Post number five in the exoplanet series, covering this time the important aspect of size. (In case you are interested to other variables examined so far, here they are: habitability, age, distance, temperature).
After that in April 2014 a Earth-size planet was discovered in its star’s HD zone, there has been a lot of excitement in the scientific community. With excellent reasons: apart from being in the habitable zone range, with all the known consequences, size is the other variable that immediately comes to mind when thinking about a planet that might harbour life. Not enough mass, not sufficient gravitational pull, and you won’t have an atmosphere. Too big, and you will accumulate a thick atmosphere of hydrogen and helium as gas giants like Jupiter and Saturn have – not exactly welcoming either, for life as we know it.
Kepler-37b (Credit: NASA/JPL-Caltech)
But Earth-size planets, whilst more common it was initially thought, are only a fraction of the worlds existing out there, some of them so extreme in dimensions to make us wonder if they can be included at all in the exoplanet category. It’s worth having a look at the others.
Starting at the bottom, there are a few “smallest ones” – depending the characteristics this selection is based upon: in absolute terms of mass, this title seems to belong to a rocky world called Kepler 37-b, with a radius slightly greater than that of the Moon and smaller in size than Mercury (closer to the Sun, though. Its temperature is about 400 C. Gosh).
Kepler 10-B (Credit:NASA)
Before Kepler-37b was located in 2013, this title was held by Kepler-10b, just 1.4 times larger than Earth. Discovered in 2011, this was also the first rocky exoplanet to be confirmed by NASA’s Kepler mission using data collected between May 2009 and early January 2010.
If we want an exoplanet where water vapour has been detected, the smallest one title is instead to be given to the recently found HAT-P-11b, about the size of our ice-giant Neptune, therefore quickly labelled an Exo-Neptune. It is located 120 light-years away in the constellation of Cygnus but, differently from its benchmark, orbits much closer to its parent star.
HAT-P-11b (Credit: ESA/Hubble)
What about the other side of the range? Huge worlds do exist. However, here the problem is where the exoplanet stops being a planet and become a brown dwarf – i.e., a failed star. After all, brown dwarves are quite close to gas giants in terms of composition. This is so true that the most massive planet recorded in the NASA archive, DENIS-P J082303.1-491201 b, with around 30 Jupiter mass, probably qualifies more as a brown dwarf. CT Cha, listed in the Planetary Habitability Laboratory, with the largest radius and with around 17 Jupiter mass, is another example.
TrES-4 (Credit:NASA)
An interesting case of a massive planet is TrES-4, a strange planet in its own right. Why? Because it is enormous in size (70% bigger than Jupiter) but lower in density, and defies all current theories of superheated giant planets. It’s located in the constellation of Hercules, and orbits a star that it’s bigger and hotter than the Sun but only 10 times larger than the planet itself. All these factors make that a small fraction of its outer atmosphere manages to escape from the planet’s gravitational pull and forms an envelope, or a comet-like tail around the planet. Weird indeed.
TrES-4 and its lower density is a reminder that this variable is relevant as well when classifying exoplanets. And if we consider density (i.e., the ratio between mass and volume) we can easily notice that in the Solar System gas giants vary a big deal under this regard – with Saturn claimed to be floating on an hypothetical ocean for being less dense than water itself (technically speaking, however, it won’t: read this).
CoRoT-Exo-3b (Credit:NASA)
One of the densest exoplanet known to date is a remote celestial body called COROT-exo-3b. It’s about the size of Jupiter, but more than 20 times Jupiter’s mass, making it twice as dense as lead. Another good candidate for our brown dwarf catalogue.
Compact as it can be, however, COROT-exo-3b is not remotely as dense as PSR J1719-1438 b – this one without any doubt among the most fantastic objects ever detected in this category. Also known as the pulsar planet, with a mass that is nearly the same as Jupiter’s but only 40% the size of the planet, this strange world is largely composed of crystalline carbon, but with a density far greater than diamond. That’s it – a huge diamond planet.
I would suggest that a diamond world is reason to fund the research and engineering needed for interstellar travel to mine it. But of course dumping that much diamond on the market would drop the price and make the effort self defeating…
Hello, thanks for your comment! You’re actually quite right about this, and I know calculations have been made in this sense.
On the same topic, have a look at this article: http://www.forbes.com/sites/petercohan/2012/10/12/diamond-planet-worth-26-9-nonillion/