Le Panoptique

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Hunting for Planets in the Starry Wild

Publié le 1 août, 2008 | Pas de commentaires
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How many planets are there in our solar system? This would be a relatively easy question, were we to forget the demotion of Pluto to minor planet status in 2006. What about the planets outside of our solar system? Could any of those planets support life as we know it here on earth?

Freya’s Planets
Ian Davis, Freya’s Planets, 2008
Certains droits réservés.

According to exoplanets.org (1), at last count there were 228 exoplanets (i.e., planets outside of our solar system) known to orbit other stars in our galaxy. This may come as a surprise, but it should be mentioned that most of these planets have very little in common with Earth, and as such may seem of little interest, at least until we give them a closer look. With recent galactic surveys uncovering exoplanets that weigh in at only a few Earth masses, or that show traces of water in their atmospheres, the rock we call home may in fact have distant cousins out there. If, then, we were to find life on those planets, not only would we no longer be alone in the universe, we might also better understand the origin and evolution of life on our planet.

The laws of gravity dictate that any object in orbit around a star causes a « wobble » in the star’s motion. FOR example, imagine spinning around while holding a string with a bowling ball attached at the other end. Because of the difference in mass, at first glance it seems as if the ball rotates around you while you simply spin on the same spot. Your friend however, who is there to make sure you don’t hurt yourself, notices that your position does change slightly as you are spinning, although much less than that of the ball. What is in fact happening is that both you and the ball are moving around a common center of mass.

Since we are not able to directly observe planets around other stars, even with our most powerful telescopes, we must rely on the oscillations they induce in the motion of their host stars. In theory, one would see a star orbited by one or more planets as slowly moving back and forth in the sky. In practice however, our technology is only now beginning to resolve such tiny displacements, and no planets have yet been discovered by this approach. The acuity of this method is equivalent to a football fan watching his favorite team play on the Moon while sitting in his backyard on Earth.

Instead, astronomers use a less direct, yet efficient, technique to measure the movement induced on the star by a planet, called the radial velocity method. When measuring the luminosity of a « wobbling » star, they can distinguish between the light emitted when the star is farthest from and closest to us. The information gathered through this technique serves to infer the speed at which a given star rotates around the center of mass of its system, which in turn allows astronomers to calculate the mass of the planet responsible for the wobble and its distance from the star. The main instruments necessary to achieve this are a large telescope and a really good charge-coupled device (CCD). Composed of a grid of pixels, this electronic image sensor—which can also be found in any regular digital camera—converts incoming light into electrons. The amount of charge in each pixel is measured and ultimately translated into a photographic image, the quality of which varies depending on the sensitivity of the CCD (i.e., its ability to pick up faint light) and its susceptibility to electronic noise. Modern astronomical CCDs, which are more advanced (and more expensive) than those in digital cameras, are able to detect variations in a star’s rotation of as little as 3 m/s.

Exotic New Planets

The radial velocity method has meant the discovery of more than 90% of exoplanets, most of which are gas giants comparable to Jupiter. But unlike Jupiter, they are often located very close to their host stars (about one hundredth of the 150 million kilometer distance between the Earth and Sun). Their resulting high temperatures have landed them the name of « hot Jupiters », and their very existence has shaken up previously accepted planet formation models, based usually on our own solar system. In response, theorists developed new models, but so far none that can accurately explain all the exoplanets that are being discovered.

Transit photometry is another method used in the detection of exoplanets that provides a way to estimate the volume of planets. By definition, planets are not massive enough to emit their own radiation. Therefore, when a planet transits its host star, it creates a tiny eclipse and causes the luminosity of the star to dim. Based on the extent of this dimming, it is possible to estimate the approximate size of the transiting planet. However, it should be noted that this method may only be employed if the given orbital plane lies along our line of sight. The effectiveness of the method of transit photometry is therefore weaker when compared to that of radial velocity.

For systems where both methods can be used, the mass as well as the volume of planets can be obtained. One such planet, TrES-4 (2), discovered last spring, is the least dense planet currently known. Having the density of cork, TrES-4 would float if placed in a large enough body of water. Beyond its singular characteristics, the planet actually defies current planet formation models which predict a much smaller size for its mass. Georgi Mandushev, who is part of the team that discovered TrES-4, believes that its large volume has to do with extreme heat. Indeed, the planet revolves around its star at only a twentieth of the Earth-Sun distance, and does so in about 3 days, resulting in a predicted temperature of 1260 degrees Celsius! (To compare: the surface temperature of the Sun is about 6000 degrees Celsius.)

