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

Since the first planets orbiting other stars (exoplanets) were discovered in the 1980′s, the field has come a long way.  Already several hundred have been discovered- in fact, they are being found so quickly that we are hesitant to give an exact number, for fear that it may be outdated by the time this article gets posted.

Unfortunately, the early discoveries of exoplanets were limited by the type of method that was used.  Popularly called the “wobble method,” this technique took advantage of a simple fact: if there is a very large planet orbiting a star, the gravity of that planet causes the star to move.  As the big planet swings around in its orbit, the star wobbles back and forth.  If you have instruments of sufficient sensitivity, you can measure this motion and deduce the presence of the planet.

There is only one problem with this method.  Only extremely big planets have enough gravity to cause a star to wobble.  Smaller planets like Earth just don’t have enough pull to move a star to any measurable degree.  Because of this, they are invisible to the wobble method.

This limitation has dictated the kind of planets that could be found.  To date, they have all been enormous worlds with powerful gravity.  Some of them are many times larger than Jupiter, and orbit very close to their primary stars.  Scientists have invented a new category into which these bodies fall: “hot Jupiters.”  At the other extreme, they have also found some huge ice worlds.  These monstrous planets were certainly interesting from a scientific point of view, but we knew that we weren’t really seeing a representative sample.  We knew that there were probably smaller planets out there, some of them possibly similar to Earth, but this method just couldn’t show them.

Obviously, we needed a new detection method, something that wasn’t so dependent on size.  To answer this need, the transit method was invented.  This method uses a curious fact that was noticed by astronomers looking at certain stars.  When these stars were observed continuously over a long period of time, their total output of light would occasionally drop for a little while, then jump back up to its former level.  If you waited long enough, this dimming would happen again and again, and it would always be exactly the same; the light would decrease by the same percentage and remain at that level for the same amount of time.

The explanation was clear.  There was a planet orbiting that star, and its path happened to pass through our line of sight (in other words, the planet was “transiting” the star).  When it came between our telescope and the star, of course this caused a slight decrease in the amount of light reaching us. Theoretically, if we could get a precise enough measurement, we should be able to tell the size of the planet.  The recurring pattern would tell us the orbital speed of the planet, and from that, we could calculate the planet’s distance from the star.  Measuring the brightness of the star would tell us how hot it was, and that fact, plus our knowledge of the planet’s distance from the star, could give us an idea of how hot the planet was.  Knowing that, we would also know whether water could exist there in liquid form.

What luck!  Suddenly we had a single measurement that could tell us how big a planet was, how long its year was, how hot it was, and therefore, whether it could have liquid water.   In the world of science, it rarely works out so neatly.

Unfortunately, this method also had its limitations.  There were two big ones here, one that could be remedied by technology, the other inescapable.  The first one was the fact that such sensitive measurements could not be performed from the Earth’s surface.  Wind currents in the upper atmosphere have a blurring and dimming effect on starlight, and the kind of precision necessary for these measurements would not be possible.  To use this method to its best advantage, the observations would have to be made from a space-based telescope.  The other limitation has to do with the nature of the method itself.  Obviously, it can only work if the planet’s path happened to pass between our telescope and the primary star.  If that didn’t happen, we would never know the planet was there.

On the other hand, the big problem that we had with the wobble method, the fact that it’s only capable of detecting very large planets, would not be an issue with the transit method.  Theoretically, if your instruments were sensitive enough, you could detect even very small planets with this method.

So now we have two methods for finding planets, and they each have their strengths and their limitations.  We will continue to use both of these methods in the future; they can both tell us things, and by using both, perhaps we can get a more complete view of the universe.

However, the transit method has one thing that the wobble method doesn’t have: the ability to find planets like Earth.  It can tell us how big they are and how hot they are- two crucial factors in determining whether they can support something that matches our Terrestrial definition of life.

Of course, there could be some exotic lifeform out there that drinks molten iron for breakfast, and thinks a hot Jupiter is the ideal place to live.  If we explore enough planets, we may eventually find such extremophiles.  But if we are looking for worlds like ours, with lifeforms that bear at least some basic similarity to ourselves, these are the planets where we’ll find it.

