Most of us at some point had to learn the names of the nine planets, but for some reason few of us know anything about the geography of our immediate galactic neighborhood. You’re probably aware that the Alpha-Centauri system is the closest to our sun, but can you name any other close stars?

I couldn’t. Since we geeks have been dreaming of traveling the stars at least since the golden age of sci-fi, you’d think we’d know more about where we’d like to travel.

Here are the closest 10 star systems (from closest on out):

1. Alpha Centauri is a triple star system located about 4.3 light years away. The closest of the three stars is the tiny red dwarf, Proxima Centauri. The other two stars rotate around each other at the distance Saturn is from our Sun. They are both close to our sun in size and temperature.
   

   …most astronomers believe that stars of spectral types from about F5 to K5 are hot enough, but long-lived and stable enough, to support potential Earthlike worlds.

   Both Alpha Centauri A and B fall within this range. Alpha Centauri A is a G star like our Sun, Alpha Centauri B is a K1 star. The star types, from hottest to coolest are O, B, A, F, G, K, M.

   According to Wikipedia, Alpha Centauri A and B rotate at a sufficient distance to allow rocky earth-like worlds with liquid water to rotate either star. Such worlds rotating either star would have a secondary sun at night for half of its year – a prolonged twilight.

   Outer gas giants like Jupiter could not exist in this binary system, and that could be a problem because gas giants are thought to be responsible for bringing comets in to seed inner planets with water and other biological building blocks. But if the twin stars served the same purpose for each other, this might not be a problem at all.

   Viewed from near Alpha Centauri, the sky (other than the Alpha Centauri stars) would appear very much as it does to observers on Earth, with most of the constellations such as Ursa Major and Orion being almost unchanged. However, Centaurus would be missing its brightest star and our Sun would appear as a 0.5-magnitude star in Cassiopeia. Roughly speaking, the // of Cassiopeia would become a ///, with the Sun at the leftmost end…

   You know…in case you ever find yourself in the Alpha Centauri system and need to find your way back home. It’s only “news you can use” around here my friends.

2. Barnard’s Star is a red dwarf located 5.96 light years away. A red dwarf is a class M star – a star that is too cool to be considered a good candidate for life-bearing worlds.
3. Wolf 359 is a red dwarf 7.78 light years away.
4. Lalande 21185 is a red dwarf 8.29 light years away. It is known to have two gas giant planets. A third large planet is suspected.
5. Sirius is a binary system containing one star that’s thought to be too hot for life (Sirius A) and a white dwarf (Sirius B) that’s far too cool. White dwarves are even cooler than red dwarves. This binary system is 8.58 light years away.
6. Luyten 726-8 is a binary system of two red dwarves.
7. Ross 154 – a red dwarf.
8. Ross 248 is another red dwarf.
9. Epsilon Eridani is a K1 star, so it falls within the class of stars thought to be capable of supporting life. It is not thought to be a good candidate for complex life though because the star is young, it has an extremely variable spectrum (burning hot then cold), and is orbited by a Jupiter-like gas giant called Epsilon Eridani B in a highly eccentric orbit.

   This star is 10.52 light years away.

10. Lacaille 9352 is yet another red dwarf 10.74 light years away.

This large number of red dwarves is not unusual. Four out of five stars in the universe are red dwarves. In fact, the only other stars within 13 light years that fall within the F5 to K5 range are Epsilon Indi and Tau Ceti.

Epsilon Indi is orbited at a great distance by a pair of brown dwarves that orbit each other. It is on the cool side of stars capable of supporting Earth-like planets.

Since Epsilon Indi is sort of like a distant cousin to Sol, some speculate whether it might just be bright enough to support Earth-type life on a planet lucky enough to orbit in its water zone. The distance from Epsilon Indi where an Earth-type planet could possibly have liquid water on its surface is centered around only 0.38 AU — around Mercury’s orbital distance in the Solar System.

[an AU, or Astronomical Unit, is the distance between the Earth and the Sun]

Tidal lock might begin to be a problem that close to a star. A planet within the liquid water zone might tend to keep one side toward it’s star like the Moon to the Earth. Such a planet would be burned on one side, rare on the other.

Tau Ceti has a different set of problems. Because it is a metal deficient star it is thought to be unlikely to harbor rocky planets. Tau Ceti is surrounded by a dusty disk filled with comets.

Though the star Tau Ceti is similar to the Sun, any planets it has are unlikely to be havens for life, say a team of UK astronomers. Using submillimeter images of the disk of material surrounding Tau Ceti, they found that it must contain more than ten times as many comets and asteroids than there are in the Solar System.

Bottom line: our best bet for a close Earth-like neighbor lies within the Alpha Centauri system.

This is pretty cool:

An international team of astronomers reportedly has discovered the smallest, Earth-like planet outside our Solar System.

The planet, known as OGLE-2005-BLG-390Lb, orbits a star one-fifth the mass of the Sun, and has an estimated surface temperature of minus 364 Fahrenheit (minus 200 Celsius), astronomers Michael Albrow and Karen Pollard of Canterbury University told Television New Zealand. They said the discovery might one day lead to finding a twin Earth.

The planet is located about 20,000 light years from Earth; it orbits a star in the constellation Sagittarius. The astronomers who discovered it did so via a technique called gravitational microlensing, which enables distant objects to be detected through the light intensification caused by their gravity. This technique seems a little counter-intuitive. I would have though that a planet passing between us and a distant star would obscure the star somewhat and reduce the amount of light that reaches us. And in fact, maybe it does — but apparently we’ll never find any planets looking for that. Instead, we go looking for the tiny light spike that the planet’s gravity causes, and voila! Now we’re locating Earth-like planets.

I reiterate: pretty cool.