This is the second article in our examination of the newly-emerging science of exoplanet detection and study. Last week, we took a look at NASA’s Kepler telescope, which recently completed its first impressive year of service. (That article is still posted at this site.) In this article, we will talk about the COROT satellite, launched by the French space agency, Centre d’Etudes Spatiales (CNES).
The COROT Space Telescope has been in orbit around Earth since 2006. The satellite was launched by CNES with equipment contributed by other ESA members, and is only one of a growing array of satellites designed to find planets orbiting other stars. So far, it has been highly successful, performing even better than expected and locating an interesting assortment of planets which will be studied for years to come. COROT is also using a method of star study called asteroseismology, which was first employed in the study of our own sun and is now being used to study others.
The detection of exoplanets has come a long way in a short time. It’s hard to believe it’s only been 15 years since the first planets outside our solar system were found. Since then, hundreds of new exoplanets have been spotted, most of them very different from the ones we are more familiar with. In fact, it’s already time for the next step in this field, the use of new techniques and the development of new equipment that will greatly expand our planet-finding capabilities. Already the equipment used for the early detections is looking antiquated, and is rapidly being outstripped by a new generation of devices and methods. While the old techniques will still be used, they are becoming much more sensitive than they were in the beginning, allowing the detection of bodies that could not have been seen before, and giving us more detailed information about them.
As we saw in last week’s Kepler article, the early efforts to find planets orbiting other suns were hampered by the limitations of the technology available. Early detections were made by using the wobble (radial velocity) method, which takes advantage of the fact that very large planets cause their primary stars to wobble as they orbit them. This worked great for finding monster planets that orbit very close to their primaries, because they make the most pronounced wobble in the star. Smaller planets like Earth could not be seen by this method, since they did not cause a big enough wobble to be detected.
Already this situation is changing, in two ways. As we saw in the Kepler article, the wobble method is now being used in conjunction with the transit method, by which planets are found because of the dimming of the light of their primaries as they pass in front of them. If a star is found that dims periodically, and the amount of dimming is the same every time, we can be sure that there’s a planet there, passing between us and the star. This method allows the detection of smaller bodies, since it is not dependent on the mass of the planet.
The other way in which the situation is changing is that the wobble method is getting more precise and sensitive now. In our earlier article, we asserted that in the early years of planet detection, it was impossible to see planets the size of Earth with this method, since they are not big enough to cause a perceptible wobble in their star. That was true back then- but in doing the research for this article, we found that the wobble method is now becoming sensitive enough to pick up the movement of a star as an Earth-size planet moves around it. We stand corrected; science is moving fast, and sometimes it’s hard to keep up.
These changes are now embodied in COROT and the program of which it is a part. The new generation of exoplanet detection starts here.
COROT was launched onboard a Soyuz-Fregat rocket from the Baikonur cosmodrome in Kazakhstan on December 27, 2006, and entered a near-circular orbit which ranges from 537 to 544 miles above Earth’s surface. Its method of observation was to point itself at a specific section of the sky and simply keep pointing at it for 150 days. During this time, hopefully some of the stars in that section would experience a planetary transit, and COROT would detect them. When that observation period was over, the satellite would turn itself toward another part of the sky and observe it for another 150 days.
As we said earlier, COROT actually has two scientific goals. While it’s observing a star in search of that telltale dimming, it will also be conducting asteroseismological measurements of the star. This is an exciting new area of research which promises to give us much knowledge about the inner workings of stars. Here’s how it works:
The inside of a star is a turbulent place. Matter there is subject to intense gravitational forces, Coriolis forces and pressure. As these factors interact with each other, they cause the star to vibrate in a changing series of patterns, or modes. While the forces that generate these vibrational modes may be happening deep inside the star, the vibrations reach all the way to its surface, and cause slight changes in the star’s brightness. The frequency, amplitude and duration of these modes, as revealed by the changes in the light of the star, can tell us things about the star’s mass, age and chemical composition.
This technique provides us with a source of information on the inner workings of stars, and as we learn to read that information better, it promises to give us much data on stellar evolution. For some years, the ESA’s Solar and Heliospheric Observatory (SOHO) has been using asteroseismology to study our own sun. What we need to do now is conduct the same kind of study of other stars, to find out how typical our sun is, and to compile a catalogue of different vibrational signatures for a wide variety of stars.
