Some interesting astrophysics news this week, from Nature: scientists have used “microlensing” to discover a extrasolar planet only five times Earth’s mass:


Planet OGLE-2005-BLG-390Lb looks much more like home. It lies about 390 million kilometres from its star: if it were inside our Solar System, the planet would sit between Mars and Jupiter.

It takes ten years for the planet to orbit its parent star, a common-or-garden red dwarf that lies about 28,000 light years from Earth, close to the centre of our Galaxy.

Of course, it’s not quite time to start buying tickets for the colony ships: at that orbit, around a red dwarf star, the surface temperature is a bit below the boiling point of liquid nitrogen.

The important thing about this is not so much the fact of the discovery, but the technique that was used. Previous extrasolar planet detections have used the Doppler effect to measure a slight change in the star’s velocity as it’s tugged back and forth by an orbiting planet. As you might expect, this is most effective at detecting very heavy planets in fairly fast orbits, and that’s mostly what they’ve found.

The new detection used a technique based on the fact that gravity can bend light. The researchers working on the project monitored a large number of stars, waiting for something small and heavy to pass between us and them. A heavy object in the path that light takes from a distant star can act like a lens, bending some rays that otherwise would’ve missed the Earth into our line of sight, and increasing the brightness of the star.

The lensing effect can be used to detect planets bacause it’s a pretty short-range effect, and things orbiting around some distance from the star will be magnified at different times, sort of like the effect you get as you slide a lens across a line of text, with each letter being magnified in turn. A star with no planets should show a smooth, symmetric profile, getting brighter then dimmer as the lensing object moves across it, while a star with an orbiting planet should show a characteristic “bump” on one side or another, as light from the planet gets bent into view. What they’re reporting in Nature is the observation of such a “bump.”

The “microlensing” technique has the potential to be much more sensitive than the Doppler method– there’s a nice graph in the Nature news story showing the sensitivity comparison. There isn’t the same sort of size limit that you get with the Doppler detection, where the planet has to be relatively big to move the star enough to detect (there’s probably some limit, but the news story doesn’t go into that). The down side is, it’s kind of a crapshoot– you have to look at a bunch of stars (millions, according to the New York Times story), and hope that something heavy passes in the path while you’re looking. There’s an active research effort underway to do this, though (I’ve heard talks mentioning the idea for a couple of years now), and a fair number of heavy things wandering around the galaxy, so expect more such discoveries to be announced in the future.

(Disclaimers: I am not an astronomer, so this is strictly a layman-level description of what little I retain from hearing a few astronomers discuss gravitational lensing. Clarifications from people who actually know stuff are, of course, welcome in comments. I’m also not usually this quick to jump on a news story, but like Razib, I was really into astronomy as a kid, and what SF fan wouldn’t be excited by hearing about distant planets?)

Comments

  1. #1 NL
    January 26, 2006

    The light curve graph is really pretty…

  2. #2 Chad Orzel
    January 26, 2006

    I should note that Mark Trodden’s post on this suggests that I have the explanation partly backwards– the planet is orbiting the star that’s doing the lensing, not the object being lensed. That’s what I get for posting late at night while waiting for cold medicine to kick in…

  3. #3 P.M.Bryant
    January 26, 2006

    Excellent summary, and you beat me to the correction. But I’ll also add that these teams using micro-lensing to probe the galaxy have been at work for over a decade.

    This is the third planet discovered by the technique apparently, but the first relatively small one.

    Unfortunately, the technique is of course not suited to followup observations, so it doesn’t look like we’ll be able to learn more about this particular system.

  4. #4 Fred
    January 26, 2006

    Blade Runner! What a geek (meant in a good way.)

  5. #5 Anthony Kendall
    January 26, 2006

    Nice description of the actual dynamic process of microlensing, making the analogy to a lens magnifying letters of text.

