But ours is not the only planetary system out
there.
Alien worlds have long been embedded in our
collective consciousness be it through film, TV, or literature. Yet it was not
until 1995 that science fiction truly became science fact when Mayor and Queloz
discovered the first alien planet orbiting a star similar to our Sun just 50
light years away. But this new discovery was not quite what everyone was
expecting.
As a stars gravity pulls a planet around in
its orbit, the planet also pulls on the star. The bigger the planet and the
closer it is to the star the greater the pull it has. This causes the star to
wobble backwards and forwards as the planet makes its way from one side to the
other side of the star each time it orbits. By observing the stars spectrum we
can see the shift in the absorption limes as the star is forced backwards and
forwards by the planet.
 |
| The Radial Velocity method |
This exoplanet, 51 Pegasi b, sparked
interest around the world and provided a number of scientists, who had strange
ideas about observing planets orbiting other stars, some hard data to stand on.
One of those scientists was Dr William
Borucki, who since 1984 had been designing and submitting a mission proposal to
NASA for a space telescope dedicated to hunting for exoplanets. I would,
however, not be until 1995 with the first concrete observations and a long
development period that Borucki’s vision would be accepted and launched by
NASA.
That mission was the Kepler space telescope
which, since its launch in 2009 has been the most prolific source of headlines
getting exoplanets in the media and out to the general public.
Kepler relies on the chance phenomenon of
the exoplanetary system being aligned 90 degrees to our own, meaning that part
of the planets orbit takes it between its star and us, similar to the transits
of Mercury and Venus in our own solar system. As the exoplanet transits it
blocks out a small fraction of the starlight. The amount of light that is
blocked can tell us a lot about the planet and its environment such as its
radius from the amount of light blocked and the length of its orbit from
observations of multiple transits.
This method, however, still favors the
close-in giant planets that appear to populate our galaxy, as the larger the
planet relative to the size of the star the more light they will block out and
the easier it is to observer, and the closer they are to their star the shorter
their year is so the greater number of transits that can be observed in a short
amount of instrument time.
This means that in order to observer an
exoplanet roughly the same size as the Earth orbiting a star similar to the Sun
with a one year orbit, we need continuous observations of a large number of
stars over a number of years. That is exactly what Kepler is doing now and with
every additional year of the mission we explore a different sub-set of
potential exoplanets.
The transit of exoplanets offers us a
unique opportunity to observe the planets atmosphere. As the planet passes
between its star and us, some of the starlight shines through the planets upper
atmosphere before reaching us. Like on the Earth, different parts of the
atmosphere absorb different amounts of the starlight at certain wavelengths. So
imprinted on the observed starlight, as weak absorption lines, are the
tale-tale signatures of various gasses allowing us to build up a spectrum of
the exoplanets atmosphere. Ultimately helping us determine the nature of the
environment in which it exists and the potential to work out how it formed.
This is unfortunately not as easy as it
sounds and although a number of chemical species have been identified in a
handful of worlds the application of this technique to exoplanets is still in
its infancy.
The study of exoplanets is at a fantastic
stage of discovery, where curiosity and ingenuity combine, taking us from the
Wright brothers flying a few feet across a field to Amy Johnson flying solo
half way around the world.
Be assured there are going to be more
unbelievable discoveries coming our way from the exoplanet community in the not
so distant future so keep an ear out.
WHAT'S NEXT?
NASA/Kepler image can be found here: http://kepler.nasa.gov/images/201205planet_size_comparison-full.jpg
D. K. Sing University of Exeter
