A Habitable Planets Primer

In the spirit of Christmas and appreciating all that we have, today’s blog is about what makes Earth such an awesome place to live. It’s also about where we might expect to find life elsewhere in the Universe. You see, near as we can tell, Earth is a unique nexus (at least, within the Solar System) of life-friendly traits. Until our telescopes get a lot better (hello, James Webb!), or we make contact with an alien race, we won’t really know how unique in the Universe we really are.

Of course, all that I’m about to say here holds true only for our own “style” of life. Having a grand total of one origin of life to study and draw on, we have no idea what other kinds of biochemistry (or bio-unchemistry?) might be able to give rise to reproduction and heredity, the two basic traits that are probably required to (eventually) evolve consciousness.

I went into more detail about specific potentially earthlike planets in my previous post on the potentially earthlike planet Kepler-22b. This post is intended to be more general and more comprehensive about what to look for in a potentially habitable planet.

And now, in listwise fashion, a bit about the most important factors that make Earth so awesome:

1. Earth is a Terrestrial Planet
If we’re going to properly appreciate what we have as Earthlings, let’s take nothing for granted. Earth is, of course, a terrestrial planet. That is to say it’s solid–not a gas giant like Jupiter (diagram of all our Solar System’s terrestrial planets plus possible Kepler-22b at top right). It’s worth noting that current estimates suggest about 15% of all stars might host terrestrial planets; that’s the percentage of stars that have the heavy terrestrial elements “missing” from their spectrums, implying that those elements wound up somewhere other than inside the star.

There is a great deal of debate about whether gas giants may in fact be hospitable to life. It’s not out of the question; at just the right distance from its parent star, the atmosphere of a gas giant could become a warm, ever-churning soup of liquid droplets containing dissolved organic molecules. In the atmosphere of a gas giant, the ionizing radiation and crackling lightning thought to be instrumental in forming life on Earth could also be found in spades. In short, it could be a brilliant recipe for life.

But, there’s a problem: the ionizing radiation. Although not all gas giants have this little wrinkle, Jupiter, for example, produces such a strong magnetic field due to its massive size that standing on the surface of Europa, for example, could kill you in five minutes. Cause of death: shredding of the DNA and other cellular matter via pelting with high-energy particles. So for now, terrestrial planets like Earth, or terrestrial moons of the less radioactive gas giants, remain our surest guess at where to find earth-style life.

2. Earth Orbits a Sunlike Star
Our Sun is a yellow dwarf star. That means its surface temperature is about 5,505 degress Celsius, a nice middling temperature, and that it is quite small compared to the largest stars in the sky. Yellow dwarfs like our Sun are among the brightest 10% of stars in the galaxy, making them somewhat rare. Our yellow dwarf parent star is hot enough to allow us to orbit at a safe distance, but cool enough not to sterilize our surface with high-energy radiation.

There are problems with stars bigger and brighter than our own. One is time: the larger a star, the faster it burns through its fuel. The largest stars live only a few million years, or a tiny fraction of a percent of our Sun’s ten projected billion years. And it’s a good thing the Sun will live so long; it has already taken our parent star half of its lifespan to evolve any intelligent life at all! Now we have another billion years to figure out how to get off of this planet and onto others before our Sun’s aging makes life on earth uncomfortable.

Bigger and hotter stars also have the pesky problem of high-energy radiation; they produce more of the DNA-shredding UV radiation that we humans use to kill bacteria and keep environments sterile. It may be possible for life to evolve in such conditions, but it seems far less likely if you have to worry about your DNA getting shredded every five minutes.

There are also problems with smaller, cooler stars. Red dwarfs, for example, make up a good 75% of all stars in the galaxy–but they’re so cool that in order to receive enough light to keep surface water liquid, a planet would likely have to orbit so close that it would become tidally locked, with the same side always facing the star. This could lead to its atmosphere freezing and leaking off into space from the planet’s dark side, among other unpleasantries. Some scientists think that life around red dwarfs may be possible, which opens up the vast majority of stellar terrain to the possibility of life; but until James Webb comes online, we won’t have enough information about red dwarfs and their planets to know for sure.

