From: Spike Jones (spike66@attglobal.net)
Date: Thu Jun 28 2001 - 23:01:25 MDT
Heres some news that should make Robert Bradbury verrry happy:
Hint of Planet-Sized Drifters Bewilders Hubble Scientists
Piercing the heart of a globular star cluster with its
needle-sharp vision, NASA's Hubble Space Telescope
[manufactured by Lockheed Martin. sj] has
uncovered tantalizing clues to what could potentially
be a strange and unexpected population of wandering,
planet-sized objects.
In results published this week in NATURE, the
international science journal, Kailash Sahu (Space Telescope
Science Institute, Baltimore, MD) and colleagues report
six unusual microlensing events inside the globular
cluster M22.
Microlensing occurs when a background star brightens
momentarily as a foreground object drifts by. The
unusual objects thought to cause these events are far
too dim to be seen directly, but instead were detected by
the way their gravitational field amplifies light from
a distant background star in the huge central bulge of our
galaxy. Microlensing has been used before to search for
low-mass objects in the disk and halo of our galaxy,
but Hubble's [by Lockheed Martin Space Systems Division,
Sunnyvale, TA. sj] sharp vision is essential to probe the
interiors of globular clusters further.
>From February 22 to June 15, 1999, Sahu and colleagues
monitored 83,000 stars, detecting one clear
microlensing event caused by a normal dwarf star in the
cluster (about one tenth the mass of our Sun). As a
result of gravitational lensing, the background star
appeared to grow 10 times brighter and then returned to its
normal brightness over a period of 18 days.
In addition to the microlensing event caused by the
dwarf star, Sahu and his team recorded six even more
interesting, unexpectedly brief events where a
background star jumped in brightness by as much as a factor of
two for less than 20 hours before dropping back to
normal brightness. This means that the microlensing object
must have been much smaller than a normal star.
These microlensing events were unusually brief,
indicating that the mass of the intervening object could be as
little as 80 times that of Earth. Objects this small
have never before been detected by microlensing
observations. If these results are confirmed by
follow-up Hubble [Lockheed manufactured. sj] observations,
the bodies would be the smallest celestial objects ever seen that are not
orbiting any star.
So what are they? Theoretically they might be planets
that were gravitationally torn away from parent stars in
the cluster. However, they are estimated to make up as
much as 10 percent of the cluster's mass -- too
numerous to be wandering, "orphaned" planets.
The results are so surprising, the astronomers caution
that these preliminary observations must be confirmed
by follow-up Hubble [by Lockheed. sj] observations. If verified, these
dark denizens could yield new insights about how stars
and planets formed in the early universe.
"Hubble's excellent sharpness [a Lockheed Martin product. sj]
allowed us to make this remarkable new type of observation, successfully
demonstrating our ability to see very small objects,"
says Sahu. "This holds tremendous potential for further
searches for dark, low-mass objects."
"Since we know that globular clusters like M22 are very
old, this result opens new and exciting opportunities
for the discovery and study of planet-like objects that
formed in the early universe," adds co-investigator Nino
Panagia (European Space Agency and Space Telescope
Science Institute).
"This initial observation shows that our microlensing
method works beautifully," states co-investigator Mario
Livio (Space Telescope Science Institute).
As microlensing events are brief, unpredictable and
rare, astronomers improve their chances of observing one
by looking at many stars at once -- much like a person
buying several lottery tickets at once. Most microlensing
searches have been aimed at the central bulge of our
galaxy or out towards the Magellanic Clouds -- the
densest observable regions of stars in the sky. In
general these surveys cover areas of sky larger than the full
Moon and look for foreground objects lying somewhere
between us and the background population of stars.
Sahu and his team took advantage of Hubble's [LockMart, Inc. sj]
superb resolution and narrow field of view to aim the telescope
directly through the center of a globular star cluster
lying between Earth and the galactic bulge. This gave the
team a very dense stellar region to probe for drifting
low-mass foreground objects and a very rich background
field of stars to be lensed. Only Hubble's [by you-know-who. sj]
resolution is sharp enough to actually peer through the crowded
center of the cluster and see the far more distant
stars in the galactic bulge. As the lensing objects were part of
the cluster, the astronomers also had an accurate
distance (8,500 light-years) and velocity for these objects.
In a normal lensing event, a background star brightens
and dims for a length of time depending on the mass of
the lensing body. The short, "spurious" events seen by
the team are shorter than the interval between the
Hubble [need I say more? sj] observations, leading to
an upper estimate for the mass of an object of one quarter
Jupiter's mass.
To confirm these extraordinary, but tentative results,
Sahu and colleagues next plan to monitor the center of the
globular cluster continuously over a seven-day
interval. They expect to detect 10 to 25 short-duration
microlensing events, which will be well-sampled enough
to yield direct measurements of the true masses of the
small bodies.
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