It’s always good to have good news to report, and now is a great time for that. Following the recent discovery of an Earth-mass planet around our stellar neighbor Alpha Centauri, astronomers have recently announced the potential discovery of five near Earth-mass planets around another neighboring Sun-like star, Tau Ceti; of which, two may be able to support life as we know it (paper here).
Tau Ceti, after Alpha Centauri, is the next nearest Sun-like star, being of similar mass, age, composition, and temperature to our own Sun. It is situated 11.9 light years away (70.1 trillion miles), as can be seen on this map of nearby stars (within 12.5 light years):
Tau Ceti is near the bottom of the image.
The team used special filtering techniques to remove the “noise” in the star’s light, and by carefully analyzing its motion via the radial velocity method, they detected “wobbles” that are most consistent with a system of five planets. Assuming that these planets are close to their minimum masses (that is, if the plane of the system is not horribly inclined to our line of sight), these planets all appear to be in the vicinity of Earth’s mass. However, they are all on the heavy side (ranging from 2.0-6.6 times Earth’s mass), making them “super Earths”, bigger and heavier than our world.
This is a schematic comparison of the Tau Ceti system to our own (inner) Solar System (outer gas giant planets excluded). The sizes of the planets and the central stars are NOT to scale with the radius of the planets’ orbits, but the orbits are to scale with each other and the planets are scaled to their relative sizes (assuming that the Tau Ceti planets are the same density as Earth.) The outer two planets are suspected to be in Tau Ceti’s “habitable zone”; that is, the orbital distance from the star where the temperature should be suitable for liquid water to exist on a planet’s surface. My own calculations shows that the habitable zone of Tau Ceti is actually a lot narrower than I’ve shown here, but I’ve artificially expanded it to include these planets. (In reality, Tau Ceti e is likely too close to the star for there to be liquid water on the surface.) Additional observations will be needed to confirm these planets, and their orbital parameters and bulk characteristics will be subject to refinement.
There are other issues with this system, however. One is that these planets are more massive than Earth, and it’s unclear what conditions would arise on planets with many Earth masses. Also, Tau Ceti possesses a much larger and more massive “Kuiper Belt” than our own Solar System. This is an extended region of large icy bodies at the outer fringes of the system—of which Pluto is a member in own Solar System. This massive disk of objects likely serves as a source of impacting bodies, which may lead to much more frequent large impacts of the type that caused the extinction of the dinosaurs here on Earth.
In any case, the fact that two of the nearby Sun-like stars are now known to harbor near Earth-mass planets—and indeed, one appears to have an entire planetary system, where one, and perhaps two of these planets may be life-bearing—is a testament to the likely ubiquity of such planets in the Galaxy. The confirmed discovery of a life-bearing planet elsewhere in the cosmos seems to be an eventuality that grows ever more imminent.
Since, presumably, intelligent, technological life should also be common in the Galaxy, it naturally follows to ask why we have not spotted any. A recent such discussion took place over at Mangan’s. While a variety of ideas were put forward (as they typically are in such discussions), the bottom line is we just don’t know; only time may (hopefully) tell.
Edit, 12/25/12: See this animation of the putative Tau Ceti system over at Sol Station (Java required).