As part of an ongoing practice, I am asking scientists to speak out about the details of their field that most people get wrong. From time to time the general public can become misinformed about certain scientific subjects. It might be for reasons of being misinformed, misunderstanding, or even a popular movie that chooses to value entertainment over scientific accuracy. These misconceptions can lead to larger issues when it comes to funding research, creating laws, and even electing public officials that just plain have it wrong. Therefore, I am going to let the actual scientists speak their minds.
For this article, I had a talk with Andrew Rushby – super cool guy/astrobiologist at NASA Ames Research Center, about the habitable zone and today he is going to set the record straight. Most people would tell you that in astronomy the habitable zone is the area in a solar system that is capable of supporting life. But that’s not exactly true, in fact, it’s pretty far from the truth.
First, let me point out that the definition of the habitable zone is simply the area around a star at which liquid water could exist on a planet that is similar to Earth. Scientists use simple climate models to determine just where this zone is located in different solar systems. These models use Earth as a basic template and they include a few assumptions about chemistry and the nature of a planet’s atmosphere.
“The habitable zone is primarily useful as a parameter or characteristic of a star system (like the number of planets, their orbit, their size etc.) that can help us target our search for interesting planets and learn more about planet formation, as opposed to definitive coordinates at which to find life. My supervisor once described it as an ‘astronomically observable property of a planetary system’, and nothing to do with life at all!” Andrew Rushby.
In our solar system
Even in our own solar system, the habitable zone is not the best indicator of life. Most versions put Mars in the zone or at the far edge, but of course, this does not mean there is life there. If you are looking for the best chance at alien life in our system the moons of larger planets like Saturn seem to be the most likely. Scientists believe that places like Europa, Enceladus, and Titan could support life and they are between 3.5 and 7.5 AU from the outer edge of the habitable zone.
There is more than one version
There are many different types of habitable zones depending on the criteria involved. These versions can take into account different planets sizes, moons, and a range of atmospheric gases. In fact, there are even versions that include dry planets, free-floating planets, planets spewing volcanic hydrogen, and for the entire galaxy. As we continue to understand more about other solar systems we will probably get more and more versions.
It changes shape
The shape of the habitable zone changes over time. The shape is determined by the amount of starlight that can heat a planet. As a star ages its physical size and luminosity expand outward and as it does its heat will travel further. Planets that were in the zone for billions of years will find themselves to hot to support life and planets that were too cold will find themselves right in the middle of the zone. As an example, Earth now sits in our Sun’s habitable zone but in about 1.5 billion years the Sun will expand and become about 15% brighter. After that Earth will not be in the zone and it’s unlikely that it will be able to support life at all.
Hopefully, that clears things up a little bit and the next time you are talking about or reading about the habitable zone you will be a little bit wiser. If that happens, send me a message. I would love to hear about it.
To learn more about habitable zone check out Andrew’s podcast, Exocast.
