Since the advent of space exploration, we’ve discovered an astounding number of celestial structures that wasn’t even known to us a couple decades ago. Exoplanets are one them, those newly discovered planets are light-years away from us, which we might never be able to reach them.
But what if we could live on one, could we ?
What ? Exoplanet ?
You might or not have heard the term. In short, an exoplanet is a planet that resides outside of our solar system, orbiting another sun and having it’s own structure. Some of them are really (really) hot, burning at high temperatures others are freezing at extremely low temperatures. We can find a wide variety of exoplanets in space. Like KELT-9b, which is the hottest extrasolar planet ever discovered. Having twice the size of Jupiter, it has an incredible day-side temperature of 4600K(4326°C), which makes it hotter than most stars ! It’s so close to its host star KELT-9 that it orbits every 1½ days. Other planets like 2MASS J2126−8140 orbits its host star each 900 thousand years, which is quite amazing !
Here we won’t focus on those insanely hot and dangerously toxic planets. For a planet to be habitable, it has to possess some important features similar to Earth’s. As we can only use our own planet as a basis of comparison because you are actually alive and reading this article. There are multiple factors in play when considering a planets habitability. It’s surface temperature or it’s average temperature is one that quickly comes up to mind. It has to be in the habitable zone, we don’t want to die of extreme cold nor die in high temperatures melting away, we are looking for something not too hot, not too cold. Another important one is mass, since the determining factor for gravity is mass, the more mass, the stronger the gravitational field, that’s usually the case. But keep in mind that mass is not the only factor affecting gravitational attraction. There many more aspects to consider when assessing habitability on an alien planet, like it’s distance from its host star, its eccentricity, its atmospheric composition and much more.
Here in our analysis our main focus will be on the surface temperature and the planetary mass. Since both of them are important factors when evaluating habitableness. We will also be exploring other features like the exoplanet’s distance from its star and much more.
A quick introduction to the planetary features we will be exploring
The surface temperature here will be calculated in Kelvins, which is an absolute thermometric scale used in astrophysics and in other physical sciences. 0 Kelvin is equivalent to −273.15 °C (−459.67 °F) which is when thermal motion ceases. In astronomy, when calculating a distant planet’s temperature we use a concept called the planetary equilibrium temperature or the equivalent blackbody temperature which is a theoretical temperature which the planet is at when considering it as a black body being heated only by its main star. The presence of an atmosphere isn’t taken into consideration here, therefore no greenhouse effect.
The planetary mass will be calculated in Jupiter mass MJpt, which is the mass relative to Jupiter’s mass. Jupiter is 1 MJpt, Earth is 0.00315 MJpt and Mars is 0.00034 MJpt.
The distance from our Sun to other stars is calculated in Parsec(pc), one pc is approximately 3.26 light-years. It is a pretty sophisticated method of calculating astronomical distances, so i won’t get into too much details.
The period is how long an astronomical object takes to complete one orbit around another object. Here the period will be in days, Earth’s orbital period around the Sun is 365 days.
The semi-major axis is defined as the radius of an orbit at the two most distant points. It is calculated in astronomical unit or AU, which is the distance from Earth to the Sun. 1 AU is approximately 150 million kilometers.
We’ll be exploring the Open Exoplanet Catalogue inside Jupyter Notebook for our research, as the names says it, it’s an open catalogue of all discovered exoplanets. Filled with useful and interesting information regarding extra-solar planets.
Surface temperature and planetary mass relation
Our main focus will be on the surface temperature and the planet’s mass, but first let’s see how both of those variables interact in the data set.
Complete data set
Cleaned data set SurfaceTempK(0-2000), PlanetaryMassJpt(0-10)
Let’s contain all the data between 0 and 2000 on the X-axis(SurfaceTempK) and 0 and 10 on the Y-axis(PlanetaryMassJpt). So now we’ll gather all the exoplanets having a surface temperature in the interval of 0 and 2000 Kelvin and a planetary mass between the interval of 0 and 10 Mjpt.
Interestingly (and obviously), we now get a different correlation score
Cleaned data set SurfaceTempK(0-2000), PlanetaryMassJpt(0-2)
Here we’ll reduce the interval and gather all the exoplanets having a planetary mass(PlanetaryMassJpt) between 0 and 2 Mjpt. Surface temperature stays at the same interval of 0 and 2000 Kelvin.
(A p-value of 0.11 basically means that if our null-hypothesis(i.e no correlation between surface temperature and planetary mass) is true we would’ve get the observed effect or more (i.e surface temperature and planetary mass correlation) at a rate of 11 % in the population of exoplanets, which is a little too high. We would need a p-value low as 0.05 to really say that there’s a correlation between surface temperature and planetary mass.)
Data of interest
We can see a bunch of them that are relatively similar to Earth in terms of temperature and mass.
We won’t go though each one them, we’ll be focusing only on the TRAPPIST-1 exoplanets and Proxima Centauri b.
