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BOX 2-1 Other Worlds Around Other Stars
The detection and study of exoplanets—planets orbiting other stars⎯is expanding into the realm of
Earth-like planets, less than 15 years after the discovery of the very first planet orbiting a star like the Sun.
Over 400 planets are known, most discovered by the ground-based “Doppler spectroscopic” technique, in
which telescopes look for a slight “radial velocity” variation in stars like the Sun and smaller. An operating
“transit” telescope is in space today capable of detecting planets the size of our own and smaller (Figure 2-1-
1). NASA’s Kepler mission, launched March 6, 2009, observes over 100,000 stars in the “Orion arm” of our
Milky Way galaxy for a tell-tale dip in their light output which, if regular and repeatable, represents the
passage or transit of a planet in front of the star. A French- and European Space Agency precursor to Kepler,
called COROT, has already detected planets as small as about 1.7 times the diameter of the Earth during its
2½ years of observations. With these missions in operation, we will know in the next five years just how
common Earth-sized planets might be in the Galactic neighborhood of our own solar system.
Meanwhile, exoplanets ranging in size from Jupiter to Neptune are being studied from ground- and
space-based observatories, revealing exotic weather systems and strange chemical patterns that differ from
those in our solar system (Figure 2-1-2). On HD189733b, in a close circular orbit around its star, day-night
temperatures are so extreme that supersonic winds may flow around the Jupiter-sized planet. The Spitzer
infrared space telescope has measured the light from a number of Jupiter-class exoplanets, hence
determining atmospheric compositions. HD80606b, a giant planet observed by Spitzer, has an elliptical orbit
that brings it alternately close to and far from its parent star so that that its atmospheric temperatures change
by many hundreds of degrees Celsius over 6 hours. Planet sizes, when combined with ground-based
measurements of the planetary masses, yield densities. Many of these planets are less dense than gaseous
Jupiter, while others are much denser, indicating a range of interior compositions and structures. Spitzer has
the capability to see planets less than twice the diameter of the Earth transiting the smallest stars, or M
dwarfs, and its successor the James Webb Space Telescope will be even more sensitive when launched in
2015. The era of study of the properties of rocky planets around other stars, cousins of the Earth, is
underway.
FIGURE 2‐1‐1 Kepler measurements of the light from HAT‐P‐7. The larger dip is that due to a planet about 1.4 times the
radius of Jupiter transiting in front of the star, reducing the light of the star by about 0.7%. Such a drop has been
observed from ground‐based telescopes. However, the smaller drop, about 0.013% of the light of the star, is seen by
Kepler as the planet itself passes behind the star—hence Kepler is directly detecting the light of the planet itself. Such
accuracy and precision is beyond ground‐based telescopes and sufficient to detect an Earth in transit across Sun‐like
stars. (Source: NASA Press release and Borucki, W.J. et al., Science 325, p. 709, 2009).
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