First extrasolar planets, now extrasolar moons! (Forwarded)

ESA News
http://www.esa.int

8 October 2003

First extrasolar planets, now extrasolar moons!

ESA is now planning a mission that can detect moons around planets outside our
Solar System, those orbiting other stars.

Everyone knows our Moon: lovers stare at it, wolves howl at it, and ESA recently
sent SMART-1 to study it. But there are over a hundred other moons in our Solar
System, each a world in its own right.

A moon is a natural body that travels around a planet. Moons are a by-product of
planetary formation and can range in size from small asteroid-sized bodies of a
few kilometres in diameter to several thousand kilometres, larger even than the
planets Mercury and Pluto.

Landing on another moon

One such large moon is Titan, the target for ESA's daring Huygens mission that
in 2005 will become the first spacecraft ever to land on a moon of another
planet. Titan is slightly bigger than the planet Mercury, and is only called a
moon because it orbits the giant planet Saturn rather than the Sun.

Four other large moons can be found around another of our neighbours, Jupiter.
These are Io, Europa, Ganymede and Callisto. Europa has captured attention
because beneath its icy surface, scientists think that an ocean covers the
entire moon. Some scientists have even speculated that microscopic life might be
found in that ocean.

Habitable moons?

In 2008, ESA plans to launch its 'rocky planet' finder Eddington. By detecting
the drop in light seen when a world passes in front of its parent star,
Eddington will be capable of discovering planets the size of Jupiter, and also
those smaller than Mars.

That means, if our own Solar System is anything to go by, it will be capable of
detecting moons similar in size to Titan and the four large moons of Jupiter.

It would be particularly exciting if such combinations of planets and moons were
found orbiting a star at Earth's distance from the Sun. Perhaps then the
surfaces of the moons would be warmed to habitable levels.

Orbital dancing

What about moons similar to our own? An equivalent of Earth's moon would be too
small to be detected directly by Eddington, but such a body would affect the way
its planet moves and it is that movement which Eddington could detect.

The Earth and the Moon orbit the Sun like ballroom dancers who move around the
floor, simultaneously twirling about one another. This means the Earth does not
follow a strictly circular path through space, sometimes it will be leading the
Moon and sometimes trailing.

This causes variations of up to five minutes from where the Earth would be if it
did not possess a moon. By precisely timing when a rocky planet passes in front
of its star, Eddington will be able to show if a moon is pulling its planet out
of a strictly circular path around the star.

So, how many moons can Eddington expect to find circling planets around other
stars? If we make an estimate based on our own Solar System, several thousands
will be found -- however, no one knows for sure. That's what makes the quest so
exciting!

More about ...

* Eddington overview
http://www.esa.int/esaSC/120365_index_0_m.html
* SMART-1 overview
http://www.esa.int/esaSC/120371_index_0_m.html
* Huygens overview
http://www.esa.int/esaSC/120378_index_0_m.html

Related articles

* How to find an extrasolar planet
http://www.esa.int/esaSC/SEMYZF9YFDD_index_0.html
* Studying the stars, testing relativity: Sir Arthur Eddington
http://www.esa.int/esaSC/SEMDYPXO4HD_people_0.html
* How many planets are outside our Solar System?
http://www.esa.int/esaSC/SEMZVI5V9ED_exploring_0.html
* Gone to the Moon ...
http://www.esa.int/esaSC/SEM7AD0P4HD_exploring_0.html

IMAGE CAPTIONS:
[http://www.esa.int/export/esaCP/SEM1...html#subhead3]
ESA's Eddington mission is designed to detect habitable planets across a wide
range of stellar types.

Credits: ESA 2001. Illustration by Medialab.