TRAPPIST-1 Planets

TRAPPIST-1 is an M8 dwarf star which is one of the coldest stars on the Hertzsprung-Russell Diagram. It has a surface temperature of about 2550 K (the Sun, by comparison has a surface temperature of 5800 K) and a mass of approximately 0.08 Solar masses.


Recently, astronomers led by Michael Gillon of the University of Liege announced that they discovered three planets orbiting TRAPPIST-1 using the transit method. The two closest are tidally locked to their parent star, much like the Moon is tidally locked to Earth. However, the third planet lies just at the outer edge or beyond the habitable zone of the star.


Theoretically, this planet could harbor life, but in all likelihood, it would nothing like Earth life. By being in the habitable zone, this means that water would be in liquid form, which astronomers believe would be necessary for life to exist.

The Drake Equation

The Drake equation was proposed by Frank Drake in 1961 to give a probability of life existing in the Milky Way Galaxy. The equation is a product of fractions and numbers that are not well known and are only estimated based on what we know. It is given by:

N = R_* * f_p * n_e * f_l * f_i * f_c * L

What do these variables mean? Let's look at each one individually.

R_* is the average rate of star formation in our galaxy. It tells us how many stars are born every year. When the formula was first published, a conservative estimate of 1 star per year was formed. Now, we know the number is around 7 stars per year.

f_p is the fraction of those stars that may have at least one planet. Originally, it was believe that a fifth to a half of all stars could have planets. Now, this number can range from 0.4 to 1.0, depending on the parameters. It is very likely that all stars will have planets form from their stellar nebula, so 1.0 is a reasonable assumption. To be conservative, however, many think that only 40% of all stars will have at least one planet around it.

n_e is the number of planets in that system that are Earth-like. These would be planets that are terrestrial and are in the habitable zone around their parent star. Originally, they thought that 1 to 5 planets around a star with planets would be in the habitable zone. Now, it may be estimated that one out of every five planetary systems would have a terrestrial planet in its habitable zone, or n_e = 0.4.

f_l is the fraction of those planets that will develop life. This number is very hard to estimate. The development of life could arise as soon as the right conditions on the planet exist. However, it may be that that primordial life could easily be snuffed out if the conditions change quickly. Original estimates suggest that if the planet is terrestrial and in the habitable zone, this fraction is 1.0. Modern analysis suggests it could be 13%.

f_i is the fraction of those planets that has life that develops intelligence. This fraction is subjective as we should define what is meant by intelligence. Beyond humans, some animals can display intelligence in the use of tools and language, however, we would not consider them intelligent in the same way humans are intelligent. Most animals are not self-aware, have no form of written communication, or use logic in any way. This number could be extremely low, 10^-10, or high, 1 (meaning that any planet that develops life will eventually have an intelligent species evolve.

f_c is the fraction of those intelligences that develop communication that reaches beyond their home planet. We have this capability already in the form of radio signals, but have only had it for the last 100 years. This is estimated to be between 10% to 20%.

L is the average lifetime of the species after developing communication. Factors that could effect this would be natural disasters or self-annihilation. Using all the above criteria, our lifetime is only 100 years. Estimates range between 1000 years and 1 billion years.

Combining these all together, conservative estimates give the number of intelligent species in the Milky Way to be 8x10^-20, which means that in all likelihood we are alone in the Milky Way and possibly, the entire Universe. Using the more hopeful statistics, estimates give that there are about 36.4 million intelligent species in the Milky Way (side note: there are about 100 billion stars in the Milky Way, which means that only 0.000364% of all stars may have intelligent life on one of their planets.) I’m more likely to believe in the higher number than the lower number, but I wouldn’t be surprised if there were less than 1 million intelligent species in the Milky Way. Also, if other intelligences exist, they are probably in the same location in the Milky Way as ours, the disk. The reason why is the age of the disk and the presence of metals in the disk as compared to the bulge and the halo. Also, in the disk, things are not as compact, which means that planets are probably far apart, on average.



Earth-like Exoplanets

In the last few years, astronomers have been focusing on finding planets considered terrestrial and habitable. Terrestrial planets are Earth-like in terms of composition and size. But what do we mean by habitable?


By definition, a habitable planet is a terrestrial planet in the so-called Goldilocks zone, named after the fairy tale character who found the just-right porridge, chair, and bed in the three bears' house. In this case, what we mean by just right is that the temperature of the planet is just-right, i.e. it is warm enough to have liquid water on the surface.


Why is water important? Water is a strange substance, even though everyone is familiar with water. Water is the only compound in the universe that is at its most dense when it is a liquid, not in its solid state. At 4°C, a given volume of water will be heavier than the same volume of ice at 0°C. Why does this matter? If you know anything about ice fishing or frozen over bodies of water or even drinking ice water, ice floats. Drop any other solid compound in its liquid component, and that solid will sink. This is important because it is theorized that life on Earth began in water. If water were to freeze like any other substance, all bodies of water would freeze solid and life could not survive. So liquid water is important.


Another thing that is important in looking for habitable planets, is that the star that any potential Earth-like planet must have a long evolutionary process. We want to make sure that the planet does not evolve off the main sequence too quickly. We know that life on Earth didn't arise until about 3.5 billion years ago, when the Earth was only a billion years old. However, intelligent life, or maybe just human life is only about 100,000 years old. High-mass stars would not be able to host a planet that could harbor life as those stars would evolve too quickly and supernova within a few million years of forming. For that matter, planets around A-type and early F-type stars would evolve off the main sequence too rapidly for any intelligent life to form.


On the other end, planets around low mass stars, M-type stars, would have to be extremely close to the parent star to be warm enough to have liquid water. However, being so close to the parent star also has problems. Recall that Mercury is tidally locked to the Sun. A planet that close to an M star would also likely be tidally locked which would lead to extreme temperature swings on the light-side and dark-side of the planet. Possible life would be single-celled organisms or those that could survive high temperatures, but intelligent life is unlikely to form on those planets.


So what type of stars do we look for Earth-like planets around? Late F-type, G-type, and early K-type stars are the stars that are stable enough, stay on the main sequence for a few billion years, and hot enough for any Earth-like planets to be a significant distance away from the parent star, but still close enough for liquid water to exist on the surface.