Dynamics of Cats

Gliese 581 is an M3 red dwarf, it has a mass of about a third that of the Sun, slightly lower metal content and is about the same age probably a bit older than the Sun, we think.
It is about 20 light years away.

Gliese 581 was known to have three planets already:

Gl 581b - 1/20th of Jupiter mass in a 5 day orbit == ok, 16 Earth masses
Gl 581c - 1/60th of Jupiter mass in a 13 day orbit == 5 Earth masses
Gl 581d - 1/50th of Jupiter mass in 67 day eccentric orbit == 7 Earth masses

Now there is Gl 581e, discovered with high cadence monitoring of this system by the Swiss group led by Mayor

Artist impression (click to embiggen)

Gl 581e - 1/160th mass of Jupiter == 1.9 Earth masses,minimum mass, in a 3 day near circular orbit!
Dynamical stability arguments constrain the system to have inclination > 40 degrees, so the maximum mass is no more than 3 Earth masses.
There is a prior probability of about 5% that the innermost planet is transiting.

Lowest mass planet yet found around a main-sequence star.
Discovery paper: "The HARPS search for souther extra-solar planets: XVIII. An Earth-mass planet in the GJ 581 planetary system" (pdf)

This proves that it is possible to find such low mass planets, though clearly the high precision high cadence monitoring following up the existing known system helped there.
Looks like the new planet is just outside the 5:3 resonance, I would expect it to have some residual eccentricity.
It should be quite hot, and most likely tidally locked to the star, though the presence of the 5 day planet offers some possibility that it will have locked into a tidal resonance other than 1:1 depending on what its original rotation period was, maybe, I haven't calculated this formally, just going by naive intuition.
Looking at figure 2 in the paper they're doing about 2 m/s half amplitude, with about 1 m/s residuals, but the periodicity looks decent in the folded data and should of course improve.
Starspots are a possible concern, though they discuss why they don't think that is the case.
Squinting at the residuals it is tempting to infer a >> 200 day trend, with low amplitude... but that is biased by the sparse early data. We'll know in a year or three if there is a fifth planet further out.

I expect there will be a lot of papers on the stability and planet-planet interactions in the near future.

As an addendum, the outer planet, Gl 581d, has had a refinement of the estimated orbital period, moving it to the current best estimate of 67 days, which formally places it in the Habitable Zone for the star, though the orbit is eccentric, and the exact habitable zone boundaries would depend on the atmosphere and structure of the planet.

HZ at GJ 581 (click to embiggen)

Way cool.

see also: Universe Today

ESO press release

Nice little ESO video explaining detection technique

Nice little video zoom to give impression of system scale

zoom in on stellar field
it is a tenth magnitude star, easily accessible to modest amateur telescopes

PS: Starts With A Bang is also on it

On the issue of the age of GJ 581: the exoplanet encyclopedia lists it at 4.3 Gyrs, but the reference links to HD 4308...
Selsis et al estimate the age of GJ 581 at over 7 Gyrs based on the lack of x-rays in ROSAT data - this is very tentative because it is based on a lack of detection and a statistical correlation between x-ray activity and age of M stars; also plausible because it is somewhat metal poor. It is a disk star.