COROT is starting this work. Since it will be staring at the same stars for a long time, it will be able to observe the slight changes in brightness that are used for asteroseismology. By mission’s end, it should have compiled a sizable catalog of stars’ vibrational signatures, which will actually begin the science of interstellar asteroseismology.
The COROT mission started off in a spectacular way. Once in a while, you get one of those rare surprises, a piece of space equipment that actually performs better than expected. Within 60 days of its deployment, COROT was sending back data of exceptional clarity and detail, amazing even its designers. The first planet that it found, called COROT-Exo-1b, was a gas giant with about 1.78 times the radius of Jupiter, orbiting a yellow star similar to our own. The unexpected accuracy of the data made scientists hopeful that it might be able to detect planets even smaller than they had thought. While they had conservatively hoped that COROT would be able to see planets a few times bigger than Earth, these results showed that it should be able to see them all the way down to Earth-size or even smaller.
The asteroseismological part of the mission was also an immediate success. While observing this star, COROT showed large variations in the light of the star over a time scale of several days, driven by magnetic activity deep within the star. This information was of outstanding accuracy, with an error of only five parts in 100,000. While the information will be studied for years to come, scientists can already say that this star is very similar to our own sun, both in its outward appearance and its asteroseismic characteristics.
So far, so good- but there is one drawback to the transit method of detecting planets. Even if you find a star with a periodic pattern of dimming, you still can’t be sure there’s a planet there. Some stars have a variable output, which causes a very similar dimming- so when you find a star that shows this kind of behavior, you can’t be sure you’ve hit paydirt unless you get corroboration by some other method.
The stars found by COROT are examined by ground-based telescopes, and this is where we see the new sensitivity of the wobble or radial transit method, because one of the ways of corroborating these findings is to look for the slight change in the star’s velocity that is caused by the orbiting of a small, rocky planet. All stars move, of course. They are all whirling around their galactic centers, and the galaxies themselves are moving in a vast, universe-size ballet. This change of the star’s velocity that they’re talking about is really just another way of expressing the wobble: the alteration of the star’s movement caused by the gravity of an orbiting body. That’s why they call it the radial velocity method (though even the NASA literature will often use the term “wobble method”).
COROT has found several more planets to date, including one which is almost as small as Earth. It’s way too close to its star to have life like us, but it’s a big step in the right direction. Someday soon, this telescope may find a tiny, wet pebble surrounded by a bubble of air- and when we look at it, maybe something similar to ourselves will be looking back at us.
Exoplanet detection is an exploding field, and new findings are constantly coming in. Each telescope is more sensitive than the last, and we can now say with confidence that it is only a matter of time before we find Earth analogs orbiting other stars. If that happens, you know we’ll tell you about it, right here.
Sources:
COROT: “COROT Team Announces the Detection of Smallest Exoplanet to Date” at website of the European Space Agency: sci.esa.int/science-e/www/object/index.cfm?fobjectid=44131
ESA News: “COROT Surprises a Year After Launch” at website of the European Space Agency: esa.int/esaCP/SEMF0C2MDAF_index_0.html
Ellison, Doug: The Planetary Society blog: “Europlanet: COROT- Preliminary Results” at the website of the Plantary Society: planetary.org/blog/article/00001089/
Lakdawalla, Emily: The Planetary Society blog: “COROT Has Bagged Its First Planet” at website of the Planetary Society: planetary.org/blog/article/00000960/
Planetary News: Extrasolar Planets (2007): “COROT Sees First Light” at website of the Planetary Society: planetary.org/news/2007/0124_COROT_Sees_First_Light.html
Cowen, Ron: “The Hunt For Habitable Planets” in Science News magazine, December 20, 2008: sciencenews.org/view/feature/id/39031/title/The_Hunt_for_Habitable_Planets
ESA Space Science: “COROT Objectives” at website of the European Space Agency: esa.int/SPECIALS/COROT/SEM20ZC4VUE_0.html
ESA Space Science: “COROT Overview” at website of the European Space Agency: esa.int/esaSC/120372_index_0_m.html
“COROT Astronomy Mission: From Stars to Habitable Planets” at the website of the Centre National d’Etudes Spatiales: smsc.cnes.fr/COROT/index.htm