Is Luna a Celestial Rarity?

by Paul Gilster on November 23, 2007

Having just written about dust formation around HD 23514, a Sun-like star in the Pleiades, I was drawn to this quote by Nadya Gorlova (University of Florida, Gainesville), whose recent work suggests that if moons like our own were common, we’d be seeing more dust than we do around other stars. “When a moon forms from a violent collision, dust should be blasted everywhere,” says Gorlova. “If there were lots of moons forming, we would have seen dust around lots of stars — but we didn’t.” By contrast, the UCLA study on the Pleiades sees major collisions as common in young solar systems, though to be sure it didn’t focus its conclusions on the 30 million year age range, as the Florida study did.

Gorlova’s team used data from the Spitzer Space Telescope and operated under current assumptions about lunar formation, in which an impactor the size of Mars is thought to have struck the Earth, creating a vast debris field that fell into Earth orbit and eventually became the Moon. The theory sets Earth’s Moon apart from the rest of the Solar System, where planetary satellites seem to have formed at the same time as their planet or else were captured by that planet’s gravity. If our Moon is anomalous, then how often will we find exoplanets orbited by similar moons?

Earth and Moon

The study looked at four hundred stars in the range of 30 million years old, which is the age our Sun is thought to have been when the Moon formed. The result seems stark: Only one of the four hundred shows the kind of dust cloud that flags such a collision. From this, the team calculates a five to ten percent chance that a given solar system will create a moon like Earth’s, and that’s a high estimate. “We don’t know that the collision we witnessed around the one star is definitely going to produce a moon, so moon-forming events could be much less frequent than our calculation suggests,” says co-author George Rieke of the University of Arizona, Tucson.

Image: Our Earth-moon system, photographed here by NASA’s Galileo spacecraft in 1992, might be somewhat uncommon in the universe. New evidence from NASA’s Spitzer Space Telescope suggests that moons that formed like ours — out of colossal collisions between rocky bodies — might arise in, at most, 5 to 10 percent of planetary systems. Credit: NASA/JPL-Caltech.

HD 23514, the star examined by Benjamin Zuckerman’s team at UCLA, is about 100 million years old and compared to most stars of its age, virtually choked by dust. But the UCLA work on this star (and earlier on the Sun-like BD+20 307) suggested that younger stars — in the 10 million year range or younger — are hotbeds of collisional activity of the sort that forms planets and, one would assume, breaks them apart to form moons. Perhaps our Moon is simply a late arrival, with other moons like it more likely to form earlier in the history of a given star’s development.

In any case, the Florida study makes a strong case that by the time a star is 30 million years old, it’s likely to have finished the planet formation process, so collision-generated moons from that point on are less likely. It would be interesting to have these two teams in the same room for debate, something conference organizers should note for future reference. Meanwhile, the paper is Gorlova et al., “Debris Disks in NGC 2547,” The Astrophysical Journal 670 (November 20 2007), pp. 516-535 (abstract).

andy November 23, 2007 at 12:27

I’m not entirely sure I buy this. Given the existence of the Pluto-Charon system which may also be result of a giant impact, the apparent stripping of the mantle of Mercury (another possible giant impact event), the outer system object Orcus which may be another Pluto-Charon type system, I am sceptical of the conclusion that only a small fraction of solar systems undergo such events.

On the other hand, perhaps this indicates that our moon formed unusually late in the solar system’s history, when most of the accretion had stopped. Presumably the majority of the major collisions occurred during the accretion time… what the consequences on a planet’s evolution of earlier or later moon-forming events would be, I don’t know.

Adam November 23, 2007 at 20:05

Hi andy

You’ve taken the words right out of my mouth – with so much evidence for collision in our System’s past why do other systems seem glaringly different? And your answer is probably as good as any – the Moon is a latecomer. Perhaps it really was a Trojan planet that became the impactor that made the Moon, and it took perhaps 50 million years for its instability to crash it into Earth?

dad2059 November 24, 2007 at 11:15

Very interesting observation to be sure, one that should have follow-up investigations.

This can figure definitely into the Drake Equation and the Fermi Paradox.

ljk November 29, 2007 at 1:09

Youthful Star Sprouts Planets Early

A stellar prodigy has been spotted about 450 light-years away in a system called UX Tau A by NASA’s Spitzer Space Telescope. Astronomers suspect this system’s central sun-like star, which is just one million years old, may already be surrounded by young planets. Scientists hope the finding will provide insight into when planets began to form in our own solar system.

“This result is exciting because we see a gap, potentially carved out by planets, around a dusty sun-like star. In almost all other star systems of this age, we typically see a primordial disk – a thick disk of dust, without any clearings, ” said Catherine Espaillat, a graduate student at the University of Michigan, Ann Arbor.

Prior to the Spitzer observations, Espaillat and her teammates knew that a sun-like star sat at the center of UX Tau A. Now, using the telescope’s infrared spectrometer instrument, they have discerned details about the dusty disk swirling around the central star.

Such dusty disks are where planets are thought to be born. Dust grains clump together like snowballs to form larger rocks, and then the bigger rocks collide to form the cores of planets. When rocks revolve around their central star, they act like cosmic vacuum cleaners, picking up all the gas and dust in their path and creating gaps.

Spitzer saw a gap in UX Tau A’s disk that extends from 0.2 to 56 astronomical units (an astronomical unit is the distance between the sun and Earth). In our solar system, this gap would occupy the space between Mercury and Saturn. Espaillat notes that the formation of one or more planets could be responsible for carving out the gap.

Although gaps have been detected in disks swirling around young stars before, Espaillat notes that UX Tau A is special because the gap is sandwiched between two thick disks of dust. An inner thick dusty disk hugs the central star, then, moving outward, there is a gap, followed by another thick doughnut-shaped disk. Other systems with gaps contain very little to no dust near the central star. In other words, those gaps are more like big holes in the centers of disks.

Some scientists suspect that these holes could have been carved out by a process called photoevaporation. Photoevaporation occurs when radiation from the central star heats up the gas and dust around it to the point where it evaporates away. The fact that there is thick disk swirling extremely close to UX Tau A’s central star rules out the photoevaporation scenario. If photoevaporation from the star played a role, then large amounts of dust would not be floating so close to the star.

“This finding definitely affects the way astronomers look at planet formation. Spitzer’s infrared spectrometer was able to see a gap in this system, but future, more sensitive telescopes maybe able to search for Earth-like planets in UX Tau A,” said Espaillat.

Her paper will be published in the December 2007 issue of Astrophysical Journal Letters. Other authors on the paper include Nuria Calvet, Jesus Hernández and Lee Hartmann, also from the University of Michigan; Paola D’Alessio of the Universidad Nacional Autónoma de México, Michoacán; Chunhua Qi of the Harvard-Smithsonian Institute for Astrophysics, Cambridge, Mass.; Elise Furlan of the NASA Astrobiology Institute at the University of California at Los Angeles; and Dan Watson of the University of Rochester, N.Y.

Previous results from Calvet and Watson are online at http://www.spitzer.caltech.edu/features/articles/20050912.shtml .

For more information about Spitzer, visit http://www.spitzer.caltech.edu and http://www.nasa.gov/spitzer .

Written by Linda Vu

ljk February 28, 2008 at 7:47

The identical oxygen isotopic composition of Earth and Moon might be explained by exchange of material between the proto-Earth and the surrounding proto-lunar disk.

FULL ARTICLE at:

http://www.psrd.hawaii.edu/Feb08/EarthMoonFormation.html

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