420 PART 4^ |^ THE SOLAR SYSTEM
■ Figure 19-1 7
(a) Visible-wavelength images of 3 planets at apparent distances of 25, 40, and 70 AU from the star HR 8799. (C. Marois et al./
National Research Council of Canada/AURA/Keck Observatories) (b) Near-infrared image of a planet orbiting about 120 AU from the
star Fomalhaut (alpha Piscis Austrinus), at the inside edge of that star’s previously known debris ring. The inset shows motion of
the object between 2004 and 2006, at the correct rate for a planet at that position orbiting a star of that mass. (P. Kalas et al./
STScI/NASA)surface temperature that would permit liquid water, it is clearly
not a Jovian planet.
Among the planets found so far, most are orbiting stars that
are metal rich rather than metal poor. Th is supports the scenario
of planet formation by the accretion of a core of solids and the
later accumulation of gas. It is evidence against formation by
direct collapse, which does not require the presence of solids such
as metals and silicates to start planet formation.
Actually getting an image of a planet orbiting another star is
about as easy as photographing a bug crawling on the bulb of a
searchlight miles away. Planets are small and dim and get lost in the
glare of the stars they orbit. Nevertheless, during 2008, astronomers
managed to image three planets around the A-type star HR 8799,
using adaptive optics and coronagraph instruments mounted on
the Gemini and Keck telescopes atop Mauna Kea, and 1 planet
around the A-type star Fomalhaut (alpha Piscis Austrinis), using the
Hubble Space Telescope’s near-infrared camera (■ Figure 19-17).
Searches for more extrasolar planets are being conducted.
Th e Kepler space telescope mission, launched in 2009, will be
sensitive enough to detect transits of planets as small as Earth.
Space observatories will be able eventually to image Jovian and
even Terrestrial planets directly around sunlike stars. Th e
discovery of extrasolar planets gives astronomers added confi -
dence in the solar nebula theory. Th e theory predicts that
planets are common, and astronomers are fi nding them orbit-
ing many stars.SCIENTIFIC ARGUMENT
Why are debris disks evidence that planets have already
formed?
Sometimes a good scientifi c argument combines evidence, theory,
and an astronomer’s past experience, a kind of scientifi c common
sense. Certainly the cold debris disks seen around stars like Vega
are not places where planets are forming. They are not hot enough
or dense enough to be young disks. Rather, the debris disks must be
older, and the dust is being produced by collisions among comets,asteroids, and Kuiper belt objects. Small dust particles would be
blown away or destroyed relatively quickly, so these collisions must
be a continuing process. The successful solar nebula theory gives
astronomers reason to believe that where you fi nd comets, aster-
oids, and Kuiper belt objects, you should also fi nd planets, so the
dust disks seem to be evidence that planets have already formed
in such systems.
Now build a new argument. What direct evidence can you cite
that planets orbit other stars?