Gliese 581 b

Gliese 581 b or Gl 581 b is an extrasolar planet orbiting the star Gliese 581. It is the first planet of three discovered in the system so far, and the second in order from the star.

Discovery
The planet was discovered by a team of French and Swiss astronomers, who announced their findings on November 30, 2005 as a discovery of one of the smallest extrasolar planets ever found, with one conclusion being that planets may be more common around the smallest stars. It was the fifth planet found around a red dwarf star (after Gliese 876's planets and Gliese 436 b).

The planet was discovered using the HARPS instrument, with which they found the host star to have a wobble that implied the existence of the planet.

The astronomers published their results in Astronomy and Astrophysics Letters.

Orbit and mass
Gliese 581 b is at a minimum, approximately 15.8 times the Earth's mass, similar to Neptune's mass. It does not transit its star, implying that its inclination is less than 88.1 degrees. It is rather close to Gliese 581 and completes a full orbit in only 5.4 days at a mean distance of about 6 million kilometers (0.041 AU). By comparison, Mercury is at a distance of 58 million kilometers (0.387 AU) and completes an orbit in 88 days.

Characteristics
Gliese 581 b is about 0.04 AU from its sun. It is likely close to Gliese 436 b in mass, temperature, and (with Gliese 876 d) susceptibility to solar effects such as coronal mass ejection. Since Gliese 581 b does not transit, nothing more can be said of it yet. At the least, given that Gliese 581 b orbits alongside two other planets (Gliese 581 c and e) and that Gliese 436 b (thus far) stands alone, their formation must have differed.