Detecting planets around other stars is a difficult project requiring very careful observations. At first finding planets might seem a simple thing to do---take pictures of stars and look for small faint things orbiting them. A planet would indeed be a faint: a billion or more times fainter than a star in the visible band---the glare of the starlight would wash out the feeble light of a planet. Direct imaging of planets would be better accomplished in the infrared band because the planet's thermal spectrum would have maximum emission in the infrared band. Also, stars produce less infrared energy than visible band energy---a planet would only be ten to a hundred thousand times fainter than the star. The planet would still be very faint, but at least the contrast ratio is improved by many thousands of times. The direct imaging technique is able to find jovian planets far from their parent stars. Eleven planets (as of mid-May 2009) have been found this way.
Some of the planets imaged are very young and still quite warm from their formation. Therefore, the young planets are quite bright in the infrared and easier to detect. Some planets have been imaged by blocking the light from the much brighter star with a device called a "coronograph" so that the feeble light from the planet can be detected. Use of a coronograph was essential to create the only visible light (optical) image of a planet (as of the time of writing): that orbiting the very bright star, Fomalhaut, shown below. The black area in the center is the coronograph, the white dot shows the location of the star, the ring is a dusty debris disk analogous to our solar system's Kuiper Belt (but much further out), the small white box shows the location of the planet some 115 AU from its star, and the inset shows its motion over two years of its entire 872-year orbit. Its motion proved it was an object orbiting the star.
Astronomers have detected disks of dust and gas around young stars using sensitive infrared detectors on the largest telescopes in the world. An equivalent amount of material locked up into a single object will have a smaller total surface area than if it was broken up into many tiny particles. The disks have a lot of surface area and, therefore, can emit a lot of infrared energy. Some bright stars in our sky have dust around them: Vega, Beta Pictoris, and Fomalhaut. These are systems possibly in the beginning stages of forming planets. One disk around the star HR 4796A appears to be in between the dust disk stage and a fully-fledged planet system. The inner part of the disk has been cleared away. Presumably, the dust material has now coalesced into larger things like planets. The planets would have a smaller surface area than if the material was still in tiny particles form, so the planets will be much fainter. The Hubble Space Telescope has also detected disks of gas and dust around 50% of the stars still forming in the Orion Nebula. It appears that the formation of planet systems is a common process in the universe.
Another way to look for planets around other stars is to notice their gravitational effect on the stars they orbit. One signature of a planet would be that the star would appear to wobble about as the star and the planet orbit a point situated between them, proportionally closer to the more massive star, called the center of mass. This technique is called the astrometric technique. Our Sun wobbles because of the gravity of the planets orbiting it. Most of the wobble is due to Jupiter which contains more mass than all of the other planets combined. However, the wobble is tiny! Because the Sun is over a thousand times more massive than Jupiter, the center of mass is over a thousand times closer to the Sun, or about 47,000 kilometers above the surface of the Sun (this distance is less than 7% the radius of the Sun). Despite the tiny wobble, astronomers on planets orbiting nearby stars could detect this wobble using the same technology we have here on Earth if they observed the Sun's motion very carefully over a couple of decades. The stronger the gravity between the star and planet, the larger will be the wobble of the star and the easier to detect. Therefore, the astrometric technique is well-suited to find massive jovian planets close to their parent stars. One planet has been found using this technique (at the time of writing). The SIM Lite mission will use this technique in the next 10 to 15 years.
Another signature of a planet would be doppler shifts in the star's spectral lines as they orbit their common center of mass. The doppler shift technique (also sometimes called the radial velocity technique) has been the easiest and most prolific way to find planets around other stars so far. At of the time of writing 325 planets have been found using the doppler shift technique. The searches have so far focussed on stars similar to the Sun, though a couple of systems have planets orbiting a pulsar (a type of ultra-compact, dead star discussed in the stellar evolution chapter---planets found using a variation of the doppler shift technique called the timing technique) and nine systems have M-type red dwarf stars (including one that has a terrestrial-sized planet in its habitable zone). Like the astrometric technique, the doppler shift technique is well suited to find massive jovian planets close to their parent stars. The number of systems discovered and the details about them changes so rapidly that the best place to find up-to-date information on extrasolar planets in on the internet. Some websites are given at the end of this chapter.
