Which type of star should you consider first when looking for stars that might have life-supporting worlds around them?

Answer 1

We know of only one planet with intelligent life; Earth. Earth orbits the Sun, a Type G0 star.

We suspect that "life like ours" is probably most likely around stars like ours.

A sample size of "one" isn't really enough to make any conclusions.

However, there could easily be "life very much UN-like ours", which might live on or near planets orbiting very different stars. We do not know.

Answer 2

The answer to this question depends very much on your definitions of the terms "suitable" and "life as we know it." By suitable you could mean simply surviving one season and leaving offspring for the next non-harsh season. A season could be any where from a few days to several thousand years. Or by suitable you could mean the temperatures calculated by Isaac Asimov and Stephen Dole in their work "Habitable Planets for man" where they quoted 0°C to 30°C as a global average habitable temperature range, or if refering to the limits of extremophile life that could be -60°C to 180°C temperature range. You could mean the right gravity range (less than 1.5g), or the right atmosphere constituents. Oxygen breathing & nothing toxic in nothing more than very trace amounts. By the term "as we know it" there is are still a possibility range that covers anything from extremophile bacteria to Whales the size of 16 elephants. A gas giant could have layers of water and oxygen at the right distance from its star, and have "life as we know it" in the form of jelly fish or birds. Aside from these variables, since you are still going to mainly discuss the characteristics of a star: Some stars are too small and at the same time give off harsh X-ray radiation. Some are too large and spin so fast that they are not likely to have any planets at all. There are several ways to calculate how far a planet needs to be from a star to have the amount of irradiance to be heated just right, to not freeze or boil. The simplest is taking the square root of the luminosity of the star, in solar units, which gives you the distance in Astronomical units (AU, 1 being the Earth-Sun distance).

Another is to use the Radius of the star (m) and the Temperature of the star (K), the Stephan Boltzman constant, and the solar constant of the Earth, or average irradiance:

d = ((R2 x (5.67051e-8) x T4) / 1366.078) (0.5) / 149597876600 meters.

As an example, a planet in the Gliese 581 system would have to be...

d = ((0.38x6.955e8)2 x (5.67051e-8) x (34804) /1366.078) (0.5) / 149597876600 meters.

... 0.1378 AU to get the same heat as the Earth. Gliese 581 c is actually 0.07 AU (hotter then Venus), and Gliese 581 e is 0.22 AU (colder than Mars). This red dwarf was a strong X-ray and EUV emitter of a long part of its life and, at that distance, would have killed "life as we know it."

Once you have the right distance you can eliminate stars that would give you harsh radiation too close, and stars that would not have planets that far out.

As an example that largest star known, VY Canis Majoris, would have an Earth-like irradiance at... d = ((600x6.955e8)2 x (5.67051e-8) x (300004) /1366.078) (0.5) / 149597876600

16,175 AU from the star. but there isn't likely to be planets forming that far out.

Isaac Asimov and Stephen Dole quoted the following star types as good habitable planet prospects:

F2=1.06%,

F3=1.92%,

F4=3.03%,

F5=3.44%,

F6=4.18%,

F7=4.57%,

F8=4.99%,

F9=5.29%,

G0=5.45%,

G1=5.45%,

G2=5.45%,

G3=5.45%,

G4=5.45%,

G5=5.20%,

G6=5.11%,

G7=3.94%,

G8=2.21%,

G9=3.12%,

K0=1.47%,

K1=0.61%

Answer 3

Stars that are similar to the Sun, single stars that are rotating relatively slowly, would be the most likely to have planets round them similar to the solar system.