Fermi and Drake

The world is buzzing this week about what could be called the most important discovery of the decade in astrophysics–the discovery of a planet within the habitable zone of a red dwarf star, Gliese 581. I want to talk about this and explain why it is truly important for our understanding of our role in the universe as human beings.

Let’s start, first of all, with one of my favorite physicists, Enrico Fermi. I’ve heard a lot about him over the years, especially since the head of High-Energy Physics at the university when I began as a contractor was his graduate student on the Chicago Pile Project, Al Wattenburg. Fermi was an amazing man without whom the Manhattan Project probably would not have succeeded. He’s currently best known among astronomers, though, for a characteristic way of thinking and the question that came from it.

Fermi was the king of the back-of-the-envelope calculation. He used to ask questions in his courses like, “Calculate the number of grains of sand on all of the beaches of Earth.” These came to be known as “Fermi Questions.”

Although it seems far-fetched, it is possible to solve Fermi Questions to about an order of magnitude or so by making some logical assumptions. To solve the one above, you need an order of magnitude approximation of the following items: the volume of a grain of sand, the packing efficiency of sand grains on sand beaches, the length of all the coastlines of the planet, the percentage of such coastlines with sand beaches and the average width and depth of the sand on those beaches. I’ll leave it to you engineers out in the reading audience to come up with an answer to the question if you want a break from your study for finals. Here’s a few more.

Fermi used assumptions like this to calculate the yield of the Trinity A-bomb. He stood at a safe distance with his back to the tower. When he saw the flash, he opened his hand and allowed small pieces of paper to fall from them. By the distance that they were tossed by the shockwave, he was able to calculate the power of the weapon.

He also used this method to calculate, over lunch with his graduate students, that it would take an average of 100 million years to colonize the galaxy using self-replicating machines if they could not go faster than light. This led to the famous Fermi paradox, “if this is so, there should have been 45 alien civilizations colonizing our planet by now–where are they?”

Now, we move forward about ten years to Green Bank Radio observatory, where scientists are listening for intelligent signals from alien civilizations. An astrophysicist came up with a Fermi-type question: “How many civilizations are there in the Milky Way Galaxy with whom we can communicate?” The Fermi solution to this became known as the Drake Equation.

Over the years, information has been plugged into it, getting estimates of anywhere from 10,000 to .0000001, depending on the assumptions going into it. A range this wide in a Fermi solution means that there’s not enough knowledge to make a good guess, since a Fermi solution should be reasonable within only a factor of 10 either way.

[I want to take a moment and mention that the Drake Equation is not a scientific hypothesis. A hypothesis must be verifiable by experimentation. Since that is impossible, it falls closer to a philosophical concept than a hypothesis. Michael Crichton pointed this out years ago.]

Now we get to the center of the relevance of Gliese 581c. Until this past week, we knew of exactly four rocky worlds within the habitable zone of a star at present–Venus, Earth, the Moon and Mars (with one more, Titan, that would be at the right temperature after the Sun became a Red Giant). There was absolutely no way to tell whether or not there was something completely unique that prevented such worlds from occuring in other systems.

There were many theorists that believed just that–that gas giants spiralled into their sun destroying rocky planets as they go unless something truly unusual (like the resonances in our solar system) prevented it. There were a lot of Hot Jupiters orbiting very close to their suns that seemed to endorse this view.

Gliese 581c has a typical mass for a rocky world at 5 Earth masses (abbreviated Me henceforth.) The transition between a rocky world like Earth and a gas giant is theorized to be about 10 Mes.

It also gives a new number to plug into the Fermi Question. We had not, until now, had any example of a small planet within the habitable zone of a different star. Now we do and it’s close–only about 20.5 light-years away. I did a quick calculation yesterday and if we assume that this distance is typical (a valid assumption because even though such planets could be more rare, we have a very low sampling rate within the solar neighborhood) I come up with a total estimate of 300 million rocky planets within the habitable zone of their stars outside the central third of the galaxy where the radiation from the core’s black hole and the high frequency of supernovae would be problematic.

