Greg Laughlin from UCO Lick pointed out that there is a transit possible of Gliese 581c on May7. Looking at the ephemeris, its at 17:13 UT, though: not very good for those of us in Ireland. Yesterdays Nature (subscription required) shows that someone is going one better:

On 26 April, just three days after the planet's existence was revealed, Dimitar Sasselov of Harvard University began using the small Canadian space telescope MOST (Microvariability and Oscillations of Stars) to observe Gliese 581. Sasselov predicts that, if the geometry is right, his team should see the star dim on 7 May as the planet passes in front of it.

They reckon on 50/1 against a transit, on geometrical grounds. Good luck to them though.

The point of all of this is that the current planetary detections of Gliese 581b and 581c are Doppler radial velocity measurements. While they tell us the period, they only give us a lower bound on the mass : 581c is at least 5 Earth masses, but could be much more (its M Sin(i), where i is the inclination relative to us is unknown). We also don't know the radius of the planet : the figure quoted in the news articles to date is just a guess based on 5 Earth masses and current theory. A transit measurement, where the planet passes in front of the star, could answer these questions.

But it occurs to be we could get numbers tonight: there is a transit (alignment) of Gliese 581b tonight at 22:40 UT. Much more favourable to those of us in Europe.

A transit of GL 581b, the big neighbour of Gl 581c, would tell is that a transit of 581c is likely, and would give us an accurate mass for 581b, and from this, a measure of the inclination. So we could also calculate the mass of 581b.

Unfortunately I don't have a photometer or CCD. Anyone else up for it?

Clear Skies over Ireland forecast , too ...