The orbital motion of the planets can be derived from the shifting spectal lines and the information about the orbits can be used to derive the masses of the planets. However, the doppler effect tells you about the motion along the line of sight only. The planet orbits are undoubtedly inclined, or tipped, to our line of sight and the amount of inclination is uncertain. This introduces an uncertainty in the derived masses of the planets. Usually, astronomers will quote the masses as ``mass×sin(orbit inclination angle)'', so the actual planet masses could be higher.
Astronomers cannot yet determine the diameters of most of the giant extrasolar planets so their densities, and, therefore, their composition is still unknown. Fifty-nine of the giant planets has been observed to move in front of their stars and cause an eclipse or dimming of the starlight (this is called a transit). This means that the planet's orbit is aligned with our line of sight (and the inclination angle is nearly 90 degrees). From the planet transit, astronomers have been able to accurately measure the total mass and diameter of the planet (and therefore, determine the density). Careful observations of the spectrum of the star while the planet is transitting across will enable astronomers to determine the chemical composition of the planet's atmosphere using spectroscopy. In other cases, the planet's spectrum is found from taking the spectrum of the star plus planet, then taking the spectrum of just the star when the planet is behind the star and subtracting it from the star plus planet spectrum. One planet, HD 189733b, has water, methane and carbon dioxide in its atmosphere but the planet is much too hot and massive to support life.
Besides looking for wobbles in stars, the transit technique is another way of searching for planets around other stars. However, most planetary systems will not have their orbits so exquisitely aligned with our line of sight so a lot of stars would need to be looked at to improve the chances of finding even a few transits. One advantage of the transit method over the star-wobble methods for planet detection is that you would be able to detect terrestrial-diameter planets (i.e., small planets). Small planets like the Earth produce too small a wobble in their parent star because of their small mass to be detected by the star-wobble methods. The COROT mission (ESA) has found a planet less than twice the diameter of the Earth. However, this planet is so close to its star that the planet's surface temperature is 1000 to 1500 deg C! The NASA/JPL spacecraft mission called Kepler is looking at 100,000 stars simultaneously to search for Earth-sized planets during a 4-year period of time. The spacecraft will be focussing on planets that could be in the stars' habitable zones (where liquid water could exist on a planet surface). Only 0.5% of the stars are expected to have their planets orbits in the habitable zones properly aligned for detection by Kepler. A terrestrial planet with mass between 0.5 to 10 Earth masses will cause its star to dim by a fractional amount of between 0.00005 to 0.0004, respectively, and the transits will last just a few hours.
Another method of planet detection uses the gravitational lensing effect discussed in the Einstein's Relativity chapter. When a star passes almost in front of another more distant star as seen from the Earth (the stars are not orbiting each other), the light from the distant star can be warped and focussed toward us by the gravity of the nearer star to produce multiple images of the distant star or even a ring of light if they are aligned exactly right. This lensing effect is too small and the resolving powers of telescopes are too small too see the multiple images. The multiple images will blend together into a single blurry blob that is brighter than when the multiple images are not present (a microlens event). As the nearer star moves in front of the distant star, the nearer star's blurry blob will appear to brighten and then dim as the nearer star moves out of alignment. The microlens event for typical stars in our galaxy moving at typical speeds will last a few weeks to a few months and the amount of the brightness magnification will depend on how closely the near and distant stars are aligned with our line of sight.
The animation above shows an extemely-magnified view of two possible microlens events (what you would see if you had an optical telescope several hundred meters across in space). The brightness of the ring and the combined brightness of the two distorted images exceed the distant star's brightness when it is not lensed. This animation is adapted from a figure by Penny Sackett in a talk about the search for planetary systems using microlenses.
