Find Seven New Planet–Forming Discs
mammoth sky survey led by University of Florida astronomers
has uncovered seven planet-forming disks in clusters
of young stars, doubling the number of such disks discovered
and expanding the territory that might yield new planets.
cluster of infant stars in the constellation Perseus
is home to several hundred young and forming stars
and seven planet-forming disks. The image was obtained
with a University of Florida-built near-infrared
camera and multi-object spectrometer at the National
Science Foundation’s Kitt Peak National Observatory
The disks, composed of giant clouds of gas and dust
that surround infant stars, are about 1,000 light-years
away — about four times farther away than most
disks seen previously. They also are by far the biggest
yet observed — which suggests that planets, known
to coalesce in such disks as they rotate, may exist
at much greater distances from stars than any yet discovered.
This observation could lead astronomers to expand the
areas in which they search for new planets — a
search that has so far been confined to stars’
“You might be able to look much farther out than
people have been looking and find planets,” said
Richard Elston, a UF professor of astronomy who conducted
the survey with his colleague and wife, UF astronomy
Professor Elizabeth Lada.
Elston and Lada presented their findings at the American
Astronomical Society meeting in May.
Lada also presented research showing that planets may
come together and form in the disks in far less time
than currently believed. Her findings suggest planets
may form in the first 3 million years of a star’s
life, much earlier than the 10 to 12 million years thought
previously. Although 3 million years may seem like a
long time, it is actually brief for stars, which can
live tens of billions of years. “If you think
of our Sun as a middle-age star and that middle-age
people are about 36, it would seem planet formation
occurs within 1 week of stellar birth,” Lada said.
As a cloud of molecular gas collapses under the pull
of gravity to form a star, it rotates and the dust,
gas and debris gradually gel in the shape of a two-dimensional
disk. The material in these disks both feeds into the
forming star and steadily coagulates into bigger and
bigger chunks, which eventually form planets. The remnants
of this process are visible in our own solar system,
where all the planets line up, more or less, along the
same two-dimensional plane.
Elston and Lada found seven such disks as part of a
major survey for newborn celestial objects in “giant
molecular clouds” in the constellations Orion
and Perseus. These clouds, which contain the raw material
for stars and planets, are the largest features of our
galaxy, stretching hundreds of light-years across.
The astronomers worked at the National Science Foundation’s
2.1-meter, or 88-inch, telescope at the Kitt Peak National
Observatory in Arizona, using a UF-developed near-infrared
spectrometer and imager. The Florida Multi-object Imaging
Grism Spectrometer, or FLAMINGOS, can image tens of
thousands of stars in a cloud in the near-infrared each
night — many more than could have been examined
without the instrument. Such observations must be made
in the near-infrared because visible light from young
stars is nearly completely absorbed by dust in the molecular
clouds, rendering the forming stars invisible to the
Computers can encode the infrared light in optical
wavelengths, creating visible images. Most of these
“snapshots” reveal only mature stars, and
the resulting images appear similar to the sky on a
clear night. However, the survey also revealed in several
positions, in Elston’s words, “wild places”
— startling clusters of infant stars in varied
stages of formation. These stars appear in the image
as colorful balls of light, with each of the seven disks
resembling a dark swath surrounding each star.
Astronomers used to think stars formed in relative
isolation. But over the past two decades, research by
Lada and others has shown that stars usually form in
clusters — celestial birthing grounds. Lada and
her colleagues have shown that the majority of stars
in such clusters are formed with circumstellar disks.
So while it wasn’t unusual to find the disks,
the surprise was that they ranged in size from 10 to
100 times larger than any of the handful of similar
disks yet seen and imaged — with each disk stretching
thousands of astronomical units in diameter.
One astronomical unit, the distance from the sun to
Earth, measures 93 million miles. The diameter of our
solar system is approximately 60 astronomical units.
The fact that these disks extend many times farther
than that suggests that planets, too, could extend well
beyond the relatively close proximity observed in our
solar system and elsewhere. That would be good news
for astronomers because the further planets are from
stars, the easier they are to detect, Elston said.