What Really Makes A Habitable Planet?
May 4, 2013

Re-Imagining Habitable Planets To Expand Our Search For Life

John P. Millis, Ph.D. for redOrbit.com — Your Universe Online

The Holy Grail in the search for planets outside our solar system is finding one that could potentially support life. Traditionally, this means finding a planet with features similar to Earth, which is just the right distance from its host star that liquid water could be supported on the surface.

Scientists believe the presence of liquid water on the surface is a necessary component for the proliferation of life. And a planet like Earth is able to maintain its oceans of water because of the heat derived from our Sun.

A New Vision For Habitable Worlds

But theoretical physicist Sara Seager from the Massachusetts Institute of Technology argues perhaps we are being too restrictive on what habitable planets could possibly look like, according to National Geographic Magazine's Marc Kaufman. Specifically, we look to stars to provide the warmth necessary for life, but the truth is even our home world is more complex than that. Heat that arrives on our planet warms the surface, and is re-radiated toward the atmosphere. Then greenhouse gases trap some of that heat, sustaining the temperatures we have come to expect.

Seager argues other worlds could sustain liquid water, even if they were further from their host star — outside of the traditional habitable zone. These planets would simply require excess greenhouse gases, such as hydrogen, in their atmospheres to maintain high enough temperatures.

Seager told Kaufman, “What primarily controls surface temperature is the greenhouse effect, what types of gases are in the atmosphere, and how massive a planet's atmosphere is. That's what we really have to understand."

The goal should be to find a planet with the right balance of atmospheric gases, atmospheric moisture — the most powerful greenhouse agent — and distance from its star. If we approached searching for habitable worlds with these criteria in mind, we may find a wealth of such planets. As it is, we have struggled to find worlds that meet our current, narrow criteria for habitability.

She further suggests life could potentially exist on worlds that orbit no star at all. If these so-called Orphan Planets had sufficient radioactive decay beneath their surface, they could potentially maintain a high enough temperature for liquid water to exist.

Challenges Ahead

The key to this new approach would be to image the atmospheres of these planets, searching for certain chemical markers that would indicate either the presence of life — elements like oxygen are believed to be signs of life, since they would otherwise be bound up in other compounds if they were not continually being replenished by life on the surface — or at the very least the potential for it to exist.

However, the task of analyzing planetary atmospheres is beyond our capabilities in most cases with current technology. And most proposed projects that would allow such measurements are simply too high cost to receive funding.

Two projects set to launch later this decade, the Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope, will work together to investigate planetary atmospheres around cooler M class stars. These include some of our nearest neighbors in the galactic neighborhood. While our ability would still be limited to the nearest systems, we could finally begin to probe worlds outside the usual habitable zone.

It may take some time, but "the field of exoplanet characterization is on track to understand habitability and to find habitable worlds."