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Finding Earth-Like Planets

Astronomers have discovered far more planets outside our solar system, known as extrasolar planets, than within it. Astronomers can not only detect a planet's presence, they can deduce a lower limit to its mass and distance from its parent star, and the length of its year.

Artist's concept of a possible Neptune-mass planet orbiting Rho 1 Cancri.

How do astronomers find planets outside our solar system?

The most successful search method used to date is called the radial velocity method. As a star is tugged to and fro by a planet's gravitational pull, astronomers measure a slight shift in the frequency of the star’s light.

Astrometry is another detection method. It is sometimes called positional astronomy. Astronomers measure a tiny shift in a star's position on the sky caused by the gravitational pull of a planet. They can use this information to calculate the planet's mass and orbit.

If a planet passes directly between a star and the observer, it blocks out a tiny portion of the star's light. This so-called transit method looks for repeated dips in a star’s light to confirm the presence of an orbiting planet.

A fourth detection method, called gravitational microlensing, comes from one of Einstein's insights in his theory of general relativity: Gravity bends space. When a planet passes in front of a more-distant star, the planet's gravity will behave like a lens to temporarily focus light from the star. This should cause a sharp increase in brightness and a change in the apparent position of the star.

Finding Earth-Like Planets

Most of the planets discovered outside our own solar system are behemoths -- worlds that are at least as massive as Jupiter, the solar system's giant.

A planet discovered by a team of astronomers led by Barbara McArthur, however, is a relative lightweight -- and perhaps a step toward detecting worlds as small as Earth. The planet orbits the star known as Rho 1 Cancri or 55 Cancri, and is the fourth planet discovered in the system.

Unlike the others in the system, which are as massive as Jupiter or even bigger, this planet probably is only around 18 times as massive as Earth -- a fraction of Jupiter's mass. Also unlike the others, it probably is rocky, and not a ball of gas like Jupiter. The planet was discovered with the Hobby-Eberly Telescope at McDonald Observatory.