    For those interested in more discussion, the Bad Astronomer has a description of the story. Also, on my blog (follow the link on my name) I go through the actual Nature article and summarize their results.

    It’s interesting that the mass of the planet, so confidently announced to be 5.5 Earth masses by the media, is probably somewhere between 2 and 7 earth masses, but 5.5 is simply the most likely number. The microlensing technique doesn’t actually tell you the mass, distance to the star, or the orbital distance of the planet around the lensing star. It just gives you ratios that depend on the mass of the star which we do not know. Anyway, for more explanation, check out my blog entry.

    Another note, the fact that this technique is much more sensitive to Jupiter-sized planets than it is sub-Neptune mass ones means that many more larger planets should be detected. But so far, two Jupiter-size have been detected and one sub-Neptune. The statistics are small, but they suggest that contrary to what we’ve seen with doppler or brightness techniques, sub-Neptune planets are much more common throughout the galaxy than are larger gas giants.

  6. #6 paul
    January 26, 2006

    It’s not surprising that the doppler and brightness data are skewed toward larger planets (and close-in large planets at that) since thats about all you can do with current combinations of sensitivity and observing time. (At worst, Nyquist says you need at least half a planetary year just to detect anything.)

    I wonder how many centuries it will be before microlensing gets used as a directed observational too. It’s been used for a while to detect Kuiper-belt objects, and with either a large enough database of objects or a big enough rocket you could do some nice astronomy. Although anomalous lensing typically is generally limited to discovering companions to the lensing star, with enough time and spectral resolution you can also get observations of the source star and any companions (search under microlensing and caustics).

  7. #7 wheatdogg
    January 27, 2006

    Nice allusion to Bladerunner. I’ll catch the next one … maybe.

  8. #8 Chad Orzel
    January 27, 2006

    It’s not surprising that the doppler and brightness data are skewed toward larger planets (and close-in large planets at that) since thats about all you can do with current combinations of sensitivity and observing time. (At worst, Nyquist says you need at least half a planetary year just to detect anything.)

    Yeah, I’ve always been a little bemused by pop-science articles making extravagent claims about the rarity of Earth-like worlds, based on the nature of the extrasolar planets that have been detected. Given that our primary method for detecting planets can only detect huge, fast-moving planets, it’s not exactly shocking that we’ve discovered lots of huge, fast-moving planets…

    The slightly different concern within the astronomical community is more understandable– existing models of planet formation don’t (or didn’t– I don’t know the status) really allow for any of the sort of planets that were first discovered, so their existence is a bit of a surprise. I don’t think the data we have really give any information about the real abundance of different types of planets, though.

  9. #9 P.M.Bryant
    January 27, 2006

    paul wrote:

    I wonder how many centuries it will be before microlensing gets used as a directed observational too. It’s been used for a while to detect Kuiper-belt objects, and with either a large enough database of objects or a big enough rocket you could do some nice astronomy.

    Microlensing detecting Kuiper belt objects? This would be quite surprising to me, given the very small mass of KBOs. Do you have more info on this? I haven’t found anything.

    The current groups doing microlensing surveys would likely say they are doing some nice astronomy already. :-)

    Chad Orzel wrote:

    [E]xisting models of planet formation don’t (or didn’t– I don’t know the status) really allow for any of the sort of planets that were first discovered, so their existence is a bit of a surprise.

    I think this was true at first, but after ten years it looks like the theorists have come up with models to explain the giant planets in close orbits.

    See this link and scroll about 3/4 of the wayd down for the paragraph starting, “Simulations of planet formation and migration based on core-accretion models (Ida & Lin, 2004, ApJ 616, 567) show a remarkable agreement with the observed distribution of semi-major axes and planet masses in the region to which radial-velocity surveys are sensitive.”

  10. #10 Jake
    January 27, 2006

    Assuming it survives, the Kepler mission aims to detect earth-size planets by watching stars for the decrease in brightness caused by the planet passing in front of the star.