Sunday, February 17, 2008

Solar System Analog Revealed by Microlensing

Normally I don't blog about astronomy in the news, usually because it's either boring or poorly spun or everyone else has already done a more thorough job of it than I'd be willing to anyhow. So I'm stunned that even though it was announced three days ago, I have seen no mention on my portion of the blogosphere about the first discovery of a solar system analog—even the Bad Astronomer only mentioned the independently discovered and announced merely Jupiter analog, though the solar system analog is arguably a much bigger deal since it has both roughly Jupiter and Saturn mass planets.

So how was this system found, why is it important—and why do I suddenly care about planets? The last question is easy: the lead author on the paper is Scott Gaudi, a professor here at Ohio State, and in fact much of the modelling and work that went into deciphering the event's lighcurve was done by one of my officemates. I say "event" because these ~0.7 and ~0.3 Jupiter mass planets around a roughly half solar mass star were found via gravitational microlensing; the star system happened to pass immediately in front of another star relative to Earth, thus magnifying the background star. The change in apparent brightness of this background star is affected by how the mass in the lensing star system is arranged, and so a detailed analysis of the lightcurve can reveal planets in the system. (If you want real details, I think the press release does a fairly good job.) What is so exciting about this particular event is that this is the first time there has been a bright enough microlensing event that has been followed closely enough to be sensitive to a Jupiter+Saturn planetary arrangement—which strongly suggests that such systems (such as our own!) are extremely common.

UPDATE (02/19/2008): Apparently I didn't do a very good job explaining why this is exciting. An astronomer asked me:

I'll grant that it's interesting, but is it really unexpected? The first extrasolar planet discovered was potentially exciting. Now, hundreds are known, and it's clear that their discovery is limited only by the amount of time and money devoted to finding them. Extrasolar multiple-planet systems are also known. Or to put it another way, I'd perhaps be more shocked if a Jupiter-Saturn analogue hadn't been discovered after a little more than a decade of finding massive extrasolar planets.
My reply: Surprising depends on who you ask, but while there have been plenty of exoplanets found (and maybe 2 dozen systems with multiple planets), most of them have been close-in planets, so-called "hot Jupiters" ... planets with periods of only a few days, maybe up to several tens of days. Radial velocity studies have only been going on for about ten years now, which is why they are just now (as in, last week) announcing planets in roughly 10-year orbits: they are restricted to monitoring nearly full orbits to be sure what they are detecting is a planet. Microlensing is the only planet-finding techinque that is actually more sensitive to far-out planets than close-in ones (both the radial velocity and the transitting signal are higher for close-in planets), but people haven't been using it to look look for planets as extensively and as long as they have RV and transits. So it is more comforting than "surprising" that we have found a solar system analog: for years the only systems found were crazy things that looked nothing like our own solar system.


BJ said...

That's too cool!

I think solar systems that have earth like planets supporting complex life would have to have large outer planets like these to sweep up cometary and other debris

Chuck said...

So the planets were around the lensing star? For some reason, I thought they were around the lensed star. Thanks for this.

mollishka said...

Yeah, they are around the *lensing* star, not the background star. The magnification pattern is sensitive to the mass distribution of the lens, and so by modeling the light curve, they were able to figure out that the lens star has planets around it.