Post number four in the exoplanet series. Among variables considered so far, planetary habitability, age and distance have taken the spotlight. Today I’m going to talk about another important factor, especially because it has a direct impact on habitability: temperature.
There are a few cold planets we have spotted thanks to Kepler and other telescopes, but this one wins them all. Its name is OGLE-2005-BLG-390L b (I do hope the ongoing contest to name exoplanets comes out with something fancier) and it has been discovered in 2005 by the Danish 1.54m telescope at ESO La Silla (Chile). Its parent star is a red dwarf star about 28,000 light-years away, nearer to the centre of the Milky Way.
The planet itself is about 5.5 times as massive as Earth, one orbit lasts about 10 years, and it supposed to have a rocky surface. Also, with a surface temperature of -220 degrees Celsius (or -364 degrees Fahrenheit), it’s so far the coldest alien world (i.e, outside our Solar System) ever discovered.
Incidentally, there are other things worth mentioning about this planet. For example, “OGLE-2005-BLG-390Lb is only the third extra-solar planet discovered so far through microlensing searches “, said Jean-Philippe Beaulieu (Institut d’Astrophysique de Paris, France). “While the other two microlensing planets have masses of a few times that of Jupiter, the discovery of a 5 Earth mass planet – though much harder to detect than more massive ones – is a strong hint that these lower-mass objects are very common.”
It is also the case to observe that, while planet’s distance to the parent star has a direct relation to its temperature, as it could be expected, this is not always true. Sometimes planets farther away are actually comparatively “hotter”. This should not come as a total surprise, since temperature is function of numerous variables. You want a familiar example? Take our Solar System.
The coldest planet’s atmosphere, for example, doesn’t belong to Neptune, but to Uranus, whose distance to the Sun is about 19.2 AU (Neptune is 30 AU on average). With a temperature of -216 C (or -224 C as a minimum, even though sources differ on this one. I am using NASA and BBC here), it is the coldest recorded for full fledged planets. (Pluto has been now demoted to a dwarf planet, therefore I haven’t considered it here.). Why so? There are different theories, one of which hinting at Uranus’ tilt. If a giant impact long ago was the one responsible, it might have also caused the heat from within its core to spill out into space. Also, Uranus lacks the tidal interaction of Neptune with its moon Triton, which converts eventually in heat.
Just to clarify, planets are by no means the coldest objects in the universe. The coldest spot ever found by astronomers is instead a pretty, spooky nebula, the Boomerang Nebula, located 5,000 light-years away from Earth in the constellation Centaurus.
With its freezing – 272 degrees Celsius (-458 degrees Fahrenheit), it’s even colder than the background temperature of space!
And what about the contrary – the hottest exoplanet known so far? It is common knowledge that even in this case distance is not the only important factor, and that Venus, thanks to its dense atmosphere, is hotter than Mercury. But in outer space, this record belongs to a planet called WASP-12b, at about 870 light-years from Earth. With 2,200 degrees Celsius (about 4,000 degrees Fahrenheit), a gaseous composition of about 1.5 times the mass of Jupiter, and almost twice the size, it’s the hottest planet ever discovered.
Also, with an orbiting period of one Earthian day and a distance from its parent start of a mere 2 million miles, it is the closest planet ever found so far. But it’s not going to remain this way for long. Yes, because it’s so close to its parent star that tidal forces are distorting it into an egg shape and pulling away its atmosphere.
Literally, it’s being eaten away as we speak, and NASA has given the poor planet only another 10 million years before it is completely devoured. But I am going to leave galactic horror stories for another post.