There were two pieces of news relating to space recently. Thirty years ago we knew there were stars. Now we know there are exoplanets and over 4,000 of them have been found. Many of these are much larger than Jupiter, but that may be because the bigger they are, the easier it is to find them. There are a number of planets very close to small stars for the same reason. Around one giant planet there are claims for an exomoon, that is a satellite of a giant planet, and since the moon is about the size of Neptune, i.e.the Moon is a small giant in its own right, it too might have its satellite: an exomoonmoon. However, one piece of news is going to the other extreme: we are to be visited by an exocomet. Comet Borisov will pass by within 2 A.U. of Earth in December. It is travelling well over the escape velocity of the sun, so if you miss it in December, you miss it. This is of some interest to me because in my ebook “Planetary Formation and Biogenesis” I outlined the major means I believe were involved in the formation of our solar system, but also listed some that did not leave clear evidence in our system. One was exo-seeding, where something come in from space. As this comet will be the second “visitor” we have recorded recently, perhaps they are more common than I suspected.
What will we see? So far it is not clear because it is still too far away but it appears to be developing a coma. 2 A.U. is still not particularly close (twice the distance from the sun), so it may be difficult to see anyway, at least without a telescope. Since it is its first visit, we have no real idea how active it will be. It may be that comets become better for viewing after they have had a couple of closer encounters because from our space probes to comets in recent times it appears that most of the gas and dust that forms the tail comes from below the surface, through the equivalent of fumaroles. This comet may not have had time to form these. On the other hand, there may be a lot of relatively active material quite loosely bound to the surface. We shall have to wait and see.
The second piece of news was the discovery of water vapour in the atmosphere of K2-18b, a super-Earth that is orbiting an M3 class red dwarf that is a little under half the size of our sun. The planet is about eight times the mass of earth, and has about 2.7 times the radius. There is much speculation about whether this could mean life. If it has, with the additional gravity, it is unlikely that, if it did develop technology, it would be that interested in space exploration. So far, we know there is probably another planet in the system, but that is a star-burner. K2-18b orbits its star in 33 days, so birthdays would come round frequently, and it would receive about five per cent more solar radiation than Earth does, although coming from a red dwarf, there will be a higher fraction of infra-red light and less visible.
The determination of the water could be made because first, the star is reasonably bright so good signals can be received, second, the planet transits across the star, and third, the planet is not shrouded with clouds. What has to happen is that as the planet transits, electromagnetic radiation from the star is absorbed by any molecule at the frequency determined by the bond stretching or bending energies. The size of the planet compared with its mass is suggestive of a large atmosphere, i.e.it has probably retained some of the hydrogen and helium of the accretion disk. This conclusion does have risks because if it were primarily a water or ice world (water under sufficient pressure forms ice stable at quite high temperatures) then it would be expected to have an even greater size for the mass.
The signal was not strong, in part, from what I can make out, it was recorded in the overtone region of the water stretching frequency, which is of low intensity. Accordingly, it was not possible to look for other gases, but the hope is, when the James Webb telescope becomes available and we can look for signals in the primary thermal infrared spectrum, this planet will be a good candidate.So, what does this mean for the possibilities of life? At this stage, it is too early to tell. The mechanism for forming life as outlined in my ebook, “Planetary Formation and Biogenesis” suggests that the chances of forming life do not depend on planetary size, as long as there is sufficient size to maintain conditions suitable for life, such as an adequate atmospheric pressure, liquid water, and the right components, and it is expected that there will be an upper size, but we do not know what that will be, except again, water must be liquid at temperatures similar to ours. That would eliminate giants. However, more precise limits are more a matter of guess-work. The composition of the planet may be more important. It must be able to support fumaroles and I suspect it should have pre-separated felsic material so that it can rapidly form continents, with silica-rich water emitted, i.e.the type of water that forms silica terraces. That is because the silica acts as a template to make ribose. Ribose is important for biogenesis because something has to link the nucleobases to the phosphate chain. The nucleobases are required because they alone are the materials that form with the chemicals likely to be around, and they alone form multiple hydrogen bonds that can form selectively and add as a template for copying, which is necessary for retaining useful information. Phosphate is important because it alone has three functional sites – two to form a polymer, and one to convey solubility. Only the furanose form of the sugar seems to manage the linkage, at least under conditions likely to have been around at the time and ribose is the only sugar with significant amounts of the furanose form. I believe the absence of ribose means the absence of reproduction, which means the absence of life. But whether these necessary components are there is more difficult to answer.