Planetary System around TRAPPIST-1

On February 22, 2017, scientists announced that there were at least seven terrestrial planets orbiting around TRAPPIST-1. Three of them are within the habitable zone. What does this mean for us?

We should learn a little about TRAPPIST-1. It is an M8V star in the constellation Aquarius, approximately 39.5 light-years (12.1 parsecs) from Earth. As an M8V star, it is smaller and cooler than Earth. In 2015, the first three Earth-sized exoplanets were discovered around the star. It wasn't until the recent announcement that they confirmed three of the planets were in the habitable zone. The habitable zone is the zone around a star where liquid water can be found. Look at the post about Earth-like planets to see why both are important.

For a star like TRAPPIST-1, the habitable zone is much closer to the star than it would be for Earth. It is likely that life might be on these planets? No, because these planets are so close to the parent star, and TRAPPIST-1 is very active, if these planets had any atmospheres at one time, they have likely been stripped away. If any life exists, it will be primitive in nature, one-celled lifeforms, if any.

Image courtesy of NASA

As shown in the above image, these are all considered terrestrial planets. They have radii and densities comparable to Earth. They are made up of mostly refractory elements.  

General Relativity and Astronomy

Previously, we discussed how mass can curve space(time) due to general relativity. Why is this important?

The curvature of mass leads to interesting phenomena. The first is it causes the perihelion of a planet orbiting the Sun to precess. Secondly, it causes light actually to bend - yes, gravity affects light - and this leads to really weird stuff.

Let's look at the first one. The best example of the precession of a planet at it orbits the Sun is the path of Mercury. This was discussed back in the post about Mercury and General Relativity. The highlight of the discussion was that as Mercury orbits around the Sun, at perihelion, Mercury is in the deepest part of the gravity well created by the Sun. As it continues to orbit, each successive perihelion moves farther ahead in its orbit. The perihelion of Mercury was noticed in the mid 1800s, but was thought to be caused by an inner planet. But after Einstein's Theory of General Relativity was developed, the equations were able to show why Mercury's orbit precessed around the Sun. Everything that orbits around another body shows this precession, with the amount of precession dependent on the mass of the central body.

The second one seems a little weirder. From Newton's equation for universal gravitation, we see that the force of gravity is dependent on mass. However, light and all electromagnetic radiation are massless. So how does gravity bend light?

In a nutshell - gravity wells.

If a star were behind the Sun, in Newtonian gravity, we would not be able to see it, because gravity only affects objects with mass. We would see something like this.

However, because of General Relativity, light will bend in the presence of a gravitational field. The light from the star will curve around the Sun and we can see it from Earth.

This was actually proven by Sir Arthur Eddington. In 1919, there was a solar eclipse that he observed and photographed. When the pictures were analyzed, they could see the effect of gravity on stars. This analysis only worked during an eclipse because otherwise, the Sun would be too bright and wash out the background stars. Eddington gave physical proof that General Relativity was correct!

Positive and Negative Image of Solar Eclipse of May 1919

Image Credit:



F. W. Dyson, A. S. Eddington, and C. Davidson

This phenomena of light bending around masses also is used to search for exoplanets. As a planet passes in front of star, that planet can bend the light towards us. This process is called lensing. Not only does it allow the light to reach us even if the source is behind the mass, it can also magnify the light, making it brighter. Most of the gravitational lensing seen are galactic in nature.

The curved arcs are the lensed (background) object. The centers are the lenses bending the light.

Image Credit:

NASA/Hubble Space Telescope

Einstein's Cross (Gravitation Lensing) - Quasar being lensed by a central dim galaxy

Image Credit:

ESA/Hubble/NASA

Einstein Ring - When the background object is perfectly aligned with lensing object and the Earth, a complete ring can be created

Image Credit:

NASA, ESA, A. Bolton (Harvard-Smithsonian CfA) and the SLACS Team

Hubble Site

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.