The first planets to be found around nearby stars
have never been seen. Instead, astronomers have discovered them indirectly,
inferring the existence of an unseen companion through its effects on the
So far, astronomers have only turned up huge planets that probably don't
harbor life. However, future missions such as Terrestrial Planet Finder and
its precursors will search for direct evidence of new planets as small as
The challenges of observing extrasolar planets stem from three basic facts:
- Planets don't produce any light of their own, except when young.
- They are an enormous distance from us.
- They are lost in the blinding glare of their parent stars.
For example, if there were a planet orbiting Proxima Centauri,
the nearest star, it would be 7,000 times more distant than Pluto. Trying
to observe this planet would be like standing in Boston and looking for a
moth near a spotlight in San Diego.
The following is an overview of some of the planet detection methods
that have thus far proved successful, as well as other methods currently
This method has been the most successful.
Precise measurement of the velocity or change of position of stars
tells us the extent of the star's movement induced by a planet's gravitational
tug. From that information, scientists can deduce the planet's mass and orbit.
Why does a planet cause a star to sway? If a star has a single companion,
both move in nearly circular orbits around their common center of mass. Even
if one body is much smaller, the laws of physics dictate that both will orbit
the center of the combined star and planet system. The center of mass is
the point at which the two bodies balance each other.
The radial velocity method measures slight changes in a star's velocity
as the star and the planet move about their common center of mass. In this
case, however, the motion detected is toward the observer and away from the
observer. Astronomers can detect these variances by analyzing the spectrum
of starlight. In an effect known as Doppler shift, light waves from a star
moving toward us are shifted toward the blue end of the spectrum. If the
star is moving away, the light waves shift toward the red end of the spectrum.
This happens because the waves become compressed when the star is
approaching the observer and spread out when the star is receding. The effect
is similar to the change in pitch we hear in a train's whistle as it approaches
The larger the planet and the closer it is to the host star, the
faster the star moves about the center of mass, causing a larger color shift
in the spectrum of starlight. That's why many of the first planets discovered
are Jupiter-class (300 times as massive as Earth), with orbits very close
to their parent stars.
As with the radial velocity technique, this methods depends
on the slight motion of the star caused by the orbiting planet. In this case,
however, astronomers are searching for the tiny displacements of the stars
on the sky.
The planets of our solar system have this effect on the Sun, producing
a to-and-fro motion that could be detected by an observer positioned several
light years away.
An important goal of the Space Interferometry Mission is to detect
the presence of Earth-size planets orbiting nearby solar type stars via narrow
angle astrometry. Similarly, the Keck Interferometry will conduct an astrometric
survey of hundreds of stars to search for planets with masses as small as
If a planet passes directly between a star and an observer's line of
sight, it blocks out a tiny portion of the star's light, thus reducing its
Sensitive instruments can detect this periodic dip in brightness.
From the period and depth of the transits, the orbit and size of the planetary
companions can be calculated. Smaller planets will produce a smaller effect,
and vice-versa. A terrestrial planet in an Earth-like orbit, for example,
would produce a minute dip in stellar brightness that would last just a few
This method derives from one of the insights of Einstein's theory of
general relativity: gravity bends space. We normally think of light as traveling
in a straight line, but light rays become bent when passing through space
that is warped by the presence of a massive object such as a star. This effect
has been proven by observations of the Sun's gravitational effect on starlight.
When a planet happens to pass in front of a host star along our
line of sight, the planet's gravity will behave like a lens. This focuses
the light rays and causes a temporary sharp increase in brightness and change
of the apparent position of the star.
Astronomers can use the gravitational microlensing effect to find objects that emit no light or are otherwise undetectable.
Since planets do not give off their own light, observing them directly
presents formidable challenges. Missions such as Terrestrial Planet Finder
will rely on advanced technologies that can harness special properties of
light to extend our vision. For a more detailed discussion of planet imaging,
see Technology >Planet Imaging.