Wednesday, 03 June 2009

• What did you learn today?

  
  Here are some things I've learned this morning.  You should leave a comment, send me a message, or link me to a blog you've written about what you've learned today.  I will post the best submissions tomorrow.
  
  This morning I have learned that dictionary.com has changed their front page, and that "erudite" means, "characterized by great knowledge; learned or scholarly: an erudite professor; an erudite commentary."
  
  I've also learned that one of its synonyms, "sapient", means, "having or showing great wisdom or sound judgment."
  
  I've learned that we have discovered the largest extrasolar planet to date (called VB10b, it is too massive to be a brown dwarf, but not quite massive enough to become a star) by using a method that has never worked before!  The planet is so large (about as large as the dwarf star it orbits, though the star is much more dense), roughly six times the size of Jupiter, that we can detect its presence by viewing its gravitational pull on its parent star directly! 
  
  
  

  How can we tell if a star is wobbling?
  

  
  Go outside and stand by a highway, close your eyes and really get in touch with how you know a vehicle is moving towards you or away from you.  Do you hear how the sounds the car produces are higher pitched as it approaches and lower pitched as it moves away?  Why is that?
  
  

  
  

  
  

  It's because as the vehicle moves towards you (imagine you're to the left of the image), it compresses the sound waves closer together, which bends the pitch higher.  And when it moves away, it elongates the sound waves, making them sound lower.  This is called the Doppler Effect, and it works with light too!  The difference is that when light waves get compacted, they shift to become more blue, and they become more red as something is moving away from us.  (Interesting tid bit: virtually all objects in space, from our vantage point, are red-shifted.  What conclusions can you draw from that?) 
  
  This is how we "see" wobbles in stars and detect the gravitational influence of other planets: we can see fluctuations in the star's Doppler shifts (called "radial velocity").  It'll be red, then blue, then red, then blue, and so on.  This means that the star is wobbling because something is pulling on it.  It's also why almost all the stars we've found have been Jupiter-like gas giants, which have the mass to affect their star.
  
  However, with VB10b, it is so massive and VB10 (its parent star) has such a low mass (as far as stars go, it still has much more mass than the planet), that we can actually see the wobble without using Doppler shifts! 
  
  

   
  

  
  Just viewing the wobble with the naked eye, so to speak, is really hard to do.  Light bends as gravity yanks on it, so it's very hard to get an accurate reading in which you have removed all other factors (indeed, it took the team 12 years to do it).  Thankfully, VB10 is only about 20 light years away (a distance so great that your brain cannot even come close to imagining it, but a relatively short distance in astronomical terms), which made this phenomenal achievement manageable.
  
  This discovery makes VB10 the lowest mass star known to have a planet, a record it is likely to keep for some time since if it were a little lighter it wouldn't even be a star.
  
  

  
  

  
  But here's a mystery for ya.  VB10b orbits its parent star very closely, about thirty million miles out, which is closer than Mercury's orbit to our Sun.  Take a look at the planets of our solar system drawn to scale for a moment and see if you notice anything peculiar:
  
  

  
  

  
  Do you notice how all the big ass planets are further away from the Sun (in fact, the outside planets are so massive that Jupiter's moon Ganymede and Saturn's moon Titan are both larger than Mercury, and Jupiter's moon Callisto is only slightly smaller)?  The little planets, Mercury, Venus, Earth, and Mars, are called the terrestrial planets.  They're mostly made of rock and are much more dense (though far less massive) than the big planets, which are called the Jovian planets.  The jovian planets are composed of gas (while Jupiter and Saturn have rocky cores, they lack a solid surface, as the pressure generated by their gravity compresses helium and hydrogen into a pseudo-solid!) rather than rock, which is why if you had a bath tub big enough to set Saturn in, it would float.
  
  Here's the mystery.  We know how our solar system formed, via an accretion disk around our fledgling star.  Our theories about this predicted that in such disks, terrestrial type planets would almost always form close to the star with the jovian planets coalescing further out.  However, once we started discovering extrasolar planets, we began to see that this is not the case at all!  As far as I know, we're not exactly sure why this is.  I'll be sure to ask MSU astrophysicist undergrad (atheist-to-the-stars and friend of the blog :D) Aron McCart and report back with what he tells me.
  
  Anyway, this is not the only way to detect a star, and if you really want to read about the magnificent complexity of one of the methods (gravitational microlensing), you owe it to yourself to read Aaron McCart's paper on gravitational lensing that I posted a while back. 
  
  I'm sure that's enough nerding about for one day.  What did you learn today?
  
  
  

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