Subj:	30 Billion Earths? New Estimate of Exoplanets in Our Galaxy
Date:	1/31/02 5:42:31 AM Pacific Standard Time

January 29 09:07 AM EST

30 Billion Earths? New Estimate of Exoplanets in Our Galaxy

By Robert Roy Britt
Senior Science Writer, SPACE.com

Chances are you haven't spent a whole lot of time wondering
how many Jupiter-like planets exist in our galaxy. But
Charley Lineweaver has, because it bears on a more important
question: How many potentially habitable planets are there?

New calculations by Lineweaver and Daniel Grether, both of
the University of New South Wales in Australia, provide an
encouraging answer to this question. The researchers expect
a flood of Jupiters will be found, perhaps 50 percent more
than currently expected.

Each such discovery would be significant in the hunt for
planets that could harbor life.

Why? Because much of the evolution of our own solar system,
including the formation of Earth, was orchestrated or
affected by Jupiter, the largest planet with by far the bulk
of the solar system's mass, excepting the Sun, of course.

"Our solar system is Jupiter and a bunch of junk," as
Lineweaver puts it.

Our protector

When Jupiter developed, it simply bullied other objects into
position or out of existence. Then the mighty gas giant
became Earth's protector.

Though the fledgling Earth was pummeled by asteroids and
comets, making it difficult for life to take hold, it could
have been much worse. Jupiter shielded Earth from an even
heavier bombardment of debris that made its way from the
outskirts of the new system toward its central star.

That protective role continues. In 1994, Jupiter used its
immense gravity to lure comet Shoemaker-Levy into a death
plunge. Had the comet hit Earth, it would have sterilized
much or all of the planet.

For now, no one knows whether our solar system represents a
common method of formation and evolution. In fact,
discoveries over the past six years seem to indicate
otherwise. Most of the roughly 80 planets discovered outside
our solar system are much more massive than Jupiter. They
also orbit perilously close to their host stars, locations
that would likely prevent rocky planets from forming in
so-called habitable orbits.

But experts attribute these findings to the limitations of
technology. Smaller planets in more comfortable orbits
around other stars simply can't be detected. Yet.

How many Jupiters?

All this in mind, Lineweaver and Grether worked out some new
calculations for the prevalence of planets that are about
Jupiter's size at about the same distance from their host
stars. The calculations are based on some of the most recent
extrasolar planet discoveries, in which ever-smaller objects
are being detected at ever-greater distances from their host
stars.

So how many Jupiters are out there orbiting Sun-like stars
in the Milky Way Galaxy?

"At least a billion, but probably more like 30 billion,"
Lineweaver told SPACE.com.

And the math behind that?

"There are about 300 billion stars in our galaxy. About 10
percent (or 30 billion) are roughly Sun-like," he explained.
"At least 5 percent (1.5 billion) but possibly as many as 90
percent or 100 percent (about 30 billion) of these have
Jupiter-like planets."

These estimates would vary based on exactly what you call
Jupiter-like or Sun-like, Lineweaver said.

What about Earths?

The calculations, which are part of a paper that has been
submitted to the journal Astrobiology, don't bear directly
on worlds like our own. But with what's known of planet
formation, some speculation is possible.

"A reasonable guess is the same number of Earths as
Jupiters," Lineweaver said.

That, however, depends heavily on how one defines
Earth-like. If one includes rocky planets in general, like
Mercury, Venus and Mars, "then they are probably more common
than Jupiters," he said. If, however, you mean rocky planets
with liquid water at the surface, "then we really can't
answer that very well. They may be as common as Jupiters, or
they may be much less common."

Alan Boss, an expert in planetary system formation at the
Carnegie Institution of Washington, said the new
calculations for Jovian twins seem reasonable. Trying then
to estimate the number of Earth-like planets requires "a
leap of faith, but one which appears to be plausible," he
said.

"As the veil covering the unseen portions of discovery space
is lowered in the next decade, I expect we will find that
Jupiter-like planets are commonplace," said Boss, who was
not involved in the new study. "Whether or not that also
means Earth-like planets are common can only be proven by
NASA's Kepler mission."

Kepler, recently approved to launch in 2006, will monitor
100,000 stars for telltale dips in light indicating an
Earth-sized planet in an Earth-like orbit has crossed in
front of the star. While it would not take photographs,
Kepler could provide the first census of planets that have
the potential to support life.