Basic Physics Offers Clues About Potentially Habitable Planets

Here on Earth, the air gets colder and thinner as altitude increases up until a certain height, about 45,000 feet, when it starts to warm up again. This phenomenon is also present on Jupiter, Saturn, Uranus and Neptune, as well as billions of other plants, according to a new study.

The layer of atmosphere where air gets thinner and colder with height - the layer that we live in - is called the troposphere, while the layer above it is referred to as the stratosphere. The tropopause is the zone between the troposphere and the stratosphere, which on Earth, can be found between 40,000-50,000 feet above ground.

In the 1980s, scientists confirmed the existence of tropopauses on Jupiter, Saturn, Uranus, Neptune and Titan, Saturn's largest moon. Surprisingly, the tropopauses on these worlds all occurred at about the same level in the atmosphere, at a pressure of about 0.1 bar, which is equivalent to one-tenth of the air pressure on Earth's surface.

The latest research, led by University of Washington planetary scientists, explains why this happens and goes on to suggest that tropopauses are probably common in billions of thick-atmosphere planets and moons throughout the galaxy.

The research team cites the physics of infrared radiation as the explanation, noting that at high altitudes, atmospheres become transparent to thermal radiation due to the low pressure there. Above this level, where pressure is uniformly around 0.1 bar, the absorption of visible and ultraviolet light causes the atmosphere to warm.

The planetary scientists suggest that 0.1 bar can be used as a rule of thumb for identifying the tropopause, and that the rule can be applied to any planet or moon with stratospheric gases that absorb ultraviolet or visible light.

Knowing this could enable astronomer to better identify possible habitable worlds because pinpointing the tropopause can allow them to extrapolate temperature and pressure conditions on the planetary surfaces, which are essential pieces of information when assessing whether a planet is habitable or not.

"Then we have somewhere we can start to characterize that world," said UW astronomer Tyler Robinson. "We know that temperatures are going to increase below the tropopause, and we have some models for how we think those temperatures increase -- so given that leg up, we can start to extrapolate downward toward the surface.

"It's neat that common physics not only explains what's going on in solar system atmospheres, but also might help with the search for life elsewhere," he said.

Robinson and his colleagues' research appears in the journal Nature Geoscience.