Astronomers
are pondering several possibilities for 47 Tucanae's dearth of
planets.
Possibility
#1: The stars they observed were in a globular cluster,
a compact region of up to 1 million middle-aged stars. Globular
clusters lack the heavier elements that may be necessary for building
planets.
Possibility
#2: Astronomers used Hubble to hunt for a specific type
of planet called a "hot Jupiter," a bloated, Jupiter-sized planet
that snuggles perilously close to its parent star. The results
do not rule out the possibility that 47 Tucanae could contain
normal solar systems like ours, which Hubble cannot detect. But
even if that's the case, the finding implies there is a fundamental
difference between the way planets are created in our own neighborhood
and how they're made in the cluster.
Possibility
#3:
The stars are so tightly compacted in the core of the cluster
that strong gravitational forces may strip nascent planets from
their parent stars. The high volume of stars also could disturb
a Jupiter-size planet's migration inward towards its parent star.
Astronomers were looking for these "star huggers," called hot
Jupiters, in 47 Tucanae. Scientists theorize that many Jupiter-size
planets found orbiting close to their host stars actually formed
farther away and traveled inward.
Possibility
#4:
A torrent of ultraviolet light from the cluster's earliest and
biggest stars, which were born billions of years ago, may have
boiled away fragile embryonic dust disks from which planets would
have formed.
The telescope didn't
snap pictures of planets: They're too small and dim to be imaged
by any observatory. Astronomers used the Hubble telescope's Wide
Field and Planetary Camera 2 to search for small dips in starlight,
the telltale signature of planets passing in front of stars. As
a planet crosses a star's face, it blocks a tiny amount of starlight.
This event is similar to the moon eclipsing our Sun.
