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Planet Unbound

It seems each month, I get later and later with these paper a month blog entries.

imagesThis month, I went a little out of my element. Inspired by Origins, I decided that instead of my usual monthly biology/chemistry paper, I’d give an astronomy one a shot. This month’s paper comes from Nature and relies on a very cool phenomena called gravitational lensing. The basic idea comes from the fact that light bends in the presence of gravity (see image on left). That bending, which is very similar to the bending caused by the lens in a telescope or a pair of glasses, can be used by experienced astronomers to make conclusions about the “lens”, or the object who’s gravity is causing the light to bend. This is especially useful in cases where the “lens” is impossible or difficult to observe directly, i.e. in the case of dark matter or a brown dwarf. It is, however, “relatively” rare as it requires the lens, the light source, and the earth to all be in a straight line.

imageInterestingly, gravitational lensing can also be used to find planets – which because they tend to be significantly smaller and less bright than stars cannot be directly observed from Earth. How? Picture a planet passing through the line of sight between a star and the Earth. Because the planet itself is small and dim relative to the star, its main effect on the star’s apparent brightness is the gravitational lensing effect causing the apparent brightness of the star to increase when its in front of the star (see image on right) by virtue of bending more of the light from the “background star” on its way to Earth (and, consequentially, decrease when it moves away). Charting the apparent brightness over time and studying the shape of the “light curve” that results from the increase and then decrease in brightness can then provide hints on the mass of the planet, or, to use the words of the paper, “be used as a statistical probe of the mass function of the lens objects.”

So, what happens when scientists train their gravitational lensing-sensitive telescopes on 50 million stars in the the Galactic Bulge? Over the course of a year, they detected some 474 such gravitational lensing events, 10 of which were particularly interesting to them as the duration of the gravitational lensing light curve “blip” was short enough (less than 2 days in length) that it suggested it was a planet microlensing event (see examples of such blips below – the impact on brightness is on the vertical axes and the horizontal axes is time) and a quick scan of the sky/database shows no obvious stars nearby for these planets to orbit.



This next piece of their analysis feels like cheating to me – but it was also fairly ingenious. As the astute observer will note, the shape of the “blip” is only a good “statistical probe” – we can’t actually conclude anything about the mass of the gravitational lens (the potential planet or star that’s causing the gravitational lensing) without knowing the speed at which the lens is moving and the distance of the lens from the star. However, because we generally have a good idea of how massive and abundant stars are, we can actually use statistical modeling to calculate how many planet-sized gravitational lensing events we should expect out of 474. This would in turn also allow us to calculate the ratio of the number of these planets to the stars – a mini-galactic census, if you will.

The chart below shows the basic results of this modeling where the red and the blue curves are based on different means of approximating of the abundance/mass of various stars and the solid black line shows what was actually recorded. Along the vertical axes, we have the number of gravitational lensing events (actual and theoretical) which have a “light curve blilp” of a certain duration (the number on the horizontal axes). If you observe closely, you’ll notice two things:

  1. The theoretical line on the right-half of the graph fits very nicely with the observed data – suggesting that the overall model and our understanding of the abundance/mass of stars is reasonable.
  2. There’s a fair amount more (remember, the scale is logarithmic) observed events where the duration is shorter than 2 days (the left half of the graph) than expected… by a factor of 4-7!

Conclusion: the gravitational lensing numbers suggest strongly that they found a ton of planets with no obvious star that they are orbiting around and, further statistical modeling, suggests that there may even be up to 2 times the number of these “rogue planets” than there are “standard” stars!


Just imagine:  the Milky Way galaxy is potentially teeming with planets that have no stars (or are, at worst, orbiting very far away from their stars.

So, what does this all mean? For starters, its the first validation that the galaxy is teeming with star-less/unbound planets. That, in and of itself is cool, but it begs the question: how did these planets become unbound? And, how many other planets – potentially life-supporting – can we find?

(Image credit: light bending) (other images from paper or commentary piece)

Paper: MOA & OGLE. “Unbound or Distant Planetary Mass Population Detected by Gravitational Microlensing.” Nature 473 (May 2011) — doi:10.1038/nature10092

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  1. Jennifer Tsai Jennifer Tsai

    I haven’t read your post yet, but I just wanted to say that the title made me think of “Prometheus Unbound” rather than Ayn Rand.

    • Ben Ben

      Haha, I realized I was thinking of something else (“Atlas Shrugged”)

  2. […] might have been crazy late with April, but for May, my on-timeliness when it comes to the paper a month posts is returning with a […]

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