“Recent” Planetary Collision around nearby Star

Recent, is of course, relative to the timescale involved.  In the case of planetary collisions and their aftermath, that can be a very long time, like, tens of millions of years.  So while very interesting, this isn’t a train wreck (or planetary wreck) in progress, but the aftermath of one that is still settling.

Physics Today is reporting, in an article in the Search & Discovery pages (October 2009, p.14 but alas, for subscribers only), that a group of astronomers led by Carey Lisse of the Johns Hopkins University in Baltimore, that they have found evidence of a planetary collision around the prosaically named HD 172555.  Yeah, okay, so that doesn’t roll off the tongue, but HD 172555 (HD = Henry Draper, the guy who put the star catalogue together) is a relatively nearby star at 95 light-years distant which makes it easier to spot things.  The evidence is indirect, meaning no pretty pictures of dust rings, but the spectra are consistent with hot dust composed of SiO (silicon oxide) gas (think vaporized sand/rock), tektite, obsidian, pyroxenes, and olivines, the latter four all igneous rocks.  Toss in what appears to be hot carbon dust and it sure looks like the remnants of a collision, or at least what you would expect to see from our distance.

Why does it matter? When I was a kid, everyone "knew" that planets formed by accretion, that, is the dust and gas that was spinning around the sun just gradually collapsed due to self-gravity and friction into big balls that became planets.  Over the intervening years, a lot of evidence has accumlated to show that that is probably wrong.  Planets up to about the size of Mars can form that way, but every one of the rocky inner planets in our solar system show signs of having been involved in a massive collision.  Mars is basically lopsided (at least geographically), Mercury appears to have had whatever outer crust it may have had stripped away so it is very metal rich more like the Earth’s core, Venus’ rotation axis is way out of alignment as if something whacked it, and the current best bet on how the Earth got its moon is that Earth was rammed by a Mars-sized planetary object [1,2] so that half of our crust ended up on the moon and half on Earth (which would explain why the chemistry of the two are so much alike). Having another system to study can help us to understand more about how our solar system formed.

If you are interesting in going through the original articles, there is PDF of the paper Lisse et al presented at the Lunar and Planetary Institute FTP site.

More Information:

[1] Giant Impact Hypothesis.

[2] Origin of the Moon.