By David F.
April 10, 2001
In 1993, when the Hubble Space Telescope surveyed the Orion nebula
for the first time, its images provided a substantial boost for
the argument that stars with planetary systems are commonplace in
the galaxy. Now, however, the most recent analyses of one the youngest,
closest and brightest nebulae cast doubt on that conclusion and
suggest that planets may be far rarer than astronomers have thought.
The Orion nebula is the closest example
of a stellar nursery. Stellar nurseries are special regions where
the vast majority of new stars in the galaxy are born. Interstellar
clouds of molecular gas form, produce thousands of new stars and
then gradually dissipate. The nebula in Orion is 1,500 light years
from Earth and six light years or 35 trillion miles across. It forms
the second point of light in the hunter’s scabbard in the Orion
constellation. The Trapezium Cluster at the nebula’s center contains
more than 1,500 stars. Four massive young stars illuminate the nebula,
making it possible to observe many objects that would normally be
invisible. The starlight they produce is so intense, in fact, that
it ionizes thin layers of the gas in the region, producing a rainbow
of colors. So it’s not surprising that studies of Orion have providaastronomers
with some of the best information about the process of star formation.
The first Hubble Space Telescope (HST)
images found that up to 90 percent of the young stars in the nebula
are surrounded by "protoplanetary disks"—disks of dust and gas from
which planets can form. Astronomers call such star and disk systems
"proplyds." Based on the assumption that similar conditions prevail
in other stellar nurseries, the finding strengthened the hypothesis
that planet production is a common side effect of the star formation
C. Robert O’Dell, lead scientist on
the first HST studies of Orion and now a research professor at Vanderbilt
University, has been studying the nebula since 1964. In a May 1
presentation at the annual meeting of the American Physical Society
in Washington D.C., O’Dell reports that the most recent studies
of Orion appear to have come up with a planet stopper. The youngest
and brightest stars in the cluster are so powerful that the ultraviolet
radiation they produce should blast away the dust and gas surrounding
newly formed stars before they can form planets.
"According to current estimates,
it takes about 10 million years for a planet to form," O’Dell says.
"The massive, young stars in Orion are more than 100,000 times as
luminous as the sun. Our best estimate is that these radiation levels
can destroy a protoplanetary disk in a few hundred thousand years.
So it appears that most of the disks will be gone long before planets
A critical factor in this calculation
is the length of time it takes planets to develop. If planets form
considerably faster than scientists currently think, then the percentage
of stars that develop planetary systems could be substantially higher,
Over the last eight years, O’Dell and
W. J. Henney at the National Autonomous University of Mexico at
Morelia—working with graduate students from Rice University and
UNAM—have used a combination of optical and radio telescope data
to construct a detailed, three-dimensional map of the nebula. Using
this map, he estimates that only 10 percent of the proplyds in the
nebula are shielded from the erosive star-shine.
If planetary formation times are correct,
and the conditions in the Orion nebula are typical of stellar nurseries,
then only one star in 10 is likely to form a planetary system, O’Dell
Why then does Orion contain more than
300 circumsolar disks? The answer, according to O’Dell, is quite
surprising. One of the stars in Trapezium turns out to be a binary.
By carefully measuring the properties of this pair of stars, Francesco
Palla at the Osservatio Astrofisico di Arcetri in Italy and Steven
Stahler at the University of California, Berkeley have estimated
that it can be no older than 100,000 years. Orion’s massive central
stars must be even younger, O’Dell contends because they have created
an intense radiation environment that has essentially shut down
star formation in the nebula.
"This estimate, combined with the fact
that we don’t see any evidence for depletion of the protoplanetary
disks, even those exposed to the highest radiation levels, suggests
that the central stars are even younger, perhaps only a few tens
of thousands of years old," O’Dell says.
The fact that these stars may not be
any older than mankind itself doesn’t sit well with the astronomer.
It goes against the Copernican principle. Copernicus argued that
the Earth wasn’t the center of the universe but, rather, that the
Earth orbits around the Sun. Since then this has been generalized
to the Copernican principle: There is nothing special in time or
space about Earth’s position in the universe.
"It is unlikely that homo sapiens and
the Orion nebula should be formed at just about the same time, but
perhaps we are just lucky," O’Dell says.