And if we are looking for places we can colonize someday, this is the method that will show them to us. Only with the transit method can we find little pebbles like Earth, where we can live on a long-term basis.

NASA built the Kepler space telescope with all of this in mind.  The strategy was to put a telescope in orbit around Earth, train it on a certain segment of the sky, and just keep it there for a long time.  The satellite would adjust its own orientation so that it always pointed at that one spot, and over the course of several years, hopefully it would detect the periodic dimming of stars that signals the presence of a planet.  With no atmosphere to blur the image, it was hoped that Kepler would be able to detect planets down to half the size of Earth.

Well, it’s a year on now, and Kepler still hasn’t found any Earth twins- but it has found five larger planets, which have been named Kepler 4b, 5b, 6b, 7b, and 8b.   Kepler 4b, the least massive of the planets found by Kepler, is about the same size and density as Neptune- but there the similarity ends; Kepler 4b is a hot world, blasted by 800,000 times as much solar radiation as Neptune gets.  Kepler 5b is two times Jupiter’s size.  It orbits so close to its primary star that iron is liquid there.  Kepler 6b is .7 of Jupiter’s size, and it, too, circles very close to its star, whizzing around it in only 3.23 Earth days.  Kepler 7b has an unusually low density, about the same as styrofoam.  Although it is about one and a half times the size of Jupiter, it has less than half its mass.  Rounding out the ensemble, Kepler 8b is another searing fire world, with a year that only lasts three and a half Earth days and an average temperature of more than 1700 degrees Kelvin.

Don’t worry, the smaller planets will be found later.  These big ones are orbiting close to their primaries, moving very fast, so they passed into our line of sight soon after we started looking.  The Earth-like planets will be farther from their primaries, therefore moving more slowly, and may take longer to move into our line of sight.  This is the reason why Kepler is kept pointed at the same part of the sky for its entire mission.

NASA scientists are elated at these preliminary results, and are optimistic about finding Earth-like planets later.  Of course, Kepler will miss some planets because their orbits do not cross its line of sight, but it will be surveying an unusually large segment of the sky filled with many stars.  With so many chances, it is hoped that it will find many Earth analogs.

Kepler is a mission for the far future.  We are a long way from visiting other planetary systems, and colonization is just a distant dream.  But these dreams will come true eventually, and to make that happen, the groundwork must be done now.  If we can survive long enough, we will find these Earth-like worlds and visit them.  When we do, it will be the result of the work done by Kepler.

Sources:

“Kepler: a Search for Habitable Planets- Mission Overview” at NASA website:  nasa.gov/mission_pages/kepler/overview/index.html

NASA News: “NASA’s Kepler Mission Celebrates One Year in Space” at NASA website:  nasa.gov/mission_pages/kepler/news/one_year_anniv.html

Alexander, Amir: “Planetary News 2009: First Light for Kepler” at website of the Planetary Society:  planetary.org/news/2009/0417_First_Light_for_Kepler.html

“Planet Quest- Exoplanet Exploration: Kepler” at website of the Jet Propulsion Laboratory, California Institute of Technology:  planetquest.jpl.nasa.gov/missions/keplerMission.cfm

Cowen, Ron: “Kepler Space Telescope Finds its First Extrasolar Planets,” Science News magazine, January 30, 2010:  sciencenews.org/view/generic/id/52465/title/Kepler_space_telescope_finds_its_first_extrasolar_planets

Kepler mission page at NASA website:  kepler.nasa.gov/

Kepler in Brief: “NASA’s First Mission Capable of Finding Earth-size and Smaller Planets Around Other Stars” at NASA website:  kepler.nasa.gov/Mission/QuickGuide/

NASA News: “NASA’s Kepler Space Telescope Finds its First Five Exoplanets” at NASA website:  nasa.gov/mission_pages/kepler/news/kepler-5-exoplanets.html