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.