by Maddie Meagher
The idea of finding a planet like Earth is a very exciting one to me and many others. However there are many factors that go into just making a planet suitable for life, factors such as location, temperature, water, atmosphere, and many others. Today I’m looking at some factors that play role in finding habitable exoplanets, or Earth 2.0 candidates as I like to call them. The main one I’m talking about today is the habitable zone. The habitable zone is the area around a star that a planet can have liquid water. The habitable zone is different for every solar system and is dependent on the parent star. Too close to the parent star and expect to have your oceans be boiled off. Too far and your planet would a giant ball of ice. The data I’m using for this little project comes from a site called the Habitable Zone. The site’s main purpose is to catalog planets in habitable zones and planetary equilibrium temperatures as well as many other various traits of planets.
These are the boundaries for the habitable zone for our own solar system. The two estimated ranges for habitable zone models in our solar system are the Conservative model from 0.95-1.4AU and the Optimistic model from .85-1.7AU (Note an AU is the average Earth-Sun distance).
What I wanted to compare was how the location of an exoplanet can affect the planet’s average temperature. To get my data for locations of exoplanets I looked at the optimistic model for the habitable zone. The optimistic habitable zone extends the inner/outer boundaries of a solar system by using the “Recent Venus”(closer to parent star) and “Early Mars”(farther from parent star) criteria. The more optimistic estimate refers to assuming the planet has the right atmosphere to help keep in cooler than it should be at the inner edge, and warmer than it should be at the outer edge. The picture above shows our own solar system’s habitable zone with the optimistic habitable zone being in dark green. For temperature I looked at periastron equilibrium temperatures or Teqb in the database I used. The periastron temperature is the average temperature of a planet when it is at it’s closest point to it’s parent star . It’s also important that a planet is well mixed. What that means that it has an atmosphere that can trap heat well enough and have the heat spread evenly across the planet’s surface. For context, our home planet Earth spends 100% of its time in the habitable zone, and is well mixed, with an average temperature of 290K.
Using the data from the Habitable Zone I made a graph comparing percentage of time spent in the THZO (the optimistic habitable zone,on the x axis) and the Teqb (average periastron temperatures in kelvins on the y axis). And for reference the blue dot represents where our own home planet Earth would sit on this plot. The results I found is that the more time a planet spent in the optimistic habitable zone (THZO) the more temperatures tended to be below at 500 kelvin, they also tend to fall into the 200-300 kelvin range which is habitable (300 kelvins being a nice warm 80.33 degrees Fahrenheit). This is especially true for those planets who spent 100% of their time the Optimistic habitable zone. Though I noticed three big outliers on the graph. One is a planet at 1457.4 kelvin (HD 20782 b) and the other at 1547.9 kelvin (HD 80606 b). Both these measurements are above 1400 kelvins or 2060.33 degrees Fahrenheit! However these two planets only spent very little time in the optimistic habitable zone as well as having some rather eccentric orbits (which I show below). The last outlier (HD 43197 b) spent more than 75% of its time in the habitable zone however temperature managed to reach 735.4 kelvins. The one thing all these planets have in common is that during their orbits they travel dangerously close to their parent star. This may contribute to the high periastron temperatures these planets.
Caption for three pictures above: All three of the exoplanets above for part of their orbit travel dangerously close to their parent star which is most likely the cause of the high spikes in their temperatures seen on the graph above. HD 20782 b doesn’t even spend all of it’s orbit too close to parent star or in the habitable zone. HD 20782 b spends a part of it’s orbit on the outer edge of the planets habitable zone where I would expect it temperature to drop.