It is now generally accepted that only a future space probe can tell us if life exists on other planets in our solar system. This automatically imposes on us the burden of searching for life on other planetary systems also. Such a search will naturally involve the study of some suitable nearby stars which may reveal evidence of such planetary systems.
There is reason to believe that the sun and the planets were formed at the same time. The planets arose out of the “solar nebula” each condensing from lumps which first formed a “proto planet”. (According to some astronomers the main bulk of the planets is made up of hydrogen and helium so they feel that the planets formed directly from the gas which also made up our sun. In brief, gas formed into a proto-star that shrank to form the sun, and later some of it coalesced into proto-planets that shrank to become the gaseous planets we now see). But for life to have formed on any of the planets, two basic conditions had to be fulfilled. First the solar system had to be stable; the planets had to orbit in paths that did not disturb one another even over a long period of time. Secondly the sun could not rotate too fast, for if it did it would burn up more quickly and would certainly throw off hot gaseous material; this would make it too hot and therefore likely to destroy complex molecules.
When the sun formed it would have rotated very quickly, as the gas forming it shrank from the proto-star stage down to that of a fully developed star. Something obviously had to occur to slow it down. It is likely that the sun transferred some of the energy of its rotation to the rest of the nebula thus explaining why the planets orbit around the sun; if the sun had retained its high rotation rate, then in course of time, much, if not all of the rest of the nebula would probably have fallen into it. But as things have turned out we have quite obviously a stable planetary system.
The requirement of a stable planetary system for the development of life would necessitate ruling out any planetary system in orbit around binary or multiple stars. The changing forces operating in such systems would not give a “stable” result and the radiation received on any planet would vary so much so that no settled life-system could develop. If we are to search for other planetary systems where there is any change at all of life existing we must look at single-star systems.
But what kind of single-star systems should we analyse? Are all classes of stars equally likely to have planets of the kind we need? Generally bright hot stars rotate at a faster rate than cooler yellow and red stars. Because of our requirement that the star ought to have transferred some of its rotational energy to its planetary system, we should exclude bright hot stars. Rather we should seek stars that are not so large and energetic, belonging to the G,K, and M, “spectral class”. (Astrophysicists can draw a diagram of star’s life plotting absolute magnitude against spectral type or “class” because spectral type is an indication of the surface temperature of a star. If a star is large, young and very hot, it will appear as a B or A star but if it is not so large and energetic it will appear as F or a G (like our Sun) or even a red or a deep red star, K or M). We must however bear in mind that much older stars may have lost some of their energy of rotation because it has been carried away by their stellar winds. (In 1957 a German Physicist Ludwig Biermann discovered the “solar” or “stellar” wind—a stream of electrified atomic particles constantly blown out from the Sun. The wind originates in the corona, the sun’s thin atmosphere which begins about 10000 kms above its disc and extends to several million kilometres out into space. Hot and very hot stars also behave in the same way, blowing vast amounts of material into space). And we should also exclude red giant stars because if they had possessed planetary system when they were younger, the stars would have by now expanded so much that they would have swallowed up some of their planets and ruined the steady conditions enjoyed by the others.
One last condition is that the stars should not be too far away otherwise we should have difficulty in obtaining evidence of their possessing planetary systems. So let us study stars nearby keeping to those which are very similar to our sun because this will make it more likely that they will have planets suitable for life. The so called G-type stars have a surface temperature about the same as the sun’s and are possible candidates. However it would probably safer to confine our selves to the G and K class stars and thus keep a range of between 6000 and 4000 degrees kelvin.
When we look for stars of the temperature, we notice that there are seventeen within as short a distance as 13 light-years though most are multiple systems. For this reason alpha centauri, the nearest, is of no use to us, but Epsilon Eridani at a distance of 3.3 parsecs is a possibility, even though it is class Kand 1.8 times dimmer than the sun. The star Elndi (class K) some 2-1/2 dimmer is still another possibility at 3.4 parsecs. So is T Ceti, a G—type at 3.7 parsecs with a brightness about 2- 1/2 times less than the sun. No planets have actually been observed for these stars yet though the new space telescope being built in the United States may provide the necessary evidence United States may provide the necessary evidence.
However direct observation of planets, though desirable, is not necessary. Another way of knowing whether a star has a planetary system around it is to find out whether or not a star is moving in a straight line through space . If it has a planetary system, then it may appear to have a wobbly path because both the star and its planets will be orbiting round the common centre of mass. Some nearby stars do seem to show such a wobble though the observations are not conclusive except in one case:
Bernard’s star, an M-type star some 650 times less bright than the sun lying at the distance of no more than 5.9 light years. Bernard’s star has a large motion of its own, amounting to no less than 10.3 arc seconds per year, and has been observed carefully for about 60 years. (To measure distances, astronomers use trigonometry and various terms definitions such as arc second, parallax and parasec. The angle which gives the shift of a star measured against the background of the more distant stars is calculated in arc seconds. An arc second is 1 /60th of an arc minute which is 1/ 60 th of a degree. Half this angle is used in calculating distance and is known as the star’s parallax. One parasec is that distance at which a star would have a parallax of one second of an arc. Thus a star of parallax .5 would lie at two parsecs. Centauri has a parallax of .76 arc seconds and thus its distance is 1.3 parsecs. One parasec is equal to about 3.25 light years. A light year again is the distance travelled by light at 186000 miles per second for one whole year.) During this time Bernard’s star has been observed to wobble that is to swing regularly to either side of its straight path by a few hundredths of an arc second. The wobble could be due to the presence of two orbiting planets one with a period of 11V2 years and of the same size as Jupiter and the other with a period of 20 to 25 years. This second more distant planet could have a mass of about a half that of Jupiter’s. What therefore is certain is that we are not observing a multiple-star system but a star being orbited by planetary bodies. Its discovery gives us great hope in the case of other wobbling stars and in the belief that there are plenty of other planetary systems.
The feeling that somewhere in the depths of space there is star with a planet like ours going in orbit round it and that there must be in that rich earth an advanced life form with which we can surely hope to establish contact, provides the motivation for astrophysicists to turn their telescopes towards the night sky with wonder waiting eyes.