As if astrophysics didn’t have enough problems on its hands, it seems that a new discovery has handed over a new challenge. An exoplanet pair, discovered by the exoplanet hunting Kepler Space Telescope, unimaginatively named Kepler 36-b and Kepler 36-c according to accepted catalog practices, turns out to be spectacular blow for established planet-formation theories.
The two planets are very close to one another. They orbit their parent star, but they are separated by only 1.9 million kilometers. This turns out to be 0.01 AU, an AU being an ‘Astronomical Distance’, which is the average distance between the Earth and the Sun. So the two planets are a hundred times closer to each other than the Earth is to the Sun.
Astrophysicists wouldn’t be worried, yet. True this is very odd, but it doesn’t transgress the rule books. Here comes the blow: The two planets are as different as Venus and Neptune!
How is it a problem?
In our own Solar System, Venus is a rocky planet meaning that it has a solid rocky ground to stand on. It’s not a ball of compressed gas. In fact, Venus very much has a surface and it is riddled with numerous volcanic features, like volcanic domes, large fissures on the ground and rolling planes formed by large sheets of solidified lava flows. Kepler 36-b is not quite Venus in the volcanic sense, but is definitely rocky. It’s more like Earth, minus the oceans. Only that it is four times heavier than Earth, suggesting that is very dense.
Neptune is as unlike Venus as a planet can get. It is a gas giant and has no solid ground to stand on. It’s much larger than either Venus or Earth. Kepler 36-c resembles this gas giant.
Planet formation theories suggest that when the disc of rocks, gas and dust spinning around a parent star coalesces in clumps to form planets, the gas and dust are naturally flung far away from the center, while the rockier bits of the initial soup of raw materials stay within the vicinity of the parent star. They coalesce to form rocky planets. This explains the occurrence of rocky planets to the interior and the presence of the gas giants farther out for our own Solar System.
Since the forces involved in this process are those of gravitation (due to the star) and the centripetal forces (due to the rotation of the disc of raw materials), scientists would have expected the same rule to work for any planetary system, as these forces are both ubiquitous and inescapable.
But that can’t be the whole story!
That however is not the case apparently. The atmospheres are also very fascinating. Kepler 36-c has an atmosphere which is 10 times as dense as Kepler 36-b, even though they are at almost the same distance from their parent star.
Scientists like puzzles, especially those which can rewrite books. Nothing is ever sacred, not even the physics of the heavenly bodies.