Planetary Chemistry 3

Atmospheres of Extrasolar Planets
The most remarkable tool astronomers use might not be a telescope nor a computer, but the human imagination. Geoff Marcy & Paul Butler
in final development


In the expanse of the universe, are we humans on Earth the only life?  That answer depends on whether there are other planets with a temperate climate, diverse chemicals, and stable oceans that provided the conditions for biological evolution.

As Earth inhabitants, it is unlikely that humans will in the foreseeable future be able to journey to any planet beyond our solar system in order to do on-site chemical investigations.  It is also unlikely that humans will be able to send mechanical robots to such locations to communicate back to Earth the results of such investigations.  The vast distance and the limitation of nothing traveling faster than the speed of light make such travel and transport impossible.    So we are left with capturing and analyzing the very limited information being carried from such locations by natural processes to provide clues for the chemical composition and processes occurring on extrasolar planets.

HuygensYet the creativity of the human mind and the development of our technology is now providing preliminary evidence about the chemical composition such planets.

During the 4th century BC, two Greek philosophers, Aristotle and Epicurus, expressed opposing ideas about the possible existence of worlds besides Earth.  Epicurus asserted that the universe must be infinite and hence contain an infinity of worlds.  Aristotle argued that Earth was placed at the center of the universe, making it unique in the universe.  But most of the educated peoples in the world adopted the geocentric world of Eudoxos and Ptolemy.  Not until Nicolaus Copernicus suggested that we may live in a solar system did the possibility of other planets, possibly inhabited, become plausible.  Planets beyond the solar system became plausible when astronomer Christiaan Huygens (1629-1695, portrait at right→) first proposed that every star was a distant Sun.  Such distant planets do not emit the huge amounts of light as do stars, but rather reflect roughly a billionth as much light as their neighbor star emits.  And from the earth's perspective, such planets are nearly at the same position in the sky as their much brighter star.  So detecting the much smaller, nearly dark planets orbiting stars require indirect means of detection.

Iodine cellBut the first evidence for the existence of such planets orbiting Sun-like stars was discovered in October 1995.  A planet orbiting a star causes a small wobble in the star which is detectable as a Doppler shift in the spectra of the star. 51Peg Marcy and Butler developed the world's most precise Doppler technique, capable of measuring velocity changes as tiny as 3 m/sec. (slow bicycling!).  Doppler precision is achieved by inserting Iodine vapor in an absorption cell (←shown at left) near the telescope focal point.  By comparing the Iodine absorption lines with those of the wobbling star, Doppler shifts in the star's wavelengths as small as 1 part in 100 million can be distinguished.  Swiss astronomers Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland reported finding the first planet with this technique. (See graph at right→)  They detected the Doppler shift of the star 51 Pegasi in the constellation Pegasus with a period of 4.2 days.  This meant that planet is orbiting at a mere 0.05 astronomical unit (AU) from 51 Pegasi at a speed of about 134 km/sec (83 mi/sec) or 482,000 km/h (299,000 mph), more than four times faster than Earth's velocity around the Sun.  In the decade after the first extrasolar planet was discovered, nearly 200 were found.

There are now three additional methods to successfully detect distant planets:
3 directly observed planetsA planet which transits across in front a star, blocking some of the starlight reaching us has been used to detect planets relatively close to a star.  This was first achieved in 1999 using a relatively small 4 inch amateur telescope.  It has the added benefit of providing enough information to allow for an estimate of the planet's density.

It was originally doubted that gravitational lensing would provide enough detail.  However a number of planets have been discovered when a nearer star bends light coming from a more distant star, often forming multiple images of the distant star. Larger planets which are considerable distance from the nearer star provide slight spikes on the images.

Finally it has been possible to block nearly all the light from a known star and then search nearby for faint planets.  This has provided a mechanism for directly detecting several large, bright planets. (See b, c, d at right→  + is center of mostly obscured star's light.)  Of course such planets must be confirmed as actual orbiting objects by repeat observations.

chemistry of extrasolar planets

Infrared spectra obtained by the Spitzer Space Telescope launched by NASA has been used to gather the first data on the spectra of extrasolar planets.  A team at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, subtracted the spectra of stars when their planets were behind the star from when the planets were in front of the stars.  The difference is presumably due to the spectra of the atmospheres of the planets.

Independent analysis was done by Jeremy Richardson at the Goddard Space Flight Center and by Mark Swain's team at the Jet Propulsion Laboratory of hot HD209458b, with mass similar to Jupiter, 153 light-years from Earth.  Its spectrum showed a low concentration of Sodium and no water vapor.  A similar planet analyzed David Charbonneau at the Harvard-Smithsonian Center, hot, Jupiter-sized HD189733b, 62 light years from Earth, also revealed no water.


Huygens tried to compare the distances to a star with the distance to the Sun by drilling tiny holes in a metal plate.  He tried to match his memory of star brightness to that of the light coming through the various holes when looking towards the Sun.  By then comparing the area of the hole to that of the Sun, he calculated the ratio of the distances.

Procedure 1

Huygen's procedure can be duplicated but you should avoid looking directly at the full, unshielded Sun since ultraviolet light emitted as a part of sunlight can permanently damage your eyes.  One's eyes become adjusted to the intensity of the prevailing light.  Before trying to match the intensity of sunlight through tiny holes, perhaps one should adjust ones eyes to roughly their sensitivity in the darkness of night, but keeping one's head covered with an dark cloth for a half hour or so.  Make provisions for obtaining adequate air for breathing!  Are there flaws with Huygen's method?  Can the method be improved?

Procedure 2

Consider the following graph of the wobble of planet HD70642 obtained by its Doppler shift in spectra colors.  Determine the period of the planet's orbit.  Why are the error bars tall but not wide?  This orbit is nearly circle with an eccentricity close to that of a circle (e = 0).  (Consider how the shape of the curve would be different if the orbit were very elliptical, or if the plane of the orbit was tilted from our perspective.)  This planet is orbiting a star of type G5V close (± 5%) to the mass of our Sun.  If you know some of Kepler's and Newton's physics, you might try to calculate the the distance this planet orbits from its star.  Knowing that and the amount of star wobble, the mass of this planet is estimated to be twice that of Jupiter.
planet HD70642 graph

If you need course credit, use your observations recorded in your journal to construct a formal report.



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created 3 March 2007
revised 14 October 2010
by D Trapp
Mac made