Recently, a planet was discovered that is only slightly larger than Jupiter but is remarkably eight times more massive and has a six-day orbit around its star. More baffling to astronomers is the pronounced oval orbit of HAT-P-2b (3). Observations show that planets with oval paths usually take much longer to complete the orbits and are more distant from their respective stars (up to twice the Earth-to-Sun distance), while those that are closer to their stars take less time and have circular orbits. These observations correspond to theoretical models predicting that, as planets migrate towards their stars over hundreds of thousands of years, their trajectory becomes more and more circular (4). Defying these predictions, HAT-P-2b seems to have finished its migration and yet maintains an oval orbit, and therefore constitutes an ongoing mystery.

Finally, the first direct detection of organic molecules on an exoplanet was published this year (5). The atmosphere of HD 189733b shows traces of methane as well as water. Although the planet is a hot gas giant, these results pave the way for astronomers to analyze the atmospheres of worlds that compare to Earth.

Remote Cousins of the Earth?

Detections of Earth-like candidates are becoming increasingly frequent. At just five times the Earth’s mass, rocky Gliese 581c (6) is the lightest planet currently known. While Gliese 581c resides in its parent star’s habitable zone (the region where the temperature is just right for liquid water to exist), climate simulations created by Werner von Bloh and colleagues of the Institute for Climate Impact Research in Germany (7) show that the atmosphere of the exoplanet contains large amounts of methane and carbon dioxide, greenhouse gases which maintain the planet’s surface temperature well above 100 degrees Celsius. However, Gliese 581d, a neighboring planet in the same system, may actually be cool enough for life to exist. Moreover, just as the planetary systems found so far seem to indicate that our own solar system is quite unique, potential life on exoplanets may exist in forms completely unfamiliar to us, as dictated by the specific orbital and atmospheric properties of those planets.

The question of whether life exists on some exoplanets may be settled by future space explorations, such as the upcoming Kepler and Darwin missions. Scheduled to launch in February 2009, NASA’s Kepler is a telescope with a nearly one-meter diameter. Being space-based and unbothered by atmospheric perturbations or day-night interruptions, the instrument is expected to use transit photometry to detect over a thousand Earth-sized planets in or near the habitable zone of their respective stars. Darwin, an ESA (European Space Agency) mission due to launch in 2015, is composed of three three-meter wide telescopes and a communications hub, the objective of which will be to find other potentially habitable planets and analyze their orbital and atmospheric properties.

Though any planets discovered on these missions will be far too distant for humans to reach (given our current technology), they will nonetheless bring us one step closer to determining whether or not we are alone in the universe. If nothing else, as CCD technology continues to evolve, we may eventually be snapping pictures of these planets with our own pocket digital cameras.

References

(1) California & Carnegie Planet Search. 3 Feb. 2008. The University of California. 20 Feb. 2008 .
(2) Mandushev, Georgi, et al. « TrES-4: A Transiting Hot Jupiter of Very Low Density.« The Astrophysical Journal 667.2 (2007): 195-198.
(3) Bakos, Gáspár Á., et al. « HD 147506b: A Supermassive Planet in an Eccentric Orbit Transiting a Bright Star.« The Astrophysical Journal 670.1 (2007): 826-832.
(4) Rasio, Frederic A., and Eric B. Ford. « Dynamical Instabilities and the Formation of Extrasolar Planetary Systems. » Science 274 (1996): 954-956.
(5) Charbonneau, David, et al. « The Broadband Infrared Emission Spectrum of the Exoplanet HD 189733b.« 
(6) Udry, Stephane, et al. « The HARPS Search for Southern Extrasolar Planets XI. Super-Earths (5 & 8 M_Earth) in a 3-planet System.« Astronomy and Astrophysics 469.3 (2007): 43-47.
(7) Von Bloh, Werner, et al. « Habitability of Super-Earths: Gliese 581c and 581d. » Exoplanets: Detection, Formation & Dynamics Proceedings IAU Symposium 249 (2008): 119-121.

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