ASTROBIOLOGY PART-VII EXO PLANETS

The lifespan of a star decreases rapidly the heavier it is than Sun as per the following formula.

Lifespan of the star=lifespan of the Sun* (star mass/solar mass) raised to the power of -2.5

L_St=L_S*(M_st/M_s)^-2.5

The lifespan of the sun is estimated to be 10 billion years. By this equation a star with double the suns mass would only live for 1.80 billion years. If it is 3 times then it would only live 641 million years. In the latter 2 cases, there is not enough time for intelligent life to develop.

Therefore we have to concentrate only on F, G, K & M spectral class type of stars whose masses are closer to the Sun. Our Sun is a G type star.  The G type stars have solar masses from 0.8 times  to 1.1 times. These stars live for 17 billion years at the lower end to 8 billion years at the upper end. F types are between 1.2 to 1.6 solar masses. K type stars range from 0.5 to 0.8 solar masses. M type stars have masses less than half of the Sun. The smaller they are than the Sun, the long lived they would be.

76% of the stars in the Universe are M types which have half the mass of the Sun. Consequently, they live for 56 billion years. K types account for 12%, G& F classes along with other brighter stars make another 12 %.

The M type stars have very low temperatures (which is why they live long), so low that most of their energy is radiated in the infrared than the visible spectrum. They are known as Red Dwarfs. Proxima Centauri the nearest star to us is a Red Dwarf.

Since the life of M type stars is very long they have every chance of sustaining intelligent life provided other conditions are fulfilled. Since 76% of all the stars are of this type there are high chances of finding intelligent life on them.

Even if we see Proxima Centauri through the most powerful telescope we have, still we would see only a point of light. The 2 essential points required for life as it is known to us; a source of energy and a stockpile of essential elements are present in many planets. However, the 3^rd, liquid water is more difficult to find because it is available only in a narrow range of temperature.

The earth is 150 million Km from the Sun and lies in the habitable zone of our solar system. The situation would be similar in other G type stars (like our Sun). Such a habitable planets distance from their sun shrinks in cooler K & M types of stars but increases in hotter F type stars.

However, in the cooler M & F type of stars there is another factor that has to be taken into consideration that affects life. These stars are red dwarfs which radiate a lot of their energy below the visible part of the spectrum, in the infrared. On the other hand in F type stars a lot of their energy is radiated in the visible spectrum and the ultraviolet. As ultraviolet rays are harmful to life F type stars are less likely to have life on their surrounding planets than cooler stars.

Our sun too produces ultraviolet rays which are harmful for life, but luckily we are insulated from them by our Earth’s atmosphere. The Earth’s magnetosphere also protects us from the effects of space weather. This is a fast moving wind of charged particles emitted by the Sun. Our magnetosphere deflects most of these charged particles. Space winds like these would be very bad for life on planets that do not have a magnetic field.

So, there are many ifs and buts’ for life to evolve on a planet. Now that brings us back to the concept mentioned earlier; bio signatures. The most promising approach is through the Exoplanets atmosphere in which chemicals that advertise life are known as biomarkers.

Now, Oxygen is a waste product of living things, such as bacteria and plants. Another waste product that is produced by bacteria is methane. Methane consists of only 1% of the atmosphere, but even that would not be there if it is not constantly replenished by biological processes. If not for life producing them continuously as waste products both Oxygen and Methane would combine with other elements and disappear. There are some other non biological processes that can produce Oxygen and Methane, but they are not very likely. Therefore the presence of these 2 gases on any planet is the best biomarker that we know of.

Now as we mentioned earlier, both M and F type stars are red dwarfs and consist of 88% of all the stars. They emit radiation in the form of infrared rays which the Hubble Space Telescope was unable to detect. There is a very recent development in this area where the James Webb Space Telescope was put into space on Christmas day 25^th December 2021.

With greatly improved infrared resolution and sensitivity it can view objects some 100 times as far as the Hubble Space Telescope. So we might expect some newer discoveries by this space telescope. This would be used by Sara Seager, a professor of Astronomy at the Masachusetts Institute of Technology and her associates along with data from TESS (Transiting Exoplanet Survey Satellite) launched in April 2018.

Very difficult to say whether the team would be successful because life as known to us consists of only an infinitesimal percentage of the mass of the Universe and hence is very difficult to trace with our current level of science.   

So far more than 4000 Exoplanets have been found, about two thirds of them being discovered by the Kepler Space telescope launched by NASA in 2009 to find earth like planets orbiting other stars and it is now retired.

Now 4000 is a lot to check for life and we have to narrow it down. The University of Puerto Rico has prepared an online “Habitable Planets Catalogue” which narrows it down to 50. One of the 50 belongs to a nearby star Tau Ceti (12 light years away). This is a “G” type star like the sun and one of its planets Tau Ceti “e” is in the inhabitable zone for life. This may well be the closest star with life.

Even though, the star is close, with our current day science known to us it is impossible to travel that distance and get some information from it. That is possible only if we travel at more than the speed of light. As of now there is no means of doing so, and going through a wormhole is a fabulous fiction because the conditions in the wormholes are stupendously destructive and nothing can survive passing through them leave alone living beings.

Some fundamental particles are supposed to travel over the speed of light, but that is neither here nor there and does not establish in any way that anything else can travel over the speed of light.

There are some close contenders to Tau Ceti for having habitable life and they are the much nearer Proxima Centauri(4 light years) and the Barnard’s star (6 light years). However, these 2 stars do not belong to the spectral class of the Sun which is G and are the much dimmer M type (Red Dwarf star). As these stars are cooler, the habitable planets have to be closer to their Sun.

The prospects that M type stars to have life means that a higher probability of finding intelligent life. These stars are more numerous and form about 75% of the stars of the Universe. Moreover they have very long lifespans providing more chances for intelligent life to develop.

Since M type stars are cooler, they emit more of their light in the infrared which makes it more difficult for photosynthesis to occur and therefore for sustenance of plant life.