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Starlight Measurements to Size Up a Planet

Posted by michael on from the about-the-size-of-my-thumb dept.

Overcoat writes "NASA scientists have used a technique called 'astrometry' to determine the size of a planet orbiting Gliese 876, a star 15 light years away from our solar system. By measuring tiny changes in the 'tilt' of light emitted from the star, changes which were caused by the force exterted by the orbiting planet known as Gliese 876b, the scientists were able to determine that the planet is the size of a golfball. Just kidding: the planet's a whopper, coming in at between 1.89 and 2.4 times the size of Jupiter. This marks the first time astrometry, usually used to measure the distance between stars, has been used to measure a planet."

16 of 23 comments (clear)

  1. Astronomy by rastachops · · Score: 3, Funny
    hah was I the only one to read it and think you being sarcastic:

    NASA scientists have used a technique called 'astronomy'
  2. Density? by Lord+Bitman · · Score: 3, Insightful

    Wouldnt these measurements be more effected by the planet's Density than its actual size?

    --
    -- 'The' Lord and Master Bitman On High, Master Of All
    1. Re:Density? by Henry+V+.009 · · Score: 3, Insightful

      You mean mass. The scientists are able to find the mass by these measurements. Of course, since it's a gas giant, the density (and therefore size) is fairly standard.

    2. Re:Density? by Spock+the+Baptist · · Score: 3, Interesting

      "f=ma"

      Only if mass is constant.
      Newton actually said: F = dp/dt.
      Recall that p = m*v, thus
      d/dt (m*v) = m * dv/dt + dm/dt * v.

      Now if mass is constant, then dm/dt => 0,
      & the second term goes to 0 as well; leaving only the first term: m*a,
      ergo for constant mass, & only for constant mass does F = m*a.

      My two minutes are up...

      --
      "Oh drat these computers, they're so naughty and so complex, I could pinch them." --Marvin the Martian
  3. More information here by Simon+Field · · Score: 5, Insightful


    Some other astrometry uses of the Fine Guidance Sensors can be found here: HST Astrometry Science Team.

    I was looking for something a little less lame, something that didn't talk about the "tilt" of the light. Are they measuring polarization? Refraction? Diffraction?

    Does anyone have a pointer to the article that UPI has so badly dumbed-down?

    --

    Free book: Science Toys You Can Make

    1. Re:More information here by Henry+V+.009 · · Score: 2

      I believe that they mean they are measuring the star's wobble. If so, this is old news--although they may be using some new techniques to get more accuracy this time around.

    2. Re:More information here by QuantumFTL · · Score: 2

      The original article can be found here:
      http://mcdonaldobservatory.org/news/releases/2002/ 1203.html

      That particular article seems to be almost as light on details, but at least it's from the horse's mouth.

      For another interesting read, try this article about a grad student there who believes they caught a black hole consuming an entire star. Pretty cool stuff.

      I would assume the "tilt" of the light would be polarization. Diffraction doesn't really make much sense in this context (diffraction is really only measurable if the size of the object is within a few orders of magnitude of the size of the radiation, which is not the case here) Refraction doesnt' make sense, because that would be gravitational lensing which really doesn't tell you anything except the mass of the object. The planet's atmosphere wouldn't be big enough to cause a measurable refraction from our perspective, so I think we're pretty much left with the idea that it must be polarization. (Someone correct me if I'm wrong, I'm studying the theoretical aspects of astrophysics, not the observational aspects).

      Cheers,
      Justin

    3. Re:More information here by QuantumFTL · · Score: 2

      Measuring the star's wobble via doppler shift can tell you the mass of the planet, and its period of revolution. The calculations pretty much treat the planet as a point mass, because it's roughly spherical. There's no information from dopplar shift that I'm aware of which can give you information on teh size of the planet.

      They are talking about measuring the "tilt" of the light, presumedly polarization.

