Just Right Goldilocks

by Wayne Ferrier

AA_NASA IMAGE exoplanet In the constellation of Libra is Zarmina’s World, the first habitable planet discovered outside our own solar system. Zarmina’s World orbits Gliese 581, a red dwarf star that is about a third the mass of our sun. It's about 120 trillion miles away, which in the scheme of things is right smack in our neighborhood. Using current technology, it would only take us several generations to make it there—not outside the realm of our current capabilities. The two scientists who discovered Zarmina’s World, Steven Vogt and Paul Butler, calculate that there could be as many as one out of five or ten stars in the universe that might have Earth-like planets in the habitable zone. With an estimated 200 billion stars in the Milky Way alone, there could be as many as 40 billion planets that could potentially harbor life here. However, this is all very speculative just how common these Earth-like planets really are in the Milky Way.

Temperatures on Zarmina—for convenience sake let’s call it Zarmina—get as hot as 160 degrees and as cold as 25 degrees below zero, but in between “it’s shirt-sleeve weather,” says co-discoverer Steven Vogt of the University of California at Santa Cruz. And the low-energy dwarf star Gliese 581, Zarmina’s sun, ought to continue to shine for billions of years, a lot longer than our sun will, which increases exponentially the likelihood that life could possibly develop there.

It's unknown if there is water on Zarmina, and what kind of atmosphere it actually does have. But because conditions there are ideal for liquid water, and because there always seems to be life on Earth where there is water, there is a lot of excitement being generated about the discovery of this Earth-like planet. But that’s the catch—does it have liquid water and the kind of atmosphere that really would make it really, really habitable?

Astronomers like to use the term “Goldilocks zone” to designate the area of a planet’s orbit that is neither too close nor too far from its star so that liquid water can exist on its surface. So far we only know of six Goldilocks planets, and three of them orbit Gliese 581. Two of these planets showed promise, but one planet turned out to be too hot and one planet too cold. “The [third] one bracketed right in the sweet spot in between,” Vogt said. “It's a beautiful planet,” so he named it after his wife; unofficially of course. The other three planets orbiting in a known Goldilocks zone are—you guessed it—Venus, Earth, and Mars. And like the Gliese 581 system, Venus is too hot and Mars too cold.

Mars Too Cold

Is Mars really too cold? Well it’s a tad more complicated than that. Mars could be a rather decent place to live if he were a bit bigger, if he were geologically active, if he had a denser atmosphere, and a functional magnetosphere. But because of its small size the planet cooled prematurely and shut down; volcanic eruptions slowed to a simmer, dampening most volcanic outgassing, and switching off the planetary dynamo. Although Mars today has no structured global magnetic field, observations do suggest however, that parts of the Martian surface crust have been magnetized, and that alternating pole reversals of its dipole field occurred in the Martian past. But because four billion years ago good ole Mars cooled, switched off his magnetosphere, that obnoxious solar wind now interacts with the poor Martian ionosphere, reducing the tiny planet’s atmospheric density by stripping away his atoms, one by one, and sending them flying into space. The surface pressure of Mars is equal to the pressure found twenty miles above Earth's surface—less than 1% of our Earth's surface pressure. If Mars were a bit bigger, he might have retained his internal heat longer, and this internal thermal heat would still be driving crucial geophysical processes, such as volcanic outgassing, which help build up and then maintain a dynamic hydrosphere. A larger size would have helped Mr. Mars retain his atmospheric gases, which are now being lost to space. Liquid water cannot exist on most of the Martian surface because of this low atmospheric pressure. Mars’s two polar ice caps, however, do appear to contain a fair amount of water. It’s been estimated that if the volume of water-ice in the south polar ice cap alone melted, the entire Martian surface would be flooded by 36 feet of water! But instead of being the warm, wet world it could be Mars is a cold desert, with a thin atmosphere, and its surface water locked in permafrost.

Venus Too Hot

Venus is Earth’s evil twin, both planets have a similar mass, volume, and distance from the sun. But while Earth is almost paradisiacal Venus went a little crazy, and she more closely resembles Hell than Heaven.

