Date: 01-24-92 20:29
From: David Galea
Subj: Habitable Planets, from the January 1992 JBIS.

* Originally dated 22 Jan 1992, 9:05

From: klaes@mtwain.enet.dec.com (Larry Klaes)
Organization: Digital Equipment Corporation


The following posting is a summary written by my friend and co-worker,
Drew LePage, of an article in the January 1992 issue of the JOURNAL OF
THE BRITISH INTERPLANETARY SOCIETY (JBIS), Volume 45, Number 1. Titled
"An Estimate of the Prevalence of Biocompatible and Habitable Planets",
it is authored by M. J. Fogg.

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There is a very interesting article in the January 1992 edition of
the JOURNAL OF THE BRITISH INTERPLANETARY SOCIETY (JBIS) on the
likelihood of various types of stars having habitable or biocompatible
planets. A biocompatible planet is one where the long term presence of
surface liquid water provides environmental conditions suitable for for
the origin and evolution of life. There are three subsets of
biocompatible planets:

* Juvenile Martian - As the name implies, it is a planet with condition
similar to those found on Mars early in its life. The planet would
receive between 27% and 75% of the light we presently receive from the
Sun and possess plate tectonics or some other geochemical carbon cycle.
Mars was this type for its first one billion years.

* Juvenile Terran - Again as the name implies, this is a planet with
conditions similar to those found on the early Earth. The planet would
receive between 75% and 95% of the light we presently receive from the
Sun and be geologically active. Earth was this type of planet for its
first four billion years (i.e. during the Precambrian period).

* Habitable - This is a planet with Earthlike conditions. The planet
would receive between 95% and 110% of the light we receive and be
geologically active.

The author of the study collected the results of various studies to
determine what conditions produce biocompatible and habitable planets,
the evolution of stars and the effects on planetary environ-ments, the
likely distribution of planets in other systems, as well as others.
The results of the author's simulations indicate the following:

* Habitable planets can exist around stars with 0.8 to 1.8 times the mass
of the Sun.

* Biocompatible planets can exist around stars with 0.5 to 1.8 times the
mass of the Sun.

* Habitable planets may occur around >3% of the stars between 0.85 and
1.45 times the mass of the Sun.

* Biocompatible planets may occur around >30% of the stars between 0.8
and 1.25 time the mass of the Sun.

If only single stars possess planets:

* There would be one habitable planet for every 413 stars.

* The mean distance between systems with habitable planets would be 31
light years.

* There would be one biocompatible planet for every 39 stars.

* The mean distance between systems with biocompatible planets would be
14 light years.

* There would be about 362 biocompatible (of which 34 would be habitable)
planets within 100 light years of us.

If planets could form in multiple star systems:

* There would be one habitable planet for every 196 stars.

* The mean distance between systems with habitable planets would be 24
light years.

* There would be one biocompatible planet for every 18 stars.

* The mean distance between systems with biocompatible planets would be
11 light years.

* There would be about 763 biocompatible (of which 71 would be habitable)
planets within 100 light years of us.

The author goes further and calculates the probability of the nearer
stars having biocompatible or habitable planets. Assuming that planets
can form in multiple star systems the following probabilities were
calculated:

Name Distance (LY) Type Habitable Biocompatible

Alpha Centauri A 4.38 G2V 7.8% 44%
Alpha Centauri B 4.38 K6V 4.4% 38%
Epsilon Eridani 10.69 K2V 0.6% 34%
61 Cygni A 11.17 K5V 0.0% 5.8%
61 Cygni B 11.17 K7V 0.0% 0.3%
Epsilon Indi 11.21 K5V 0.0% 18%
Lacille 9352 11.69 M2 0.0% <0.3%
Tau Ceti 11.95 G8V 1.5% 35%
Lacille 8760 12.54 M1V 0.0% 1.5%
Groombridge 1618 15.03 K7 0.0% 2.5%
70 Ophiuchi A 16.73 K1 4.4% 38%
70 Ophiuchi B 16.73 K6 0.0% 16%
36 Ophiuchi A 17.73 K0V 0.0% 28%
36 Ophiuchi B 17.73 K1V 0.0% 27%
36 Ophiuchi C 17.73 K5V 0.0% 9.0%
HR 7703 A 18.43 K3V 0.0% 27%
Sigma Draconis 18.53 K0V 1.5% 35%
Delta Pavonis 18.64 G5 5.1% 39%
Eta Cassiopeiae A 19.19 G0V 3.9% 38%
Eta Cassiopeiae B 19.19 M0 0.0% 0.7%
HD 36395 19.19 M1V 0.0% 0.5%
Wolf 294 19.41 M4 0.0% <0.3%
+5301320 A 19.65 M0 0.0% 0.6%
+5301320 B 19.65 M0 0.0% 0.5%
-45013677 20.6 M0 0.0% <0.3%
82 Eridani 20.9 G5 4.4% 38%
Beta Hydri 21.3 G1 7.5% 35%
HR 8832 21.4 K3 0.0% 23%

Assuming that the author's simulations and calculations are correct,
there could be as many as 5.6 BILLION biocompatible planets in our
galaxy of which about 500 MILLION are habitable. And, as the above
table shows, the nearest biocompatible planet could only be 4.38 light
years away.

Drew LePage
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