Earth, Barely Habitable?

by Fraser Cain on January 9, 2008

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Artist impression of a super Earth. Image credit: CfA
Our home planet has been often described in glowing, nurturing terms. A cradle for life, right in the goldilocks zone. But our planet is actually right on the edge of habitability. If it were any smaller, and a little less massive, plate tectonics might never have gotten started. It turns out, life needs plate tectonics.

Astronomers at the Harvard-Smithsonian Center for Astrophysics announced their research today at the Winter meeting of the American Astronomical Society. According to the team, plate tectonics only really get going when a planet gathers enough mass. And the Earth has just barely enough mass to enjoy plate tectonics.

“Plate tectonics are essential to life as we know it,” said Diana Valencia of Harvard University. “Our calculations show that bigger is better when it comes to the habitability of rocky planets.”

When a planet reaches a large enough size, huge chunks of the planet’s surface can float atop an ocean of boiling magma. These plates spread apart and crash into one another, lifting up gigantic mountain ranges like the Himalayas.

And without plate tectonics, we wouldn’t be here. The process enables complex chemistry and recycles carbon dioxide, which acts like a blanket to keep the Earth warm and hospitable for life. Carbon dioxide is locked into rocks, and then returned to the atmosphere when the rocks melt. Without this cycle, carbon dioxide would get locked away in rocks forever.

The researchers examined what would happen on different rocky planets. They looked at a range of planets, smaller than our planet, up to the so-called “super-Earths” – planets twice our size with 10 times the mass. Any bigger than that, and you start to get a gas planet.

According to their calculations, the Earth is barely habitable. If you get a planet with more mass, the plate tectonics really get rolling, and the carbon cycle becomes really active. A super-Earth could have globe-spanning rings of fire, bursting with hot springs and geysers. Life would have every opportunity to get started.

Of course, if we tried to visit a super-Earth, we’d find the gravity uncomfortable. We’d experience 3 times the gravity trying to walk around on the surface of the planet. Oh, my back.

But for native life forms, it would be paradise.

Original Source: CfA News Release

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

alastair January 9, 2008 at 11:21 AM

“We’d experience 3 times the mass”?!!

Are you sure about that statement? Don’t you mean “weight”?

orkstrtdkaos January 9, 2008 at 12:59 PM

You are correct in thinking that you’d weigh 3 times as much. But if you understand the quoted statement to mean that weight is an “experience” of mass… then the statement is true. We would “experience” what it would be like, gravitationally, if we had 3 times the mass.

Fraser Cain January 9, 2008 at 1:17 PM

No, that was poorly worded, you’re right. I fixed it.

Khyron January 9, 2008 at 2:03 PM

These claims are hard to reconcile with the fact that Venus, where plate tectonics are lacking, has a huge abundance of carbon dioxide in the atmosphere. In fact, the explanation I recall for Venus’ CO2 quantities is the lack of a tectonic carbon cycle, the opposite of the effect claimed here.

Perhaps this study still assumed an oversized moon to strip away most of the atmosphere, thus requiring atmospheric CO2 replenishment?

Greg January 10, 2008 at 12:40 AM

The size or diameter of a planet does not necessarily reflect it’s mass, depending on it’s composition and density of heavier elements. If in fact the planet had 10 times the mass as Earth, you would “weigh” 10 x as much if you were the same distance from the center of that planet as you are from the center of the Earth. Since the diameter of the larger mass planet will also be much larger, you would likely be farther from the center of gravity and not weigh so much there. Since gravity decreases at the square of the distance from the center of an object, you would need to crunch some numbers to figure out your weight on the surface of any such object. Suffice it to say your weight will be greater,but not in direct proportion to the greater mass of the planet.

Greg January 10, 2008 at 12:59 AM

The problem with Venus is a matter of balancing extremes. Earth sequesters alot of its carbon dioxide which cools it down. At some point Earth was cool enough to form liquid water which is a necessary ingredient to form minerals rocks with carbon dioxide that pulls it out of the atmosphere. Over time plate tectonics has melted these minerals which releases enough of carbon dioxide back into the atmosphere (via volcanoes) to keep Earth from freezing over. A simple way to think of it is that water is trying to cool the planet into a snowball while plate tecotnics is trying to warm it up with volcanoes. Venus underwent a runaway greenhouse event where too much carbon dioxide was released into the atmosphere warming it up so much that it became too hot for water to exist. The water evaporated and due to the atmosphere being full of carbon dioxide the water vapor leeched away into deep space rather than falling as rain like on Earth. Without water the carbon dioxide does not get pulled out of the atmosphere via rock formation with water. Venus now inescapably hot as Hades a result.

