Showing that you shouldn't draw conclusions based on too few facts, new NASA research suggests that a decades-old view of Uranus as an unusually cold planet isn't true, but that it does actually generate its own heat as other worlds do.
For a solar system that isn't exactly left wanting for oddities, Uranus pretty much sits at the top table. The seventh planet from the Sun, it's been something of an enigma ever since it was discovered by the astronomer William Herschel in 1731. For over 200 years, very little, if anything, was known for certain about it, but that all changed in the last 50 years with the flyby of Voyager 2 in January 1986 and the development of powerful space-based and earthbound telescopes.
Today, we know Uranus as a world with an axial tilt so great that it rotates on its side, with one pole and then the other facing directly at the Sun during a continuous 42-year summer. It also rotates on its axis in the opposite direction from all the other planets except Venus, while its magnetic field is tipped at 60° from the poles.
And then there's a possibility that it rains diamonds deep inside the atmosphere where the pressure could turn methane into super-hard crystals.
It's also cold. Really cold. It has the coldest atmosphere of any planet where that atmosphere isn't actually frozen, with a temperature reaching as low as -224 °C (-371.2 °F). More than that, the planet is like my feet in bed. It just can't seem to warm up.
Or, at least, that's what scientists thought.
According to the accepted view, Uranus had no internal heat. When you added up the heat it received from the Sun and subtracted the heat it radiated out, the planet's total came to zero. That's unusual because the other planets radiate more heat than they receive – especially the gas giants Jupiter, Saturn, and Neptune. Uranus was the odd planet out.
This was a bit of a poser for planetary scientists. Why would it do this? The other planets produce a surplus of heat because of heat left over from their formation 4.5 billion years ago, the decay of radioactive elements at their core, and phase changes over time in the planet's internal structure. If Uranus was so cold, it might have been due to it being much older than the other planets or perhaps the cosmic collision that laid it on its side carried off all that excess heat.
Of course, there was one big problem with all this speculation. The heat balance profile of Uranus was based entirely on one very brief set of data collected by Voyager 2. Was that enough to draw a proper conclusion?
According to the new study by NASA and Oxford University, apparently not. The team of scientists led by Amy Simon, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, took another look at the problem, only this time they added in observations of Uranus from decades of observations from ground-based and space-based telescopes, including NASA’s Hubble Space Telescope and NASA’s Infrared Telescope Facility in Hawaii. This allowed for a better idea of how much light is absorbed, reflected, and radiated by Uranus in all directions. This was fed into a new computer model that also took into account the planet’s hazes, clouds, and seasonal changes.
In other words, they ran the numbers.
What they found was that Uranus radiates 15% more heat than it receives over the course of its 84-year revolution around the Sun. That's not a big imbalance compared to neighbor Neptune, but it is significant. According to NASA, this helps put to rest questions about the age of Uranus, which firms up ideas of the timeline for when the solar system formed, how and when the planets migrated into their present orbits, the mechanisms behind this, and how they may relate to observations of Uranus-sized exoplanets orbiting other stars.
Source: NASA