* Astronomy

The planet recently discovered around Gleise 876 is unlikely to bear life because its orbit is so close to the star, giving the planet an extremely high surface temperature.

M class stars have usually considered unsuitable for SETI observations.

The "Goldilocks" analogy suggested that planets whose temperature was "just right" for liquid water, and life, would orbit so close to their star that they would be tidally locked.
As a result, the world would quickly become "too hot" on one side, and "too cold" on the other: the atmosphere would boil off on the light side, and freeze out on the dark side. These dwarf stars also flare, producing X and UV radiation that would challenge any life on the surface.

"Simple theory said that terrestrial planets in orbit around M stars will be uninhabitable, and uninhabited. But we are not confined to simple theory any more. Recent models predict sustainable atmospheres and substellar liquid water regimes…A growing appreciation for natural sun screens that developed early in the evolution of life on Earth, and a speculation that niche sterilizing events might actually speed evolution towards complex life, make M stars worth considering once again. And reconsider them we will!" - Jill Tarter, Director of the Centre for SETI Research at the SETI Institute.

Observed Properties of Exoplanets: Masses, Orbits, and Metallicities
Authors: G.Marcy, R.P.Butler, D.A.Fischer, S.S.Vogt, J.T.Wright, C. G. Tinney, H. R.A. Jones.

We review the observed properties of exoplanets found by the Doppler technique which has revealed 152 exoplanets to date. We focus on our ongoing 18-year survey of 1330 FGKM type stars at Lick, Keck, and the Anglo-Australian Telescopes carried out with a uniform Doppler precision of 3 m/s. The 104 planets detected in our survey have masses as low as 15 M_Earth orbiting between 0.03 and 5.5 AU.
The mass distribution rises toward the lowest detectable masses as dN/dM is proportional to M^-1.1. Nearly all giant planets orbiting within 2 AU of all FGK stars within 30 pc have now been discovered.
The distribution of semi-major axes rises from 0.3 - 3.0 AU (dN/dlog a)
, but remains unknown for larger orbits. Extrapolation suggests that 12% of the FGK stars harbour exoplanets within 20 AU. The median orbital eccentricity is 0.25 (excluding those tidally circularized), lower than previously measured.
Planets orbiting beyond 3 AU continue to exhibit non-zero eccentricity, suggesting that the circular orbits of giant planets in our Solar System are unusual.
The occurrence rate of ''hot Jupiters'' of FGK stars is 1.2�0.2%.
The probability of occurrence of planets varies as the square of the stellar metal abundance, P \propto N^2_Fe, ranging from 3% for stars of subsolar metallicity to 25% for stars with Fe/H (less than) +0.3.
Nearly 14% of planet-bearing stars harbour multiple-planet systems, occasionally locked in resonances. Kepler and COROT should measure the occurrence of earth-sized planets. The Space Interferometry Mission (SIM) will detect planets with masses as low as 3 M_ Earth orbiting within 2 AU of nearby stars and will measure masses, orbits and multiplicity.
These candidate rocky planets will motivate spectroscopic follow-up by the ''Terrestrial Planet Finder'' and Darwin.

Read More (PDF)

The GJ 876 Planetary System -- A Progress Report
Authors: G. Laughlin, R. P. Butler, D. A. Fischer, G. W. Marcy, S. S. Vogt, A. S. Wolf.

We present an updated analysis of the GJ 876 planetary system based on an augmented data set that incorporates 65 new high-precision radial velocities obtained with the Keck telescope from 2001 to 2004.
These new radial velocities permit a more accurate characterization of the planet-planet interactions exhibited by the system.
Self-consistent three-body orbital fits (which incorporate both the estimated instrumental uncertainties and 6 m/s Gaussian stellar jitter) continue to show that GJ 876 b and GJ 876 c are participating in a stable and symmetric 2:1 resonance condition in which the lowest order, eccentricity type mean-motion resonance variables are all librating.
The planets are also locked in a secular resonance which causes them to librate about apsidal alignment. The small libration widths of all three resonances likely point to a dissipative history of differential migration for the two planets in the system.


The red dwarf GJ 876 (RA=22 53, DEC=14 16) is observable from both hemispheres, and is distinguished by being the fortieth-nearest stellar system, with a Hipparcos-determined distance of 4.69 pc.


Read more (PDF)

The team measures a minimum mass for the planet of 5.9 Earth masses, orbiting Gliese 876 with a period of 1.94 days at a distance of 0.021 astronomical units (AU), or 2 million miles. The orbital plane is tilted 40 degrees to our line of sight.


Expand

"In a two-day orbit , it's about 200 degrees Celsius too hot for liquid water. That tends to lead us to the conclusion that the most probable composition of this thing is like the inner planets of this solar system--a nickel-iron rock, a rocky planet, a terrestrial planet." - Marcy, Butler, theoretical astronomer.


Download Movie

This animation shows the newly discovered planet and its two larger siblings as they orbit the M dwarf star Gliese 876. The star and the orbits are to scale; the planet sizes are exaggerated. In addition, the orbits are shown inclined at an angle of 50 degrees, which is how they appear in our sky.

The outermost planet, known as Gliese 876b, was discovered in 1998. It orbits the star once every 61 Earth days, and has about twice the mass of Jupiter, our own solar system's largest planet.
That makes it roughly 600 times the mass of the Earth. The middle planet, Gliese 876c, was discovered in 2001. It orbits the star once every 30 days and has about half the mass of Jupiter, or roughly 150 times the mass of the Earth.

The new planet has about twice the radius of the Earth and about seven-and-a-half times it’s mass, and orbits the star once every two days.


Download Movie

This animation takes us on a flythrough of the Gliese 876 system. The star, the orbits and the sizes of all three known planets are to scale.

We first see the outermost planet, Gliese 876b, which was discovered in 1998. It orbits the star once every 61 Earth days, and has about twice the mass of Jupiter, our own solar system's largest planet.
That makes it roughly 600 times the mass of the Earth. Next we see the middle planet, Gliese 876c, which was discovered in 2001. It orbits the star once every 30 days and has about half the mass of Jupiter, or roughly 150 times the mass of the Earth.

Finally we come to the newly discovered planet. It has about twice the radius of the Earth and about seven-and-a-half times its mass, and it orbits the star once every two days.


Expand

These three graphs show the observational data and the best theoretical fit for each of the three planets known to orbit the star Gliese 876. From top to bottom the panels are for the newly discovered planet, the middle planet and the outermost planet. The vertical axes (which are different for each) refer to the tiny back-and-forth motions the orbiting planets cause in their parent star. The colours refer to the year the observations were made. The wide variations seen in the second panel reflect the strong gravitational perturbations the planets exert on one another.