Friday, August 14, 2009

News from Other Worlds

This week, I came across some quite amazing news about planets at other stars in our galaxy. But it's not just the stories of planetary collisions and retrograde orbits that have fascinated me: It's also how all this has been learned, by closely analyzing light curves and spectra. So, here are a the plots behind the news:

Direct light from an extrasolar planet

So far, extrasolar planets have mainly been detected indirectly - by dimming the light of the star during transit for example, or by a periodically changing Doppler shift. It's very hard to see the light reflected by the planet itself. This week, the Kepler team has reported clear evidence of the light form planet HAT-P-7b, in orbit around a star in the constellation Cygnus. Here is the proof:

Light curve of the parent star of planet HAT-P-7b, shown over a bit more than one period. From Kepler’s Optical Phase Curve of the Exoplanet HAT-P-7b by W. J. Borucki et al., Science 325 (7 August 2009) 709.
The figure shows the light curve of the parent star, its apparent luminosity over a time span of about three days. As can be seen in the upper figure, the luminosity drops very clearly by about 0.6 percent every two days or so - that's when the planet transits in front of the star, and darkens a bit its disk.

But wait - there is a second slight dip in the light curve right halfway in between two eclipses. Looking at the curve on another scale in the middle of the figure, we see that the luminosity gently increases and decreases by 0.01 percent over one period, with a marked drop back to "normal" in the middle.

This gentle variation of the light curve comes from the light reflected by the planet! The drop in the middle occurs when the planet is hidden behind the star on its orbit.

If only it was possible to detect such small changes in the spectrum of the light, it may tell interesting stuff about the planet.

Planetary collisions at HD 172555

Speaking of spectra... The Spitzer Infrared Space Telescope has observed a dust cloud around the star HD 172555 in the southern constellation of Pavo. The resulting spectrum of infrared light can best be understood by assuming that quite a dramatic planetary collision has happened at this star a few thousand years ago.

Infrared Spectrum of the dust disk around star HD172555. From Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the ∼12 Myr HD172555 System by C.M. Lisse et al., Astrophys.J. 701 (2009) 2019-2032, arXiv:0906.2536v2
The spectrum - the black, noisy curve - shows a general blackbody shape, with additional features that are typical for silicate particles.

Silicates, the matter of sand and dust, contain silicon-oxygen bonds that produce infrared bands whose shape and location also depend of the specific crystal structure, and thus are very characteristic for the different types of silicates. A unique feature in the infrared spectrum of the dust cloud of HD 172555 is the sharp peak at a wavelength of about 9 micrometre. This peak can be understood as produced by a mixture of mainly two types of silicates, tektite and obsidian.

There is something special about these two silicates: They are glass-like, and they are produced by melting and rapid cooling of other silicates materials. Tektite is a telltale sign of the impact of large meteorites on the Earth's surface.

Now, for finding such large amounts of tektite and obsidian in the dust cloud of HD 172555, there is just one plausible explanation: There must have been a collision of Moon- to Mercury-sized planets orbiting the star, similar to the collision that happened to the early Earth which is supposed to be the origin of the Moon!

Retrograde planet WASP-17b

A collision might also have caused the retrograde motion of planet WASP-17b, which orbits star WASP-17 in the constellation of Scorpius.

One usually assumes that a star and its planets originate by collapsing from the same rotating cloud of dust. Hence, the rotation of the star and the revolution of its planets should be in the same sense - the spin of the star and the angular velocity of the planets should be parallel. This is the case, for example, for all the planets in the solar system. A planet revolving "in the wrong direction" is called retrograde.

Surprisingly, it is possible to check if the revolution of a planet is normal or retrograde if the planet transits the star. Due to the rotation of the star, one half of it has a component of motion towards our line of sight, while the other half is moving away from us. This implies a Doppler effect towards the blue and to the red for the different halves of the disk of the star, respectively. When averaging over the whole disk of the star, this results in a broadening of the spectral features. But when a planet transits the star and thus blocks parts of its light, a net Doppler effect can bee seen. This is the so-called Rossiter-McLaughlin effect.

Doppler shift of the light from star WASP-17 during the transit of its planet, WASP-17b. From WASP-17b: an ultra-low density planet in a probable retrograde orbit by D. R. Anderson et al, arXiv:0908.1553v1.
The curve shows the Doppler shift of the light from star WASP-17, measured with the CORALIE spectrograph shortly before, during and after the transit of its planet WASP-17b. If the planet revolves in the same direction as the star rotates, there should first be a net redshift: The planet appears in front of star on the side which is moving towards us, thus blocking the blueshifted light. For the analogous reason, there should be a net blueshift at the end of the transit. This "normal situation" corresponds to the grey, dotted spike-like curve in the figure.

But the data points evidently fit much better to the opposite case: First a blueshift, then a redshift. This is the sign of a retrograde orbit!

I do not know if there is more than mere speculation as to what has happened to planet WASP-17b in the past to make it revolve in "the wrong direction" - but maybe we can learn more from future spectra.

... and a meteorite on Mars

After all these abstract curves which have such vivid interpretations, here is very concrete photo (thanks to Andi for the link):

It is an iron meteorite, lying on the surface of mars, and stumbled upon by the Mars rover Opportunity.

