Photo courtesy NASA/STScI
Credit: Greg Bacon

Artist's concept of planet-orbiting star 79 Ceti

Are we alone in the universe, or does life exist elsewhere? We have begun to search for extraterrestrial intelligence (SETI) by looking for radio transmissions from other civilizations. An important aspect of SETI is deciding where to look for these radio transmissions -- after all, the universe is an awfully big place. To refine the SETI searches, it is helpful to know which stars have planets orbiting them.

Until 1991, our sun was the only star known to have planets around it. This changed when astronomer Alex Wolszczan discovered two planets orbiting a pulsar in the constellation Virgo. Since his discovery, there have been over 50 planets found in orbit around other stars -- these orbiting bodies are called extrasolar planets.

In this edition of HowStuffWorks, we'll find out what planets are, how they are formed, how we search for extrasolar planets (and what we've found so far) and about the new methods being developed for planet hunting.

What is a Planet?

Photo courtesy NASA
Our solar system
There are eight other planets in our solar system besides Earth. Many of these planets have moons orbiting them. But what exactly is a planet? By definition, a planet is a large body that orbits a star and shines by reflecting starlight from its surface. Planets vary in their mass, composition and distance from the star. In our solar system, the planets are divided into three major categories:

  • Inner-terrestrial planets - Mercury, Venus, Earth and Mars. These planets are made of rock and orbit close to the Sun.
  • Outer gas giants (Jovian planets) - Jupiter, Saturn, Uranus and Neptune. These planets are massive (with hundreds of times the mass of Earth). They have dense, gaseous, hydrogen-rich atmospheres that also contain helium, ammonia and methane. These atmospheres probably surround inner cores made of rock.
  • Other bodies - Pluto, comets, asteroids and Kuiper Belt objects. These bodies are made of rock and ice mixtures. Despite controversy, Pluto is still categorized as a planet, though its composition is more similar to asteroids and comets than to other planets.


Photo courtesy NASA
Formation of a solar system

The planets in our solar system were made from the disc of swirling gas and dust that formed our sun. As the hydrogen gas and dust of the early solar system fell into the center of disc, forming the protosun, the gas and dust heated up to a temperature that could sustain nuclear fusion. At the same time, smaller clumps of dust and gas, called planetismals, formed in the outer parts of the disc. When the protosun "ignited," it blew the dust and gas away from its immediate vicinity. The planetismals coalesced to form the planets (see How Stars Work: The Life of a Star for details). Scientists believe that other solar systems would be formed in the same way.

Habitable Zone
Stars are much too hot to support life, so planets or moons are the most likely places for life to develop. Light from a star warms the orbiting planet and supplies the energy necessary for life. In addition to energy, life seems to need a liquid, chemical solvent of some type in which to develop. On Earth, this solvent is water, but it is conceivable that other solvents (such as ammonia, methane or hydrogen fluoride) might also work. With this in mind, it seems that the planet must lie within a certain range of distances from the star so that the solvent can remain in liquid form -- if the planet is too close to the star, the solvent will boil away; if it is too far from the star, the solvent will freeze. For our sun, the habitable zone appears to be between the orbits of Venus and Mars.

Searching for Extrasolar Planets
It is difficult to find planets around other stars because the light from the star is so bright that the glare drowns out the light reflected from the planet. It's like trying to see a lighted birthday candle placed in front of a search light. So, the only way to detect extrasolar planets at this time is to measure their effects on their parent stars. There are two ways in which planets affect their parent stars: They tug on the star as they orbit it, and they can dim the light from the star if they pass directly between the star and our field of view (eclipsing part of the star's light). The effects of these planetary motions on the star can be detected from Earth by three methods:

  • Astrometry - measuring the star's precise position in the heavens
  • Doppler spectroscopy - measuring the wavelength spread of light emitted from the star
  • Photometry - measuring the intensity or brightness of the light emitted from the star

Astrometry
As a planet tugs on a star with its gravitational pull, it causes the star to wobble in its path across the sky. By making careful, precise measurements of the star's position in the sky, we can detect this extremely slight wobble. When we know the period of the wobble, we can calculate the period of the planet's orbit, the distance or radius of the planet's orbit and the mass of the planet.

Doppler Spectroscopy
As a planet orbits a star, it periodically pulls the star closer to and farther away from Earth (our observation point). This motion has an effect on the spectrum of light coming from the star.


