User Guide

A NEW SUN
APPENDIX 5
215
As you can see, the Sun and Alpha Centauri A are very similar to each other.
They are both G2 stars, which means that Alpha Centauri A has the same yel-
low color as the Sun. Alpha Centauri A is larger and brighter, but not by a great
deal. Anyone, or anything, comfortable with the Sun would probably be com-
fortable with Alpha Centauri A.
Alpha Centauri B is smaller and dimmer than the Sun. It is orange, rather than
yellow. Alpha Centauri C is a great deal smaller and dimmer than any of the
other three, and is red.
HABITABLE PLANETS
There are limits on the radius at which a planet habitable to mankind might orbit
a star. The brighter and hotter the star, the farther out a planet must be to be
habitable; the dimmer the star, the closer a planet must be. Alpha Centauri C can
have no habitable planets — any planet close enough for life-sustaining sunshine
would be quickly ripped apart by gravitational forces. Both Alpha Centauri A and
B could have habitable planets, if they weren’t part of a binary system.
But what effect might the two stars have on each other’s habitable planets?
As it happens, very little. If a planet were within either star’s habitable zone,
it would never come close enough to the other star to be materially affected
by it — its orbit would be stable. Take a look at the illustration of our solar
system, with the Alpha Centauri binary system superimposed on it. Mentally
replace the Sun with Alpha Centauri A. Alpha Centauri A’s habitable zone does
not reach much beyond the orbit of Mars, and any planet that close to Alpha
Centauri A is always too far from Alpha Centauri B to be affected by it.
Now mentally replace the Sun with Alpha Centauri B. Since Alpha Centauri B
is cooler than the Sun or Alpha Centauri A, its habitable zone is even closer to
Alpha Centauri B. Again, Alpha Centauri A would never come close enough to
perturb a habitable planet around Alpha Centauri B.
The same can not be said for planets at greater distances from either of the
binaries. Neither star can have a planet with the orbital radius of Jupiter,