Looking for the “wink”
as planets block light from their star
As planets orbit around
distant stars, there is a small chance that they will, occasionally,
pass directly between their parent star and the Earth. Most planets
will pass either above or below their parent star, as we look at it,
and nothing will be seen. However, for the small fraction that do pass
across the face of their star, it is possible to detect their presence
by watching for the star winking, as the planet crosses its face.
Such events are well illustrated by an example from our own Solar system.
From the Earth, we sometimes see the planets Venus and
Mercury pass between us and the Sun,
blocking some of the Sun's light. This is known as a planetary transit
(click here for
an image of the planet Venus transiting the Sun in 2004). Despite the
fact that Venus orbits the Sun every 224 days, and Mercury goes around
once every 88 days, these transits are actually surprisingly rare. A
pair of Venusian transits occur roughly once every century, while there
are around 13 Mercurian transits every century. The rest of the time,
the planets pass above or below our Sun, in the sky, and we see nothing.
Beyond our Solar system,
the odds of being aligned just right to see a planet go across the face
of its star are very low. However, we can see so many stars in the
night sky that, by watching maybe a million, non-stop, for a period of
a year or more, we could expect to detect a few tens of "transiting
exoplanets" - planets giving themselves away by passing directly
between us and their parent star, time and time again, each time the
planet completes one orbit around its star. By
regularly monitoring millions of stars, astronomers can identify those
that “wink” at us. In this way, so far, over 40 transiting extrasolar
planets have been discovered.
Image copyright Mark Garlick (markgarlick.com)
Transiting planets are special, because for them we know the
exact orientation of their orbit. The fact they transit means that
their orbit is moving exactly in our line of sight -- that's the only
way that a planet can transit! This means that, once we have found a
transiting exoplanet around a star, we can then look at that star in
more detail, in order to measure its wobble.
This allows us to precisely measure the mass of the planet,
and work out how far it is from the host star. We can also measure
their size, as bigger planets block more starlight, and therefore
create a more obvious "wink". Given the size of the planet, and it's
mass, we can work out its density, which tells us what these unseen
planets are made of. We can even measure their temperatures, and get
an indication of the composition of their atmospheres!
The easiest way to find transiting exoplanets is to watch a large area of sky for a long period of time, and see which stars wink at us. The great majority of stars that wink do so for other reasons though - they may be variable stars, brightening and fading regularly as they swell and shrink; they might be binary stars (two stars in orbit around one another), whose brightness changes as one passes in front of the other (this would give a much deeper wink, in general, than a planet moving around a star, so these are often quite easy to spot!). So, even when a star is seen to be winking, there is still a lot of work to do! Scientists working on a project called SuperWASP have been watching the sky for a number of years now, and have so far discovered 15 planets around other stars! These planets are probably just the tip of the iceberg - and the SuperWASP team expect to discover many more planets in the future!
The above video shows how the SuperWASP project discovers new worlds, and how the
observers check the discoveries on the huge 4.2 metre diameter William
Herschel telescope, on the Canary Island of La Palma. Taken from the Open University / BBC
series The
Cosmos: a Beginner's Guide, .
The fifteen planets discovered by SuperWASP
to date, with the Sun and Jupiter from our own Solar system shown to
scale. The variety of sizes of both planets and stars in the SuperWASP
sample can be clearly seen!