Discovery space

4.5 Discovery space

Figure 4.10 is complex but very instructive. It shows the range of planets, in both mass and separation from their star, which could be detected by the four meth- ods that have been discussed above. The lower horizontal axis is the distance (in AU) from the star with, above, the upper horizontal axis giving the corresponding orbital period of a planet around a 1 solar mass star. The vertical axes give the mass of a planet in both Jupiter and Earth masses. The plot shows the position, given by their mass and period, of the planets of our Solar System along with a number of extra-solar planets to give an indication of the parts of the discovery space in which the planets have been found by the differing methods.

In blue are the planets discovered by the radial velocity method. They lie to the upper left of the diagram in that they are both massive and tend to lie close to their star. The two blue lines separated by a pale blue shaded area show the discovery space with Doppler precisions of 3 m s ⫺1 , relating to the year 2006, and 1 m s ⫺1 ,

the expected precision that will be achieved by the year 2010. You will see that planets like Jupiter in mass and separation from their star could already have been detected by this method and that those like Saturn could follow. The fairly sharp cutoff at a period of 10 years is simply due to the fact that we need to observe a star for at least one orbital period to be sure of its presence and observations had only been taking place for ∼10 years at the time this plot was made.

In red are the planets discovered by Earth based transit observations. Their dis- covery space is relatively small, but will greatly increase when the two space mis- sions COROT, launched in December 2006, and Kepler, due for launch in February 2009, complete their missions. The initial planetary detections need extensive follow-up observations to confi rm their presence much of which is being done from ground based telescopes. By the end of 2007, many candidate planets found in the fi rst year of COROT transit observations were being followed up from the ground and the discovery of two planets had been announced. They are shown in green on the plot.

By 2004 one planet, shown in yellow, had been discovered by gravitational microlensing. Three further planets, shown in orange, had been discovered by the end of 2007. As mentioned above, one of these has the lowest known mass of any extra-solar planet, 5.5 Earth masses.

As yet, no planets have been detected by the astrometric method. Two lines in the upper part of the diagram show the discovery space of two ground based surveys. The upper blue line relates to a Palomar survey using the 5-m Hale

Introduction to Astronomy and Cosmology

Figure 4.10 The discovery space of extra-solar planets as described in the text. Image: Wikipedia Commons.

Extra-solar Planets

telescope, whilst the lower green line relates to the surveys to be undertaken by the VLT and Keck telescopes. The solid and dotted red lines show the discovery space that could be achieved by NASA’s SIM spacecraft after 1 and 5 years from launch, respectively. Sadly, development work on the project has slowed due to budget cuts and its launch has been delayed indefi nitely. As the mission would sig- nifi cantly increase the discovery space for planets of similar mass and separation from their star as our Earth, it is hoped that its launch will not be delayed for too long.