Exactly how and why Earth and Venus turned out so different still perplexes the scientific community. The average surface temperature on Venus is 860 F— hot enough to melt lead— and Venus has a crushing surface pressure, equivalent to the pressure found hundreds of feet below Earth’s oceans. And it rains sulfuric acid there, but it’s so hot it evaporates before it ever reaches the ground!

Because Earth and Venus share a similar size and shape, scientists assume they probably have similar planetary structures such as a core, a mantle, and a crust. But unlike Earth, which generates a strong magnetic field, Venus has only a weak magnetic field.

Like Mars, Venus may have been similar to Earth in her younger days. Researchers think that Venus may have possessed an ocean or two, but not anymore. So what happened? This miserable planet probably lost her H2O because she hasn’t a robust magnetic field; and this weak little field may have allowed hydrogen to leak from the planet’s atmosphere. In this possible scenario, water molecules floating around in Venus’s upper atmosphere would be broken down into basic hydrogen and oxygen by the ultraviolet rays from the sun, and the lighter hydrogen carried off by the solar wind, leaving the heavier oxygen molecules behind. In this way Venus may have been robbed of her water. That would also explain her lack of plate tectonics, considering that water is a key component of a successful plate tectonic system like the one we have here on Earth.

The weak magnetic field could be responsible for Venus’s hot temperatures as well. With Venus losing hydrogen all the time, all the free oxygen in the atmosphere would pair up with carbon instead, making a lot of carbon dioxide. Venus’s atmosphere is about 96 percent carbon dioxide—Earth’s atmosphere less than 1 percent. If you have learned anything about greenhouse gasses and global warming, you can imagine a 96% carbon dioxide atmosphere, and this should clue you in on why Venus is such a hellish place.

Our wayward sister's lack of a strong magnetic field is surprising given that Venus is only a few hundred miles smaller than Earth. But a good, functioning dynamo has three requirements: a conducting liquid, sufficient rotation, and convection. Venus’s core is thought to be electrically conductive, while its rotation is often thought to be too slow. Simulations hint that however slow it might be, it might be enough to drive a dynamo. This could lead us to conclude that the dynamo lacks convection in our sister’s core. On Earth, core convection occurs in the liquid outer layer because the bottom of the liquid layer is hotter than the top. On Venus, a global resurfacing event may have shut down her plate tectonics and led to a heat flux reduction throughout the crustal layer. This might have then caused her mantle temperature to increase significantly, thus reducing the heat flux out of the core. The result no dynamo! Instead heat energy is being used to reheat the crust. But if all this is true it is probably the result of a lack of surface water. Which came first? Did our twin sister lose her robust dynamo because she lost her water or did she lose her water because she lost her dynamo? We need to learn more about Venus. I do believe there may be no other place, besides the Earth itself, that can teach us more about ourselves than this world can.

Earth Just Right

The two primary factors that appear to make a planet habitable are its size and distance from its star. Distance from the Sun and an atmosphere help regulate surface temperature. For habitability, surface temperature needs to be within the range where liquid water can exist on the surface. Both factors: size and distance contribute to climatic stability on cozy, planet Earth. Unlike Mars, Earth's relatively large size allows her to retain much of her internal heat, which drives a very active geology. Plate tectonics then recycle carbon and other elements that would otherwise be trapped on the surface and recycle them back into the hydrosphere. The greenhouse gas carbon dioxide is a major thermal regulator here on the planet.

About two-thirds of the air in our atmosphere lies within 10 kilometers from the surface. This atmosphere helps protect us from short-wavelength radiation coming from the sun. Carbon dioxide, methane, and water in our atmosphere keep the Earth relatively warm. Without this greenhouse effect, the average surface temperature on Earth would be well below the freezing temperature of water.

In addition, Earth has a strong magnetosphere which deflects most of the charged particles from the solar wind. In the absence of a protective magnetosphere, solar wind can strip planets of their vital atmospheric gasses. I am really very excited about the possibilities extant in Gliese 581 of the constellation Libra. I like the juxtaposition of the Goldilocks worlds Zarmina and her two wayward siblings and Earth and her two wayward siblings. Comparing and contrasting what we know and what we have yet to learn is going to teach us more than we ever could have imagined.