Without any detailed spectroscopy, it also seems to tell us something: When the meteorite fell on Mars, the Martian atmosphere must have been much denser than it is today, because otherwise the impact on ground would have been strong enough to destroy the meteorite and create a big crater.

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  1. We may not be see/deduce a world in a grain of sand yet, (or heaven in a wild flower); but it is amazing what a few photons can tell us!

  2. Excellent post Stefan. I read with great interest, both of the fascinating discoveries and the methods.

    I look forward to your posts on LHC experiments when it finally boots up!! I want the juicy technical details too.

  3. Fascinating post. I love how the changing phase of HAT-P-7b is visible in the light curve!

    I guess Antarctica is also good for meteorites, because it's dry and there isn't much erosion or sedimentation, so it's not so surprising in retrospect that there would be some laying around on Mars. But if it's been there since the atmosphere was thicker, wouldn't it also be more eroded?

    Maybe it is eroded? I heard an interpreter say the crenelations on Willamette Meteorite were from water erosion over a few hundred years, but I thought that lumpy texture was characteristic of fresh iron meteorites as well.

  4. Hi Stefan,

    A nice piece focusing attention on the exciting things that have been going on in the search and study of exoplanets. It’s also something I’ve been discussing lately with a long time friend of mine. He pointed out to me much of what you have written about here and something else which I was completely unaware.

    This being that in at least one case they have been able to measure the temperature of such a planet and determined it have an atmosphere yet with wind speeds which are extremely high in velocity. This has been determined since its rotation. although tidally locked due to its closeness to its sun it has temperatures globally that are much more even then one would expect in such situations. Also. the zones of temperature peaks are not in areas that are at right angles to its surface, in relations to its star.

    This I find as all fascinating stuff which holds out the promise of even future discovery. Personally I can’t wait for the day they find another blue marble orbiting a distance star so that we may wonder if someone out there may be looking back perhaps thinking the same.

    Just as a related aside and in context of all these Newton quotes that where earlier thrown around, it’s interesting to speculate about what other then gravity and creation was occupying Newton’s mind when he wrote the following in his principia.

    “This most beautiful system of the sun, planets, and comets, could only proceed from the counsel and dominion of an intelligent and powerful Being. And if the fixed stars are the centres of other like systems, these, being formed by the like wise counsel, must be all subject to the dominion of One; especially since the light of the fixed stars is of the same nature with the light of the sun, and from every system light passes into all the other systems: and lest the systems of the fixed stars should, by their gravity, fall on each other mutually, he hath placed those systems at immense distances one from another.”



  5. Phil,

    As you undoubtedly know, Newton's theological efforts consumed more of his life than did physics.


  6. Thanks for the interesting summary! We also wrote previously about detection of extrasolar planets through gravitational microlensing, which I find a particularly nice technique.

  7. Dear Arun,

    ... it is amazing what a few photons can tell us!

    A propos a few photons: there is other interesting news as well, coming from single photons: Testing Einstein's special relativity with Fermi's short hard gamma-ray burst GRB090510 (arxiv:0908.1832):

    "... Even more importantly, this photon sets limits on a possible linear energy dependence of the propagation speed of photons (Lorentz-invariance violation) requiring for the first time a quantum-gravity mass scale significantly above the Planck mass."

    Cheers, Stefan

  8. I might write some lines about that GRB paper later. I'll first have to finish the inventory of my boxes...

  9. Dear Bee,

    while writing, I was indeed reminded of our "Plotl a Day" series :-)

    Cheers, Stefan

  10. Hi Rillian,

    I heard an interpreter say the crenelations on Willamette Meteorite were from water erosion over a few hundred years, but I thought that lumpy texture was characteristic of fresh iron meteorites as well.

    I haven't heard of the Williamette Meteorite before, thanks for the pointer.

    I guess if this Martian meteorite has eroded by water after landing on the planet, that would be quite exciting as well, as evidently today, there seems to be no water around.

    Best, Stefan

  11. Hi Arun,

    As you undoubtedly know, Newton's theological efforts consumed more of his life than did physics.

    Yes I’m aware of this, yet this was not my intent in quoting him in this instance. It’s more that there existed some even centuries ago who with the aid of science could imagine that other worlds existed far beyond our own. It’s also true that in all likihood if Newton had been more explicate about what he was thinking the consequences could have been quite unpleasant. Let’s then give thanks that because of discoveries like his own and those that preceded and followed we can now talk about such things without fear of the flame being put to our feet.



  12. I remember that in a previous Bee's post about another GRB they were talking about a possible modification of the dispersion relation at around 10^18. Then I guess just one photon can't tell us enough after all.

  13. The previous posts about constraints on Lorentz Invariance Violation from GRBs you are referring to are here, here and here.

  14. At the recent 7/22 solar eclipse in China, I measured a light curve of the eclipsing sun:

    My guide was Roland Zeidler, a German national living in Chengdu.

  15. Hi Stefan,

    finding a proof or a possible proof, how our earth and our moon has been developed is very exciting. I usually read Theory articles and I am so in theory that I forget to read some experimental papers. So it's good when you or Bee come up from time to time with such interesting news from the experimental front.




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