Monday, October 4, 2010

Life on Another Planet?

by Conroy


Artist rendition of Gliese 581g. No one knows what it's really like.

Last week news came out about a new extrasolar or exo-planet - the sixth - orbiting the star Gliese 581. Accompanying the story of the discovery of the planet, dubbed Gliese 581g (the "a" through "f" suffixes had already been claimed by the previous five planets and the star itself), were speculations about the potential presence of life there. Based on the early calculations, Gliese 581g appears to fall within it's parent star's "habitable zone", the distance from the star where liquid water can exist on the planet's surface. It is widely believed that liquid water is the major prerequisite for life. Further, the planet's mass, diameter, density,and surface gravity all seem to be similar enough to Earth to support an atmosphere. The planet was discovered by a team led by astronomer Steven Vogt, a professor of astrophysics at UC Santa Cruz. Vogt was so confident that the planet fit the criteria for life that he said the chances of life are "almost 100 percent."

100 percent?

Not 50 percent, 90 percent, or 99 percent, but 100 percent. Now how can Mr. Vogt be so sure? The planet is far too small and dim to be observed directly. The Gliese system is over 20 light years away. In fact, Gliese is such a dim star (a red dwarf) that it cannot be observed without a telescope. By comparison, an observer on Gliese 581g - maybe there are some right now - could easily see our sun with naked eyes. Lacking direct observation, the planet was discovered using one of the only techniques available, doppler spectroscopy. This approach utilizes careful computations of the star's movement to detect the gravitation pull of revolving satellites (planets). Other complimentary techniques can be used to estimate the orbital distance and mass of the planet. Most exo-planets have been discovered using this approach. That said, "planet hunting" is a relatively new (and let's admit, pretty amazing) project in astronomy and the discovery techniques are still being refined. In fact, there have been previous claims of life-sustaining planets orbiting Gliese 581, see Gliese 581c. More detailed analysis indicated the Gliese 581c was not a good candidate for Earth-like life. Prudence demands additional observations and analysis before judgment is made about Gliese 581g's suitability for advanced life.

To that point. In order to be within Gliese 581's habitable zone, Gliese 581g has to be very close to the star (red dwarfs radiate way less energy then stars like our yellow sun), 14 million miles away or so according to preliminary calculations (the Earth is 93 million miles from the Sun). Being so close, Gliese 581g is likely tidally locked to its parent star, meaning one side always faces the star and one side always faces away (the Moon is tidally locked to Earth). As a result one side of the planet would be exposed to blazing sun and the other to deep cold. Vogt has suggested that life could proposer in the "twilight zone" along the planets perpetual sunrise/sunset horizon. Perhaps atmospheric conditions could allow adequate heat transfer from the hot sunny side to the cold dark side. Perhaps. But tidal locking seems like a bad condition for a planet that hopes to support complex life (see more below). It's equally (or more) possible that all of the water on the planet - if there is any - is frozen in ice on the dark side and not available in liquid form at all.

Speculation about life on a single planet is one thing (literally), but generalizations about the prevalence of life in the galaxy is quite another. Mr. Vogt has postulated that the potential for life within the Gliese 581 system, a star that is so close to Earth (only 116 stars are closer), and one of the first systems where planets have actually been searched for, may mean there is a hyper-abundance of Earth-like, life-sustaining planets in the Milky Way galaxy. Vogt suggested that 10 or 20 percent of stars could have Earth-like planets. Considering that there are as many as 400 billion stars in the galaxy, that could mean tens of billions of Earth-like planets in the Milky Way alone. An astounding number, and one that would suggest that life could be common in our part of the universe.

Okay, that's one position. Call me a skeptic, but I remain unconvinced. There may be a great number of Earth-like, life-filled planets out there, but I think one must consider the myriad elements that allowed life (as we understand it) to develop and flourish on Earth before we gift other planets that distinction. The Earth, our blue marble, benefits from the following amazing confluence of phenomena:

Earth. The blue marble.
  1. Earth and the solar system are located in an outer band of the Milky Way galaxy away from other stars, gamma ray generators, and the galactic center where energy densities and radiation are inimical to life.
  2. Earth revolves around a stable, main-sequence star that has been burning for billions of years and will continue to burn for billions more, allowing life the time to develop and evolve. Further, the Sun doesn't emit gamma rays or x-rays in bursts that could jeopardize life on orbiting planets.
  3. Earth sits right in the center of the Sun's habitable zone, so liquid water can exist in the surface. The Sun's habitable zone has been extremely stable over time.
  4. Earth's orbit is far enough inside the big planets, especially Jupiter and Saturn, that the planet's formation was not disrupted by their massive gravities, while at the same time these planets act as a gravitational "shield" that attract potential planetary (and life) threatening asteroids, comets, etc. from deep space.
  5. Earth's orbit is far enough from the Sun to maintain planetary axial rotation (i.e. no tidal locking).
  6. Earth has a small enough mass and density to be rocky, i.e. have a surface for life to grow on.
  7. Earth has a large enough mass and density to (1) sustain an atmosphere and (2) maintain a geologically active metallic core, which supports plate tectonics and a planet-shielding magnetic field.
  8. Earth has a nearly circular orbit, which results in relatively consistent solar radiation reaching the planet as it revolves around the sun.
  9. Earth's axis is inclined just the right amount to promote seasons and overall higher temperature but avoid conditions where some of the planet is always facing the sun and some is always facing away (think of the heavily inclined axis of Uranus).
  10. Earth has a relatively consistent axial tilt due to the stabilizing effects of the Moon's gravity.
  11. Earth has significant tides, which may promote life through mixing of water and air, because of the proximity and size of the moon. The Moon may have formed from a freak massive collision of a small planet with the proto-Earth.
  12. Earth has a high abundance of life-supporting elements (oxygen, hydrogen, carbon, and nitrogen). Earth's crust has a substantially higher proportion of oxygen than the universe in general. The oxygen may have been delivered by early random comet/asteroid impacts (or through the collision with the small planet that resulted in the Earth-Moon system).
Consider the chance and specificity of each of these elements. What are the odds that other planets will have the same congruence of elements? Maybe life-supporting planets, and by extension life is common throughout the galaxy. Or maybe Earths are very rare and special.


  1. Very interesting post, Conroy. I especially like all of the factual detail concerning the special conditions that pertain to Earth. It certainly is a special planet. You hedged a bit, though, on an important point, referring only to life "as we understand it". What about life forms that are truly alien to us? For that matter, what is life? Need it be carbon based? Or will silicon suffice? Given different initial conditions, one wonders how strange it could possibly be.

    According to the standard model of evolution, as I understand it, all life on Earth evolved from the same source; emanating from this source, different forms of life are often represented by a branching structure, like a shrub. Branches more closely connected appear more similar; thus, when we look at other mammals, we catch a glimmer of ourselves. With reptiles, this gets more difficult. I find snakes, for example, very strange; I don't trust them at all. (But some people do.) Bugs are even worse. And bacteria is totally weird; yet, it is the dominant form of life on this planet. My point is that life — whatever it may be — is capable of assuming very strange forms even here on Earth, never mind what it might look like elsewhere in this Universe.

    But let's just go ahead and stipulate that life is both carbon-based and cellular. In that case, what are the necessary conditions for the emergence of life? Must life evolve on a planet? What about a planetary satellite such as Europa? Or a free wheeling space traveler such as a comet or an asteroid? One apparent factor is heat; like Goldilock's porridge, conditions must not be too hot or too cold. The problem here is that these concepts are relative. We have discovered extremophiles, for example, which live in extreme temperatures; thermophiles live in hot springs and thrive in deep sea hydrothermal vents; the hyperthermophile's optimal temperature is 80°C.

    The existence of such life also poses a challenge to the concept of the habitable zone, which must be expanded outward from a star, or inward, or both. But this isn't much of a challenge. There are other, more important factors complicating the habitable zone concept. For example, heat deflectors such as icecaps and heat retainers such as ozone layers allow planets to regulate their heat. More interestingly, friction-generated heat caused by tidal-locking or heat from radioactive decay, such as occurs at Earth's core, may permit the presence of life in rather far-flung, exotic places. Some have speculated that life might exist in the interior of Europa, for instance, well outside the zone of habitability typically drawn around the Sun. Encased in a thick layer of ice, Europa probably has a hot liquid core and a layer of water in between — a layer in which life as we understand it might exist now (or may have existed in the past or in the future).

    And that's just in our own solar system. Can we assign probabilities to the existence of life beyond Earth? Sure, somewhere between 0% and 100%. Anything more specific is probably just wishful thinking.

  2. Conroy,

    You display an impressive understanding of orbital dynamics and fundamental physical principals. I'd like to clarify a few points, however.

    - Points 3 and 8 as written say fundamentally the same thing. An interesting effect of our mostly circular orbit, however, is that we have days of constant length. A more eccentric orbit, combined with a constant rotation rate, would yield solar days (as opposed to sidereal days) that varied in length throughout the year, which would be interesting to try to account for.

    - Regarding point 10, our constant 23.5-degree axial tilt is due to the conservation of angular momentum, and not the moon. Your point regarding tidal locking of the Earth (as opposed to the tidally locked moon), however, is interesting -- I had never considered that possibility!