Spectroscopic technique of detecting an extrasolar planet

As the star moves toward the Earth, the light waves coming from it are compressed and shifted toward the blue (shorter-wavelength) end of the spectrum. As the star moves away from us, the light waves are stretched out toward the red (longer-wavelength) end of the spectrum. These shifts in the spectrum of light coming from the star are called Doppler shifts. By making measurements of the star's spectrum over time, we can detect shifts that would indicate the presence of a planet. We can also use Doppler shifts to measure the radial velocity of the star's movement, which is how fast the star moves toward us and away from us.


Photo courtesy European Southern Observatory
Radial-velocity measurements of the star Gliese 86 in the constellation Eriadni. The measurements indicate an extrasolar planet with an orbital period of 15.8 days. The calculated mass of the planet is about five times that of Jupiter.

Conceptually, we can deduce the size of the planet from the radial velocity. A massive planet will tug on the star with more gravitational force than a small one, causing the star to have a greater radial velocity. If we graph measured radial velocity versus time, we get a "sine" curve like the one shown above. From the period (peak-to-peak time or trough-to-trough time) and the star's mass, we can get the distance of the planet from the star -- the planet's orbital radius. From the amplitude of the curve, we can calculate its mass (see High precision Doppler measurements: The physics and techniques for finding planets and Doppler-Wobble Tutorial for details).

Photometry
If the orbit of an extrasolar planet is in a straight line of sight with Earth, the planet will pass directly between the star it's orbiting and Earth. When the planet passes in front of the star, it blocks some portion of the star's light, and the star gets slightly dimmer (by about 2 to 5 percent). The planet eclipses the star. As the planet passes behind the star, the star's normal brightness returns. By constantly measuring the star's light intensity over time, we can detect changes in its brightness that might indicate the presence of a planet or planets.


Photometric technique of detecting an extrasolar planet

What Have We Found?
In July 1995, two astronomers from the University of Geneva, Didier Queloz and Michael Mayor, found the first planet orbiting a normal star in the constellation of Pegasus by the spectroscopy method. The discovery of 51 Pegasus was confirmed by astronomers Geoff Marcy and Paul Butler of San Francisco State University. Marcy and Butler have since found numerous planets around other stars using the spectroscopy method. As of May 2000, we have found over 50 extrasolar planets. All of the planet-hunting methods tend to detect large planets -- about half the size of Jupiter to several times the size of Jupiter. These planets tend to orbit their parent stars within about 3 astronomical units (AU).


Photo courtesy NASA
Some of the extrasolar planets found.
The names of the planets are in the center of each row. The position of each planet in the row indicates its distance from its parent star in AU. The mass of the each planet is written to its right (MJ means "compared to the mass of Jupiter").

For the latest results, see The Search for Extrasolar Planets, Geoff Marcy's Web site at San Francisco State University.

Importance of Jupiter
In our solar system, Jupiter plays an important role in clearing the inner solar system of passing debris. As many comets and asteroids pass from the outer solar system into the inner solar system, Jupiter's gravity breaks them apart. If Jupiter were not located where it is in our solar system, and were not so massive, more comets and asteroids would have collided with Earth, causing mass extinctions like that of the dinosaurs.

Future Planet Hunting

Photo courtesy NASA
Terrestrial Planet Finder
NASA's Chief Administrator, Daniel Goldin, has set a major goal for NASA to find Earth-like planets orbiting other stars. As we mentioned above, it is difficult to detect Earth-like planets because they are too dim to see in the glare of a parent star, and too small to have detectable effects on that star. However, NASA plans to launch a set of telescopes, called the Terrestrial Planet Finder (TPF), to aid in achieving this goal. The TPF will be an array of four optical telescopes and a combiner instrument. Each telescope in the array will detect light from the target star. The light will be combined in such a way as to cancel out the bright glare from the star, a technique known as nulling interferometry. The baseline of the array will be at least 0.6 miles (1 kilometer). Precision flying methods, which are currently being developed at NASA, will keep the array in formation.


Photo courtesy NASA
Simulated results of the nulling-interferometry technique (the red arrow indicates a planet)

Once the star's light is cancelled, the infrared spectrum of the planet's light can be examined for the presence of substances in the planet's atmosphere that would indicate an earth-like environment.



Photo courtesy NASA
Simulated infrared-absorption spectrum of an Earth-like planet (top) and how it could be interpreted for signs of life (bottom)

The TPF mission is in development stages, and hopefully will be launched within the next decade. Once operational, this space-based telescope system will revolutionize planet hunting and the search for life in the universe.

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