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.  

Proxima Centauri B

It has been recently announced that astronomers have discovered a possible terrestrial planet in the habitable zone around the star nearest to the Sun, Proxima Centauri. We discussed a little about Proxima Centarui when we discussed the nearest star system to the Sun, Alpha Centauri.


So what exactly does this mean? Terrestrial planets are those planets much like Earth in terms of size and composition. The habitable zone are the parts of a planetary system where water can be in liquid form on the surface of the planet. Remember, that water is a important to life in the universe.


Now, having a Earth-like planet in the habitable zone does not necessarily mean that the planet has water. We won't know for a long time, either.

Is Pluto a Planet?

In the past, I know I've been adamant to say that Pluto is not a planet. You can read them again here, here, and here. However, I've thought about things and may be coming around to categorizing Pluto as a planet.


Before, I had always said that there are two types of planets: terrestrial planets, like Earth, and Jovian planets, like Jupiter. But what if we redefined a planet to have a third type: dwarf planets. These are bodies in the solar system that are too large to be asteroids or comets but are not necessarily big enough to have swept all the smaller bodies from its orbit (meaning it could have co-orbital bodies), have densities somewhere between Jovian planets and terrestrial planets, and can be found anywhere in the solar system.


I did discuss dwarf planets in more detail in a previous post, when discussing Pluto (see first link above), so I may have unwittingly decided already that Pluto is a planet. In fact, if we make Pluto a planet again (which probably will happen sooner rather than later), we will also have add a few more planets to our solar system.

Trans-Neptunian Objects (objects orbiting the Sun outside of Neptune's orbit)

Image Credit:

NASA/Hubble

Eris and Makemake will definitely be added. Haumea, Sedna, and 2007 OR10 may as well. It all depends on where the lower limit of a dwarf planet's size will be if the IAU determines that.

This is what I love about science. It's a fluid subject that can easily change based on new information. I've changed my mind about Pluto (for now), and there may be other things I might change my mind about.

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.