By now, I suspect everybody has heard of Trappist-1, a totally non-spectacular star about 39 light years from Earth, and in terms of astronomy, a really close neighbour. I have seen a number of people on the web speculating about going there some time in the not too distant future. Suppose you could average a speed of 50,000 kilometers per hr, by my calculation (hopefully not hopelessly wrong) it would take about 850,000 years to get there. Since chemical rockets cannot get significantly more velocity, don’t book your ticket. Is it possible for a person to get to such stars? It would be if you could get to a speed sufficiently close to light speed. Relativity tells us that as you approach light speed your aging process slows down, and if you went at light speed (theoretically impossible if you have mass) you would not age, even though it would take 39 years as seen by an observer on earth. (Of course, assuming an observer could see your craft, it would seem to take 78 years at light speed because the signal has to get back.) It is not just aging; everything you do slows down the same way, so if you were travelling at light speed you would think the star was surprisingly close.
The chances are you will also have seen the comment that Trappist-1 is only a little bit bigger than Jupiter. In terms of mass, Trappist-1 is about 8% the mass of the sun, and that certainly makes it a small star as stars go, but it is about 84 times the mass of Jupiter. In my book, 84 times as big is not exactly “a little bit bigger”. Trappist-1 is certainly not as hot as the sun; its surface temperature is about 40% that of the sun. The power output of the star is also much lower, because power radiated per unit area is proportional to the fourth power of the temperature, and of course the area is much less. In this context, there are a lot of planets bigger than Jupiter, many of them about 18 times as big, but they are also too small to ignite thermonuclear reactions.
Nevertheless the system has three “earth-sized” planets in the “habitable” zone, and one that would be too hot for water to be in the liquid state, with a surface temperature predicted to be about 127 degree Centigrade provided it is simply in equilibrium with incoming stellar radiation. Of course, polar temperatures could be significantly cooler. The next three out would have surface temperatures of about 68 degrees C, 15 degrees C (which is rather earth-like) and minus 22 degrees C. Such temperatures do not take into account any greenhouse effect from any atmosphere, and it may be that the planet with a temperature of 68 degrees could equally end up something like a Venus. Interestingly, in the Nature paper describing them, it is argued that it is the planets e, f and g that could have water oceans, despite having temperatures without any greenhouse effect of minus 22, minus 54, and minus 74 degrees C. This arises from certain modeling, which I find unexpected. The planets are likely to be tidally locked, i.e. like the moon, the same face will always be directed towards the star.
So, there is excitement: here we have potential habitable planets. Or do we?
In terms of size, yes we do. The planetary radii for many are quite close to Earth’s, although d, the one with the most earth-like temperatures has a radius of about 0.77 Earth’s. Most of the others are a shade larger than earth, at least in terms of radius.
Another interesting thing is there are estimates of the planetary masses. How they get these is interesting, given the complexity of the system. The planets were detected by their transiting over the face of the star, and such transits have a periodic time, or what we would call a year, i.e how long it takes to get the next transit. Thus the closest, b, has a periodic time of 1.51087081 days. The furthest out has a period of 20 days. Now, the masses can be determined by mutual gravitational effects. Thus since the planets are close, suppose one is being chased by the other around transit time. The one behind will be pulled along a bit and the one in front retarded a bit, and that will lead to the transits being not quite on time. Unfortunately, the data set meant that because of the rather significant uncertainties in just about every variable, the masses are somewhat uncertain, thus the mass of the inner one is 0.85 + 0.72 earth masses. The second one is calculated to have a density of 1.17 times that of earth, which means it has a huge iron core. However, with the exception of the outer one, they all have densities that strongly suggest rocky planets, most with iron cores.
Suppose we went there. On our most “earth-like” planet we might have trouble growing plants. The reason is the light intensity is very low, and is more like on earth just after sunset. The reason the temperatures are adequate is that the star puts out much of its energy in the form of infrared radiation, and in general that is not adequate to power any obvious photochemistry, although it is good for warming things. The web informs us that astronomers are excited by this discovery because they give us the best chance of analyzing the atmospheres of an alien planet.
The reason is the planets orbit in a plane that means they pass in front of the star from our observation point, and that gives us an excellent chance to measure their size, but eventually also to analyse their atmospheres if they have certain sorts of gases. The reason for this is that as infrared radiation passes through material, the energies corresponding to the energies of molecular vibrations get absorbed. So, if we record the spectrum of the stellar radiation, when a planet passes in front of it, besides the main part of the planet lowering the intensity of all the radiation, where there is an energy corresponding to a molecular vibration, there would be a further absorption, so there would be little spikes on the overall dip. Such absorption spectra are often used by chemists to help identify what they have. It only identifies the class of compounds, because all compounds with the same functional group will absorb the same sort of radiation, but as far as gases go, there are not very many of them and we should be able to identify the with quite a degree of confidence, with one exception. Gases such as nitrogen and oxygen do not absorb in the infrared.
So, where does that leave us? We have a system that in principle lets us analyze things in greater detail than for most other planetary systems. However, I suspect this might also be misleading. This system is quite unlike others we have seen, mainly because it is around a much smaller star, and the planets may also be different due to the different conditions around a smaller star during planetary formation.