This new planet is important because it is circling a Red Dwarf Star. Such stars have not been targets of examination in the past because they were not seen as likely candidates for life. There are billions of them in the galaxy–80% of the stellar population are Red Dwarfs. One characteristic of such stars is their extremely long lifetimes compared to other stars. Earth will have probably 5.5 billion years total in which water-based life is possible on its surface at its present distance before stellar evolution heats it beyone the boiling point of water. Gliese 581c, with a mass half that of the Sun has 5 or 6 times as long in which life could develop. In other words, we on Earth have perhaps a billion years of viabilty remaining as an ecosphere. Our equals on 581c would have over 26 billion years left for the life lottery.

Lastly, it has a Hot Jupiter inside its orbit. This means either that Hot Jupiters do not necessarily destroy rocky planets outside of their orbits as they spiral inward or that they are formed where they are currently found and do not spiral in at all.

Now, a cautionary note before we get too excited. Red Dwarfs are very different from our sun. They are cooler and dimmer and their spectrum has much less Ultraviolet light, which forms a protective ozone layer above our planet and Gliese 581 (man, we need a better name for this star and its planets now that it’s important, it’s too damn long to type each time) is what is called a “flare star.” These stars have a tendency to have much larger versions of the solar flares that our sun exhibits. Therefore, the sunward side of the planet would have a much higher dose of X-Rays than our world receives.

Why is the discovery of this new world important in a philosophical sense? Our estimated number for possible other races and civilizations has just gotten a lot larger. This reopens the Fermi Paradox for consideration, since it is proportionally more surprising that we have no evidence of other races having reached our solar system.

I’m going to have a bit of fun here at the end. I’m going to examine the parameters of the system and see if I can describe a habitable world there and what one would experience as one of its people. I am not implying that any of my conclusions are valid, merely that the science that I am employing is current.

First of all, it is highly likely that the planet is tide-locked to its sun, much as our Moon always has one side facing the Earth. This means that it has a bright-side and a dark-side. (Where’s David Gilmore when you need him?) How high Gliese would stand in the sky would depend on how close you were to the Hot Pole–the point at which the star would be overhead all the time. Gliese would appear to be about seven times larger than our sun does.

I am going to assume that the point where our planet….let’s name it Fredville, for lack of a better name….first formed was similar to where Earth formed in our solar system and that the composition of the molecular cloud was the same. Therefore, it would have a similar amount of water in its rocks and have been hit by a similar number of comets. (See why this is speculation or science-fiction? There’s way too many unknowns.)

Because of the amount of water and minerals, it would have about 70% of its surface covered with water. However, the distribution would be very different from that of the Earth. Because of the tidal effects of being so close to its star, Fredville would have a tendency to have its oceans preferentially located at the sunrise and sunset points of the planet. This would mean that the planet would have a ring of oceans completely around it, with one large continent facing the star and another one on the farside.

It was once assumed that a tide-locked planet would have its atmosphere frozen completely on the dark side. By studying Venus, which has an extremely long rotation, scientists have discovered that the atmosphere of near-locked planet circulates anyway due to differences in temperature. Venus rotates in 243 days, but its atmosphere goes completely around the planet in only 4 days. There are winds blowing off of the hotside near the equator, but the air is returned to that side by opposing prevailing winds at different latitudes. The closer you get to the Hot Pole, the higher the temperature, perhaps reaching as high as a constant 95 degrees Fahrenheit. At the sunset/sunrise lines, it’s comfortable on Fredville, and by the time you get to the Cold Pole in the middle of Farside, you’re down to Antarctic temperatures.

Most important, there’s life on Fredville. The intelligent Freddies evolved from creatures that lived in the Ring Ocean. They breathe oxygen like us, but look a lot different. When life left the ocean and crawled onto the Hotside land, the ones without shells died from the X-radiation with which Gliese regularly bombared the planet. Freddies look a lot like Earth’s hermit crabs, except that their shells are extruded. Babies are kept inside thick manufactured shells until they’ve grown their own carapace. Since the gravity on Fredville is twice that of Earth, things fall faster and it is a lot easier to be injured, so they grow up fast and tough.

On the Farside, there are different kinds of life-forms, but they’re much more primitive due to the lack of energy from the sun. They resemble Earth-like fungi and have limited vision capability. Close to the Cold Pole, there’s no life at all and it’s eternally dark except for the stars, the other planets of the system and an occasional aurora near the magnetic poles.

The Freddies have explored their planet and discovered the stars when they crossed the Ring Ocean four hundred years ago. Now that they’ve discovered astronomy, chemistry and physics they look up at the sky at the sunset line and wonder, “Is there life out there? If so, where the hell is it?”

Tom