55 Canceri is a dim, unprepossessing star in the constellation of Cancer. It's a double star, whose claim to fame is that the yellow, Sun-like star of the double is host to a solar system of at least 5 planets, one of the few stars with extensive planetary systems so far discovered.
It's fame has now increased as transits of the innermost planet, 55 Cnc e, has been observed. This makes 55 Cnc the closest system with a transiting exoplanet, and also the only transiting system whose star is visible to the unaided eye (well providing you live in the country with dark skies).
If you head out tonight (well, tomorrow night. as 55 Cnc has set now as seen from Australia), around 8 pm and look north east, You will easily see the twins, Castor and Pollux. Just above and to the right is a very dim star, just above that is a dimmer star (suburban dwellers will need binoculars to see it) which is 55 Cnc.
The planetary system was originally discovered by the radial velocity method, which fixes some parameters of the system.
The 55 Cnc system (excluding the outermost planet), 55 Cnc e is marked by the red cross near the sun.
When the inner planet ,55 Cnc e, transits were observed, the radius of the exoplanet could be determined more precisely. It turns out to be a mere 1.6 times that of Earth, placing it as one of the smallest "Super Earths" yet discovered. With a mass of nearly 9 times that of Earth, it must be a rocky world, almost as dense as lead.
The orbit was also revised, it turns out the planet screams around it's sun in a year that lasts 18 hours. It's surface temprature, depending on whether it has an atmosphere or not, can range between 2100 and 2900 degrees C.
As always, I've created a Celestia file for 55 Cnc e. Celestia already has a file with the 55 Cnc system in it. If you wish, you can comment out the old 55 Cnc e data in exoplanets.ssc or replace the data with the following.
"e" "HD 75732" # Rho1 Cnc A
Mass 8.57 # M.sin(i) = 8.57 earths
# Inclination 90.0 # inclination is differnt from rest of system as measured with radial velocity
# likely to be in captured synchronous rotation
AltSurface "limit of knowledge" "HD 75732/e"
The original paper is here, more commentary at Universe Today and Centauri Dreams.