February 06, 2006
Poison In The Ink: The Cool Art of Planet-Hunting
A few weeks ago, astronomers announced they had found a new planet in orbit around a dim red star 28,000 light years away. Called OGLE-2005-BLG-390Lb, the extrasolar world was not only the latest to be uncovered by planet-hunters, it was also the smallest to be detected around a normal star—only five and a half times as massive as Earth.
The new world was detected using a trick of Einsteinian physics called “gravitational microlensing,” whereby warped spacetime around a star acts like a lens to bend and focus light from a distant star directly behind it.
This lensing effect magnifies the distant star’s light, making it appear brighter than it normally would. If the lensing star is also host to a planet, the distant star’s light becomes brighter still. Here's a flash animation of how gravitational microlensing works.
Astronomers use this increase in brightness to calculate the ratio between the lensing star’s mass and the mass of its planet. They can also calculate the distance between the two objects.
OGLE-2005 was only the third planet—and the first rocky one—to be found with the microlensing technique. The other two were giant gas-planets that were several times larger than Jupiter.
The rest of the more than 150 planets discovered so far were found using two other techniques. One of them is variously referred to as the radial velocity or Doppler or “wobble” technique. The other is called the transit technique.
My editor often has to remind me that the general public is usually more interested in what a finding is than in the technical details behind a finding. Most of the time, my editor is right. In this case, however, I think the techniques employed by planet-hunters are pretty cool so the following is a brief primer on how the techniques work and the pros and cons of each:
If the orbit of a planet around its star just happens to be edge-on, then once during every revolution, it will pass in front of the star. If astronomers have their telescopes trained on the star when this happens, they can detect a dip in the star’s brightness.
Scientists compare this to watching a mosquito fly in front of a very large flashlight from two-hundred miles away and trying to figure out how much dimming has occurred. To get an idea of the size-relationships involved, here's a picture (credit: David Cortner) of Venus transiting our Sun:
As this picture shows, the transit technique works best for big planets. Another disadvantage of the the transit technique is that only a small percentage of planets are configured this way around their stars.
Doppler (aka “wobble”) technique:
By sheer virtue of its mass, a star will affect the movement of the planet orbiting around it. This is easy to understand. Something that may be more difficult to wrap your head around (at least for me) is the fact that planets, by virtue of their mass, can cause their host stars to move in small counter-orbits.
Astronomers can detect this tiny “wobble” in the star’s movements, and use it to determine the size and mass of an orbiting planet.
The wobble technique has been the most successful so far in finding extrasolar planets. It was this technique that found the first extrasolar planet around a normal star in 1995, called 51 Pegasi.
Because the perturbations that a planet induces in its star is so small, the wobble technique can only detect very massive gas planets or planets very close to their stars. This requirement rules out the possibility of finding rocky, Earth-like planets that lie within a star’s habitable zone, the space around a star where liquid water can exist on a planet’s surface.
Also, the wobble technique can only detect stars that are within about 160 light years of Earth and it can be slow. Astronomers must watch a planet make one complete orbit before they can be sure that what they’re seeing is the effect of a planet on its star. For small planets close to their stars, this isn’t too big a deal, but for big gas giants that are quite a distance away, orbit times can take many years.
Like the other two techniques, gravitational microlensing has its strengths and weaknesses.
Currently, microlensing is the only planet-finding technique capable of detecting low-mass, rocky planets within a star’s habitable zone. It can also find planets that are very far away. Whereas the transit and wobble technique can only find planets that are hundreds or thousands of light years from Earth, respectively, microlensing can find stars that are tens of thousands of light years away.
Microlensing is also the only one of the three techniques that can find planets around small, dim stars like red dwarfs. That's because unlike the wobble or transit technique, microlensing doesn’t rely on the detection of light from a planet’s host star.
As for its cons, microlensing yields very little information about a planet compared to the other two techniques. With the transit technique, astronomers can not only glean information about a planet’s size and its distance from its star, they can also take measurements of its atmosphere. And in the wobble technique, scientists know how long a planet takes to orbit its star.
A planet-detection made with microlensing, in contrast, yields only two types of information: the mass ratio between the star and the planet and the distance between the two objects. Critics point out that determining a star’s mass is very difficult, so if that’s off, calculations about the planet's mass will be off too.
Another common criticism of microlensing is that it relies on a precise alignment between a distant star, a planet-hosting lensing star and observers on Earth. This type of alignment occurs only once in all of cosmological history for any two stars. A microlensing experiment can therefore never be repeated or verified by other scientists at a later date.
Many astronomers view this as an acceptable trade-off, however, because of microlensing’s speed (OGLE-2005 was confirmed in a day) and the type of information it can provide.
Microlensing is the shot-gun approach to planet-hunting. It’s a quick and dirty way for astronomers to take a galactic planet census to see what types of planets exist in our Milky Way and determine how common each type is.
Astronomers also point out that what a microlensing experiment lacks in repeatability, it makes up for in simultaneous verifications made with numerous telescopes scattered around the globe. The detection of OGLE-2005, for example, involved 73 researchers and 32 institutions worldwide.
The next generation of planet-finding tools:
The holy grail for planet-hunters, of course, is to find a habitable Earth-like planet, but many scientists don’t expect this to happen until the next generation of space telescopes are deployed, which won’t be for nearly another decade at least. Two projects that have received a lot of attention are NASA’s Terrestrial Planet Finder (TPF), set for launch in 2014, and the European Space Agency’s Darwin, also expected to launch sometime around the same time.
The TPF will consist of two observatories. One will carry an ultra-sharp optical lens that will be at least 10 times more precise than the Hubble Space Telescope. This could potentially allow astronomers to directly detect an extrasolar planet for the first time.
The other TPF observatory will carry an infrared interferometer that will allow it to detect dim planets despite the bright glare of their parent stars. The interferometer will also be capable of analyzing the reflected starlight from a planet to detect the presence of gases like carbon dioxide, water vapor, ozone and methane. If scientists know the relative amounts of each gas that a planet contains, they may be able determine whether a planet is suitable for life—or if life already exists there.
Darwin is expected to carry an interferometer that works in a similar way to the TPF's. NASA and the ESA are also considering combining the two projects into a joint collaboration that they would launch and operate together.
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