The extra-solar planet revolution
The main goals of extra-solar planet research, and that of astrobiology in general,
are to understand the place of the solar system and in particular the Earth in the
universe. How common are extra-solar planets and planetary systems like our own
solar system? How often do stars have rocky Earth-like planets? What fraction of
stars have planets in the habitable zone (with a planet at such distance that
their surface temperature allows liquid water)? And the ultimate question,
how common is it for these Earth-like planets to harbour life...?
Direct detections of extra-solar planets are highly challenging due to the
enormous star-to-planet brightness contrasts, and so far have been beyond
reach. However, indirect methods, in particular the radial velocity technique
which measures the reflex motion of a star about the star-planet barycentre, have
resulted in a genuine revolution in exoplanet research. It has led to the first
discovery of exoplanets to the more than 150 systems
known to date. The planetary systems discovered so far have already resulted in
several major surprises. The first planets found were all so-called 'hot Jupiters',
planets with orbital periods of only a few days (well within the orbit of Mercury
on solar system scale), something thought to be impossible from planetary formation
theories. In addition, the large majority of the systems subsequently discovered
have orbits with eccentricities much higher than common in our own solar system.
Also, apparently, planets are more likely to form in a metal-rich environment,
with statistical studies showing that on average those stars hosting planets have
a significantly higher metal content than the average solar-type stars in the
Although the radial velocity technique has been a great success, it also
has its limitations. The orbital parameters of the systems are determined exactly,
but almost nothing is being learned about the planets themselves. The amplitude
of the radial velocity signal only provides a lower limit to the planet's mass,
because the inclinations of the orbits are unknown, and no further measurements
of the fundamental parameters are possible until technologies have advanced to
allow the planets to be detected directly. This situation is very different in
the rare case that the orbit of the planet has an inclination such that it transits
its host star, as in the case of HD209458b. Not only are the precise orbital
parameters of this planet determined, but also the planet's mass, radius, and density.
High precision follow-up observations have lead to several observational milestones,
such as the first detection of a planet's atmosphere, and detection of direct
thermal emission using the secondary eclipse.
The Transit Technique
At the moment, transits are -the- way to learn more about the extrasolar planet
population. Mid-2005, there are 8 transiting planets known.
The main challenge for using this technique is that the probably for a planet
to transit its host star (as seen from the Earth) is small, typically less
than 1% for planets in Earth-size orbits. Furthermore, such a transit would
occur only once a year, lasting in the order of only several hours. Luckily,
hot Jupiters have much shorter periods, and also have a significantly higher
probability (~10-15%) to transit their star.
The way a transit survey works is that large, densely populated areas
of sky are monitored for many nights, producing light curves for the thousands
of stars in the field. Periodic dips in the light curves of some stars
reveal the candidate transiting planets. Spectroscopic follow-up is than
needed to distinguish the genuine planets from transit mimics, caused by
e.g. blended eclipsing binary stars or grazing eclipses.
The most successful transit survey to date is
the OGLE-III survey
has so far (mid-2005) yielded 5 transiting planets. The camera+telescope
combination used by OmegaTranS is more than an order of magnitude more powerful
than that used by OGLE-III.