Tuesday , October 22 2019
Home / Miles Kimball / Leah Burrows: A ‘Goldilocks Zone’ for Planet Size

Leah Burrows: A ‘Goldilocks Zone’ for Planet Size

Summary:
The inner edge of the habitable zone is defined by how close a planet can be to a star before a runaway greenhouse effect leads to the evaporation of all surface water. But, as Arnscheidt and his colleagues demonstrated, this definition doesn’t hold for small, low-gravity planets. The runaway greenhouse effect occurs when the atmosphere absorbs more heat that it can radiate back out into space, preventing the planet from cooling and eventually leading to unstoppable warming that finally turns its oceans turn to steam. However, something important happens when planets decrease in size: As they warm, their atmospheres expand outward, becoming larger and larger relative to the size of the planet. These large atmospheres increase both the absorption and radiation of heat, allowing the

Topics:
Miles Kimball considers the following as important:

This could be interesting, too:

Paul Krugman writes Can Warren Escape the Medicare Trap?

FT Alphaville writes WeGiveIn: SoftBank to the rescue

Tyler Cowen writes Academic vita fraud?

Menzie Chinn writes Presentation: “The Economic Consequences of Trump’s War on Multilateralism”

The inner edge of the habitable zone is defined by how close a planet can be to a star before a runaway greenhouse effect leads to the evaporation of all surface water. But, as Arnscheidt and his colleagues demonstrated, this definition doesn’t hold for small, low-gravity planets.

The runaway greenhouse effect occurs when the atmosphere absorbs more heat that it can radiate back out into space, preventing the planet from cooling and eventually leading to unstoppable warming that finally turns its oceans turn to steam.

However, something important happens when planets decrease in size: As they warm, their atmospheres expand outward, becoming larger and larger relative to the size of the planet. These large atmospheres increase both the absorption and radiation of heat, allowing the planet to better maintain a stable temperature. The researchers found that atmospheric expansion prevents low-gravity planets from experiencing a runaway greenhouse effect, allowing them to maintain surface liquid water while orbiting in closer proximity to their stars.

When planets get too small, however, they lose their atmospheres altogether and the liquid surface water either freezes or vaporizes. The researchers demonstrated that there is a critical size below which a planet can never be habitable, meaning the habitable zone is bounded not only in space, but also in planet size.

The researchers found that the critical size is about 2.7 percent the mass of Earth. If an object is smaller than 2.7 percent the mass of Earth, its atmosphere will escape before it ever has the chance to develop surface liquid water, similar to what happens to comets today. To put that into context, the moon is 1.2 percent of Earth mass and Mercury is 5.53 percent.

The researchers were also able to estimate the habitable zones of these small planets around certain stars. Two scenarios were modeled for two different types of stars: a G-type star like our own sun and an M-type star modeled after a red dwarf in the constellation Leo.

The researchers solved another long-standing mystery in our own solar system. Astronomers have long wondered whether Jupiter’s icy moons Europa, Ganymede, and Callisto would be habitable if radiation from the sun increased. Based on this research, these moons are too small to maintain surface liquid water, even if they were closer to the sun.

“Low-mass water worlds are a fascinating possibility in the search for life, and this paper shows just how different their behavior is likely to be compared to that of Earth-like planets,” said Robin Wordsworth, associate professor of environmental science and engineering at SEAS and senior author of the study. “Once observations for this class of objects become possible, it’s going to be exciting to try to test these predictions directly.”

This paper was co-authored by Feng Ding, a postdoctoral fellow at the Harvard John A. Paulson School of Engineering and Applied Sciences and Graduate School of Arts and Sciences.

This work was supported by NASA Habitable Worlds grant NNX16AR86G.

Miles Kimball
Miles Kimball is Professor of Economics and Survey Research at the University of Michigan. Politically, Miles is an independent who grew up in an apolitical family. He holds many strong opinions—open to revision in response to cogent arguments—that do not line up neatly with either the Republican or Democratic Party.

Leave a Reply

Your email address will not be published. Required fields are marked *