      This is something that to my knowlege has never been done before, almost all extrasolar objects which are measured via astrometry are very large (with the notable exception of neutron stars, they are very small, but massive and bright). This is very cool and cutting-edge stuff, it's rather amazing IMHO.
      Cheers,
      Justin

    4. Re:More information here by QuantumFTL · · Score: 2

      Ah, I need to RTFA more often... apparently it *WAS* only a mass measurement. Heh. I suppose then what I said didn't make much sense, tehy were simply tracking the movement of the star very precisely (probably dopplar, there's no mention of a transit).

      Sorry about that :)

    5. Re:More information here by QuantumFTL · · Score: 2

      I need to RTFA more often... apparently it *WAS* only a mass measurement. Heh. I suppose then what I said didn't make much sense, tehy were simply tracking the movement of the star very precisely (probably dopplar, there's no mention of a transit).

      Sorry about that :)

    6. Re:More information here by Simon+Field · · Score: 3, Informative


      Thanks!

      In reading the article, it looks like the "tilt" thay are talking about is not the "tilt" of light, but of the orbit of the planet relative to the line of sight to earth.

      They knew the orbital period from the radial velocity measurements done earlier. Now that they know the orbit is nearly edge-on to earth, they can determine the mass of the planet.

      --

      Free book: Science Toys You Can Make

    7. Re:More information here by Henry+V+.009 · · Score: 2

      Hmmm, I checked the article a little more carefully, and it said they used "astrometery." A google search gave me the HST Astrometery Home Page. If the article and this site are describing the same thing, what they are doing could be a very accurate form of parallex.

      The article talks about splitting the light "by prisms," which might be newspeak for polarization measurements. I don't know--I'll look around for a better article before I make too many more guesses.

    8. Re:More information here by martyb · · Score: 3, Informative
      Does anyone have a pointer to the article that UPI has so badly dumbed-down?

      Check out NASA's Space Interferometry Mission site, especially their page on Planet Detection

      There's much more information on that site that should sate your curiousity. HTH!

  4. Hubble is Big Brother! by Tsar · · Score: 3, Funny

    "If you were in San Francisco and there was a quarter lying on the ground in New York and someone moved the coin an inch, we'd be able to tell," said George Benedict, an astronomer with the McDonald Observatory at the University of Texas in Austin, who is among an international team of scientists using the Hubble Space for astrometry research.

    Seems like an odd use of the telescope, but not as scary as this one:

    "You can't hide massive companions from the Hubble Space Telescope," said Barbara McArthur...

    What if we don't go out, and just hang out at her place and watch pay-per-view?

  5. And in Other News.. by droyad · · Score: 2

    And in other news...

    Scientists have found the composition of an unknown substance using a technique called "chemistry"

    Microsoft has developed the latest version of windows with a technique called "software engineering"

  6. What's going on by Jason+T.+Wright · · Score: 4, Informative
    Most extrasolar planets are found by the precision radial velocity technique. The orbiting planet induces a reflex motion in the star (like in track & field when the hammer thrower leans back and revolves about the center of mass a little while the hammer moves a lot). We can detect this reflex motion as a change in the star's radial velocity. These velocities have magnitudes of hundreds or even tens of meters per second.

    A limitation of this technique is that if a planet orbits its star in the plane of the sky, there will be no radial component to the star's reflex velocity, so we won't detect it. Further, unless the planet orbits with an inclination such that it passes nearly in front of the star, we will measure only a fraction of the total reflex motion.

    This means that when we detect a planet, we can only put lower limits on the mass of the planet, since the signal could be from a massive planet in a nearly face-on orbit, or a tiny planet in an edge-on orbit. This ambiguity is proportional to the sine of the inclination (the "tilt"), so what we measure to be the mass of the planet is actually M*sin(i), where M is the true mass of the planet.

    What these folks have done is use an instrument on HST to make extremely accurate measurements of the position on the sky of a star known to have planets, and used these measurements to measue the path of the star in the plane of the sky as it wobbles under the influence of the orbiting planet. This measures the missing tangential component of the reflex velocity, resolving the sin(i) ambiguity, and determining M itself. This is only the second time anyone has precisely determined the inclination of one of these planets.