Pedro January 10, 2008 at 3:29 AM

Life spreading like in a paradise in a planet as much as 10 times earth mass? Even though Greg says that gravity of sych planet isn’t a direct proportion, we could expect something around 6 or 7 times earth gravity, wouldn’t we? So, what sort of life would thrive with such weigh? I read somehere that as litle as 2-2,5 times our gravity was too much to non-water complex life forms developed. This would mainly influence carbon life forms with circulatory sistems like every mammals, wouldn’t so?

Mark January 10, 2008 at 9:26 AM

I did some rough calculations, and assuming that the density remains the same, a planet with 10x Earth’s mass will have roughly 2x the radius. g at the surface works out to 21.16 m/s/s. One would then experience 2.15x his/her regular weight.
This is, of course, assuming that I know what I’m doing.

mansie January 10, 2008 at 11:56 AM

I imagine that life-forms on a higher G planet would be smaller. As would trees, mountains and the general terrain. The atmosphere would be far thicker, as would atmospheric pressure – not sure what effect this might have on flying animals, aeroplanes and space launches…

Interesting to speculate though!

Kevin M. January 10, 2008 at 7:05 PM

Very interesting thoughts. By “smaller”, I assume you mean “less tall”, but not necessarily, if it is really tall and thin or flat and wider. This in no way means that there would be fewer life forms, just very different ways of living together. Life finds a way, I’m sure in ways that we cannot imagine yet. It is time that someone made up a simulator algorythm for biological life forms under different gravities and pressures.

By the way, does any of this imply how Venus or Mars could yet be terraformed, though without tectonics? It seems very do-able in a few lifetimes that we could harness such energies and deliver or manufacture water and gases on both planets. Without tectonics, it sounds like it might take direct intervention of materials. Even if they didn’t become habitable for a while, it would be utterly fascinating to see what effects it would have. Talk about a laboratory! It must be done.

Kevin M. January 10, 2008 at 7:24 PM

Earth barely habitable – perhaps, but it still sounds like we’re on the much calmer and nicer edge of habitability. More tectonics means far more violent catastrophes all the time, kind of like the Woody Allen movie where his childhood home was situated underneath a roller coaster.

Plus, there is the interesting question of whether more rapid concentration of life forms on a smaller smaller surface area leads to faster cultural and therefore mental development? Wider surface – slower mixing and interacting of distant cultures?

Also, if birds are heavier, but the atmosphere is thicker, does that mean that they fly just as well? Does everything move slower and slithery-er, not much jumping around? I admit I’m afraid to meet the intelligent ones.

RUF January 11, 2008 at 9:33 PM

Terra-forming Mars is not possible, as Mars has no magnetic field. any atmosphere (including the nitrogen-oxygen atmosphere that we need) would be stripped away to space by solar wind. Time to put this whole terra-forming Mars idea to rest…

Lyle January 11, 2008 at 9:48 PM

Lol, … methinks we’re applying base premises that correlate to ‘Earth’ rather than the ‘reality’ of a hypothetical planet of different properties, … such as plate tectonics or not.

Good point Greg about ‘proportionality’ and distance, … most people miss that one.

A ‘slower’ carbon cycle does not exclude other carbon cycles.

Joe February 11, 2008 at 1:37 PM

An interesting though would be the life-span of the creatures that spawn from this planet. And perhaps the rate they experience life. That would have to do with the span of one cycle of the planets season (if it has a cycle of seasons). Could it be that if the mere rotation of a planet had anything to do with the life-span of the creatures that lived upon it?

Charles Peter Watson March 29, 2008 at 2:08 PM

With the consideration that plate tectonics, being only a theory at present, the siesmic shifts caused by them have also contributed to the emission of dangerous sulfuric gases that have altered climatic conditions on Earth (Pinatubo, Krakatoa). The 2004 Indonesian earthquake not only spawn one of the deadliest tsunamis but lost Earth a miniscule fraction of its rotation time, shortening the average day but not by a recognizable amount.
As for habitability, planet mass is not so much a factor as is its core size where the molten rock that fuels the tectonics & thus the continental drifts are churned by their own volitility as well as planetary rotation. It should also be noted that there is a timline to the cooling period of that core & there will be no more movement of the crust. Despite this occurance being in the very, very extreme future, can Earth life be spared on a planetary mass of whatever exponential size without tectonic activity should human successfully colonize the Moon prior to Mars?

bonus September 13, 2008 at 3:21 AM

Please read the thread on this site “Mars once had plate tectonics”. Obviously, it ceased due to Mars cooling, but it was there once, and when Earth cool, its plate tectonics will cease, too. In other words, Earth is NOT on any “borderline of habitability”

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