3. Earth is in the Habitable Zone

You have probably gathered by now that liquid water is vitally important for biochemistry as we know it; due to the unique combination of hydrogen and oxygen’s properties, nothing is better at dissolving organic chemicals and encouraging them to react with each other to form more complicated structures like RNA and proteins.

Liquid water is so important that this is the criteria scientists use to estimate the “habitable zone” around other stars; at what distance from this star could a planet expect to enjoy temperatures that would neither freeze nor evaporate liquid water? So far of the thousand extrasolar planets discovered by the Kepler space telescope, 54 are within this sought-after “goldilocks zone” around their parent stars; only six of these are close in size to Earth, making it likely that they’re terrestrial.

Of course, this is not all-inclusive; there are other ways of getting liquid water. Jupiter’s moon Europa, for example, is well outside of the Sun’s habitable zone, but it’s still thought to have abundant oceans of liquid water. How? Jupiter’s gravity makes its tidal forces so strong that it actually stretches Europa as the moon orbits. This creates heat from friction–enough heat, based on preliminary readings to keep Europa’s interior oceans warm. This is why Europa I fully expect to find marine ecosystems on Europa (pictured at top left; the white crust is ice, and the long bands are cracks in the ice from Jupiter’s gravitational stretching).

4. Earth is Just the Right Size

Let’s take notice of our neighbors, Mars and Venus. What do they have in common? Both are thought to exist in the Sun’s habitable zone, where, all other conditions being correct, they could host liquid water. But, clearly, something went wrong with both of them. Venus is far too hot and Mars’ atmosphere far too thin to support liquid water on its surface. Why?

For Mars, the answer is clear: it’s too small. Earth’s hot, molten core turns out to be key to maintaining its atmosphere. Geologic activity, in the form of volcanoes and other outgassings, keep our planet supplied with carbon dioxide which photosynthesizers can turn into oxygen. Mars, having only 10.7% of our mass, has long since cooled to the point where geologic activity halted. Its much smaller size also means much lower gravity, which allowed its existing atmosphere to evaporate into space.

Venus is much more of a mystery; it has 81.5% of Earth’s mass, and its hot surface and thick atmosphere are the polar opposite of Mars. Yet it lacks two vital aspects of Earth’s geologic activity: Venus has no plate tectonics, and no magnetic field. Some say that Venus’ lack of plate tectonics can be blamed differences in its crust makeup due to a massive collision with another planetoid early in its history; but its lack of magnetic field indicates that its core, too, is fundamentally different from Earth’s. Could it be too cool?

If Venus’ smaller size meant a smaller core, too cool to maintain plate tectonics, this may actually explain its thick, acidic atmosphere. You see, Venus does have geologic activity; but it doesn’t happen in the form of slow and steady plate tectonics. It happens all at once in the form of a single, massive “resurfacing event.” Venus’ crust is all the same age; about 600 million years ago, it appears that the whole thing liquified and was replaced by magma, which cooled to form a single, solid crust with no cracks. This devastating amount of geological activity would have released devastating amounts of greenhouse gases over the course of a few million years; in short, this weird geology could have made the Venus we see today.

There is a lot more to be said about why Earth may be an ideal home for intelligent life. The freak collision, early in Earth’s history, that produced our Moon may also play a huge role. But, lacking that wildly unlikely set of circumstances, this is what we can expect to look for in other planets we hope to find habitable or life-bearing.

And this is a small slice of what we have to appreciate as earthlings.

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2 Responses to A Habitable Planets Primer

  1. Panchi says:

    It took me time to read the whole thing, but nicely written!! 🙂 I tried writing a small blog on Kepler 22B’s discovery but though I am an astrophysics student, your write-up looks better to me :p

    • kagmi says:

      Hey, thanks for the comment! I’m so pleased to hear you’re enjoying these posts. I always figure these topics are pretty fascinating, but I never know who shares that opinion. 😛 I will definitely be following you to see what astrophysicists write about. That’s a field I love but I never went into it because the math part of physics is not my strong suit.

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