Earth-like
planets may be more rare than thought
30 July 2004
- Nature
In
cosmic terms, our solar
system could be special after all.
We could
be alone in the Universe after all. The discovery during the past
decade of over a hundred planets around other stars has encouraged many
scientists to think that habitable planets like ours might be common.
But a recent study tells them to think again.
Martin
Beer of the University of Leicester, UK, and co-workers argue that our
Solar System may be highly unusual, compared with the planetary systems
of other stars. In a preprint published on Arxiv1, they point out that the
alien planets we have
seen so far could have been formed by a completely different process
from the one that formed ours. If that is so, says Beer, "there won't
necessarily be lots of other Earths up there".
Most of
the planets around other stars, known as extrasolar planets, are
detected from the wobble that they induce in their own sun's motion.
This wobble is caused by the gravitational tug of the planet on the
star. Because stars are much bigger than planets, the effect is tiny,
and it is only in the past decade that telescopes have been sensitive
enough to detect it.
Even
then, the wobble is detectable only for giant planets, which are those
about as big as Jupiter, the bloated ball of gas in our Solar System.
It is not possible at present to detect planets as small as the Earth.
Jupiter
is not habitable: it is too cold, and is mostly composed of dense gas.
And it is unlikely that extrasolar giant planets would support life
either. But astronomers generally assume that if they detect such a
planet in a distant solar system, it is likely to be accompanied by
other, smaller planets. And maybe some of the smaller planets in these
systems are just like Earth.
This is
what Beer and colleagues now dispute. They say that the properties of
almost all the known extrasolar planets are quite different from those
of Jupiter.
Hot
Jupiters
There
are 110 of these extrasolar planets, at the latest count, and they are
all between about a tenth and ten times as massive as Jupiter. Most of
them are, however, much closer to their sun than Jupiter is to ours:
they are known as 'hot Jupiters'. They also tend to have more elongated
orbits than those of Jupiter and the Earth, both of which orbit the Sun
on almost circular paths.
Ever
since Copernicus displaced the Earth from the centre of the Universe,
astronomers have tended to assume that there is nothing special about
our place in the cosmos. But apparently our planetary system might not
be so normal after all. Is it just chance that makes Jupiter different
from other extrasolar planets? Beer and his colleagues suspect not.
They
suggest that other planets were not formed by the same kind of process
that produced our Solar System, so they might not have smaller,
habitable companions.
Different
recipes
The
planets in our Solar System were put together from small pieces. The
cloud of gas and dust that surrounded our newly formed Sun agglomerated
into little pebbles, which then collided and stuck together to form
rocky boulders and eventually mini-planets, called planetesimals. The
coalescence of planetesimals created rocky planets such as Earth and
Mars, and the solid cores of giant planets such as Jupiter, which then
attracted thick atmospheres of gas.
But that
is not the only way to make a solar system. Giant planets can condense
directly out of the gaseous material around stars, collapsing under
their own gravity. This process, which generates giant planets with a
wide range of orbital radii and eccentricities, does not seem capable
of producing the rocky planets seen in our own Solar System, which is
why it has generally been ignored.
Yet it
might account very nicely for the known extrasolar planets. "It
wouldn't surprise me if there are two different ways that planetary
systems are formed," Beer says. But how can we know if that is the
case? "Probably the best way is just to gather more observations," says
Beer. Only then can we know how unusual we really are.
References
- Preprint, http://xxx.arxiv.org/abs/astro-ph/0407476
(2004).
Fuente: http://www.nature.com/news/2004/040726/full/040726-14.html