If the nearer star has a planetary system with a planet at the right position, a smaller and briefer microlens event will happen superimposed on top of the star's microlens. By looking for brief deviations in the otherwise smooth increase, then smooth decrease of a stellar microlens event, you could detect the presence of a planet. This method is called the microlens technique. The planet's mass and orbit size could be determined from careful measurements of the brief deviations. The microlens event method can be used to detect jovian-mass and terrestrial-mass planets near their parent stars and the parent stars are distant from the Earth. Like the transit method, a lot of stars must be monitored to pick up even a single stellar microlens event. The microlens events are due to chance alignments that are not repeatable. Eight planets have been found using the microlens technique at the time of writing.
The transit and microlens techniques are not good for looking planets around a particular star of interest. The star-wobble and direct imaging methods are better. However, the transit and microlens methods are useful for determining the statistics of planetary systems in our galaxy, particularly the number of star systems with terrestrial planets in the habitable zones.
The figure below summarizes the orbit sizes and orbit eccentricities of the 293 other planetary systems known at the time this was written from the Extra Solar Planets Encyclopedia. The figures include planet data that still needs to be confirmed. Data for 340 planets are plotted on the left and data for 318 planets with known eccentricities are plotted on the right.
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Two things to notice are how close the large planets are to their stars and the large eccentricities of some of the planet orbits. The Condensation Model outlined in the previous section predicts that large planets will only form far from the young star. Giant planets start from a core of rock and ices that were able to solidify far from the intense heat of the young star. The rock-ice cores then pull in surrounding gas by their gravity. Near the star, the temperature is too high to form the rock-ice cores.
Over a decade before the discovery of the first extrasolar planets, astronomers predicted as part of the Condensation Model that large gas/rock clumps would form far from a young star and spiral inward toward the star because of friction with the gas remaining in the disk around the forming star. The gas/rock clumps can also interact with each other sending one into a small orbit while the other is ejected out of the system. Such interactions may also explain the elliptical orbits we see. Some astronomers working on planet formation models are looking for ways to halt the inward spiral of the gas giant planets near the star through tidal interactions between the planet and star. Perhaps the gas giant planets we see are simply the ones that did not have time to spiral completely into the stars before the gas disk was cleared away by the strong T-Tauri winds that accompany the start of nuclear fusion. Perhaps in our solar system other giant planets had formed but did not survive. Observations of other star/planet formation places and other planetary systems have confirmed various features of the Condensation Model and they have also led to modifications and extensions of the theory in the continual interaction of observation-theory-testing process of error correction of science.
In the next few years, ground-based interferometers will be completed that can image large extrasolar planets. What about Earth-like planets? It is unlikely that life could arise on a gas giant planet. NASA's proposed Terrestrial Planet Finder (TPF) and ESA's Darwin, both space-based missions, should be able to obtain infrared or optical pictures of life-bearing planets. With TPF and Darwin, astronomers will also be able to analyze the spectrum of the planets to determine the composition of their atmospheres. Spectral lines from water would say that a planet has a vital ingredient for life. If oxygen, particularly ozone (a molecule of three oxygen atoms), is found in the atmosphere, then it would be very likely that life is indeed on the planet. Recall from the previous chapter that molecular oxygen quickly disappears if it is not continually replenished by the photosynthesis process of plants and algae. However, it is conceivably possible for a few non-biological processes (e.g., the runaway greenhouse effect with the photodissociation of carbon dioxide and water) to create an atmosphere rich in molecular oxygen and molecular oxygen does not produce absorption lines in the preferred infrared band that would be used in the TPF and Darwin missions. Ozone does. Ozone existing along with nitrous oxide and methane in particular ratios with carbon dioxide and water, all of which produce absorption lines in the infrared, would be very strong evidence for an inhabited world.
The setup and technologies TPF and Darwin will employ will be based on the experience gained from previous projects such as the Keck Interferometer, the Large Binocular Telescope Interferometer, Kepler, CoRot, and the Space Interferometry Mission. Current plans are to have TPF and Darwin launch sometime in the next ten to twenty years.
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last updated: June 1, 2009
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