We’ve got some interesting exoplanets from the TRAPPITS-1 system showing up on the plot. Which are TRAPPIST-1 d, TRAPPIST-1 e, TRAPPIST-1 f and TRAPPIST-1 g.
Let’s take a closer look at the TRAPPIST-1 system.
TRAPPIST-1 is an ultracool dwarf star which has a temperature of less than 2,700 Kelvin and a solar mass of 0.0802, which is a pretty low temperature and mass for a star. TRAPPIST-1 is 12.1 Parsec away from us, which is around 40 light-years away from our Sun, in the constellation of Aquarius. (That would take us 40 years traveling at the speed of light to get us there)
There are seven planets that have been detected so far orbiting the star. There is evidence of water being present on them due to a trail of hydrogen left by the closer orbiting planets when they transit in front of TRAPPIST-1. The planets that are farther away from the main star have a low equilibrium temperature, which helps in maintaining water at the surface.
TRAPPIST-1 planets are orbiting closely to one another which makes them more likely to be impacted by UV irradiation, this radiation causes water molecules to break down, producing hydrogen and oxygen. A significant amount of hydrogen could therefor be present system wide indicating a history of water loss from the planets. Each one of them would have lost water at a different rate due to the runaway greenhouse effect, which ultimately led to water boiling away.
(Hydrogen being present system-wide is due to the high levels of XUV radiation hitting the water molecules present on the exoplanets, which breaks up water molecules by a process called “ionizing radiation”.)
It is also highly likely that each of TRAPPIST-1 orbiting exoplanets are in a tidally locked orbit, having one side of their hemispheres constantly facing their parent star with one side getting light and the other shrouded in darkness. If that hypothesis was to be true, that would imply that between those two extremes there would be what we call a terminator line, which is the limit between light and complete darkness, that delimiting area would have a stable temperature suitable for liquid water to exist.
Another thing to keep in mind is that some of those planets weren’t always in the habitable zone.
Earth’s and TRAPPIST’s variables and correlations
That’s for the TRAPPIST-1 exoplanets ! Like i said in the beginning there are many factors in play when we look into habitability. Not considering the existence of a suitable atmosphere with the right properties is pretty dangerous. Even though TRAPPIST-1 habitable zone seems to be “habitable”, if the star isn’t calm and quiescent, excessive ultraviolet light could pretty much kill any living thing on those planets.
Another important aspect to consider is the distance separating us from TRAPPIST-1, the star is 12.1 Parsec away from our Sun, that’s almost 40 light-years away ! Unless we find a way to distort space-time or get to find a wormhole (A wormhole is basically a compact region in space-time which reduces time to travel between two points), traveling to TRAPPIST-1 is physically impossible.
At last we got Proxima Centauri b
Proxima Centauri which means in latin “nearest (star) of centaurus”, a red dwarf low-mass star, it is 4.25 light-years away from our Sun, it is 10 times less massive than our sun with a temperature about half of our Sun.
It is loosely orbiting the Alpha Centauri(Alpha Centauri A and Alpha Centauri B) binary star. In 2016, the European Southern Observatory (ESO) announced the presence of an Earth-like exoplanet orbiting Proxima Centauri, Proxima Centauri b.
Proxima Centauri b
Proxima Centauri b is one of the closest Earth-like exoplanet ever discovered to this date. Orbiting its sun in the habitable zone, it is highly likely that water is present on its surface. Having a surface temperature of approximately -39.15 °C with a planetary mass of 0.004089 Mjup which makes it a great candidate for a habitable planet. It has a semi-major axis of 0.0485, orbiting closely its main star. Above all, it is pretty “close” to us at 1.295 Parsec from us, that’s approximately 4.22 light-years away from our Sun. The planet also receives 30 times more XUV radiation than Earth and 250 times more X-rays.
Earth’s and Proxima’s variables and correlations
(According to some researchers Proxima Centauri b might have been hit by ‘superflares’ coming from its main star that made the planet uninhabitable long time ago.)
The TRAPPIST-1 exoplanets might be habitable in theory, but not knowing the planets exact atmospheric properties and irradiation levels opens a wide gap of uncertainty. Similarly, Proxima Centauri b atmospheric properties aren’t known, which is a crucial aspect when assessing habitability.
Our best bet would be Proxima Centauri b and TRAPPIST-1 g if we ignore the fact that those exoplanets might be highly irradiated due to the closeness to their main stars, they still share some important similarities with Earth. With a acceptable surface temperature and planetary mass. Proxima Centauri b is relatively close to us at a distance of 4.22 light-years. Unless we find a wormhole, we won’t be able to go anywhere close to it during our lifetime.
XUV radiation carry enough energy to break up electrons from atoms, high levels of radiation can break up water molecules which in turn destroys any life prospects. Which is why it is a major factor when looking for life-sustaining planets.
Looking for a new home in space isn’t that simple.
I would also like to point out that all the data we have about those extrasolar planets is largely based on theoretical assumptions and not on hard facts.