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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TAGS: astronomy, spectroscopy, extrasolar planet