we didn't."

 

It's hard to imagine Earth without a moon. Our familiar white orb has long been the subject of art, myth and poetry. Wolves howl at it, and humans have left footprints in its soil. Life itself might have evolved from the ocean to land thanks to tides induced by the moon's gravity.

 

 

See the rest: http://www.spitzer.caltech.edu/Media/releases/ssc2007-18/release.shtml

[Reaper]

What I'm wondering is if the systems that were looked at actually contained planets similar to Earth, or small rocky planets and bodies that are indeed colliding?

 

The dust could be anything by the way, for all we know it might just be asteroids and comets.

[swansont]

What I'm wondering is if the systems that were looked at actually contained planets similar to Earth, or small rocky planets and bodies that are indeed colliding?

 

The dust could be anything by the way, for all we know it might just be asteroids and comets.

 

AFAIK the resolution is only now getting to the point where an earth-mass, earth-orbit planet is detectable. So I'm guessing the answer is "no."

[Martin]

What I'm wondering is if the systems that were looked at actually contained planets similar to Earth, or small rocky planets and bodies that are indeed colliding?

...

 

Here's the paper

http://arxiv.org/abs/0707.2827

Debris Disks in NGC 2547

N. Gorlova (Univ. Florida), Z. Balog, G. H. Rieke, J. Muzerolle, K. Y. L. Su (Univ. Arizona), V. D. Ivanov (ESO), E. T. Young (Univ. Arizona)

Accepted to ApJ, 29 pages, 13 figs.

(Submitted on 19 Jul 2007 (v1)

 

"We have surveyed the 30 Myr-old cluster NGC 2547 for planetary debris disks using Spitzer. At 4.5-8 um we are sensitive to the photospheric level down to mid-M stars (0.2 Msol) and at 24 um to early-G stars (1.2 Msol). We find only two to four stars with excesses at 8 um out of ~400-500 cluster members, resulting in an excess fraction <~1 percent at this wavelength. By contrast, the excess fraction at 24 um is ~40 percent (for B-F types). Out of four late-type stars with excesses at 8 um two marginal ones are consistent with asteroid-like debris disks. Among stars with strong 8 um excesses one is possibly from a transitional disk, while another one can be a result of a catastrophic collision. Our survey demonstrates that the inner 0.1-1 AU parts of disks around solar-type stars clear out very thoroughly by 30 Myrs of age. Comparing with the much slower decay of excesses at 24 and 70 um, disks clear from the inside out, of order 10 Myr for the inner zones probed at 8 um compared with a hundred or more Myr for those probed with the two longer wavelengths."

 

Swantsont is right. they did not select a sample of stars KNOWN to have planets. they just took a cluster of stars and looked at about 500 stars.

 

It sounds to me like they found only one or two stars that COULD have had a catastrophic collision on a scale like what is thought to have formed our Moon.

 

They were not just looking for that, they were adding to understanding the whole process. they found evidence that inner dust and debris gets CLEARED while the star is still young.

 

A signal that there might be our kind of moon would be an already comparatively mature star that has a near dustcloud. Then the inner system would have first cleared, and planets like venus, earth, mars would have collected and everything looks stable but then BLAM one of these highly unlikely crashes between already-formed planets (like earth and something the size of mars)

 

then you get a second episode of dust, this time around a more mature star.

 

that is what they were reporting is kind of rare. they were not distinguishing planets (too far away, too much trouble)

 

I think this is a reasonable paraphrase. Say if you disagree. the statistics make sense to me. Moons like ours (big ones around rocky inner system planets) must be rare because their formation would show up as secondary dust episodes around already mature stars----and these secondary dust episodes are rare.

=====================

 

thanks to DrSpitzer for flagging this one. It is an iteresting article. DrSpitzer you seem to keep scanning the Spitzer telescope news releases for interesting stuff. This is really helpful. I hope you keep doing this.

[YT2095]

Neat, Thanks for that each

[SkepticLance]

Another wonderful coincidence with our moon. Does anyone know what the odds are that, assuming one moon, that moon would have exactly the apparent diameter to exactly cover the sun during eclipses - to the extent that astronomers can see the sun's corona flaring all around the outside of the moon?

 

With the exception of Charon, our moon is the largest with respect to its parent planet for our entire solar system. And Pluto and Charon are supposed to be twinned planets - and not a case of a moon that came from its parent.

[swansont]

Another wonderful coincidence with our moon. Does anyone know what the odds are that, assuming one moon, that moon would have exactly the apparent diameter to exactly cover the sun during eclipses - to the extent that astronomers can see the sun's corona flaring all around the outside of the moon?

 

The moon has been receding from the earth, though, so this is something that has not always been true. The coincidence is the right size and orbit but that also includes having sentient beings alive at the right time. Some current eclipses do not cover the sun to that extent — you can get an annular eclipse when the moon is near apogee. Total eclipses will become less common over time and will cease in a few tens or hundreds of millions of years.