|Artist rendition of Gliese 581g. No one knows what it's really like.|
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.|
- 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.
- 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.
- 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.
- 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.
- Earth's orbit is far enough from the Sun to maintain planetary axial rotation (i.e. no tidal locking).
- Earth has a small enough mass and density to be rocky, i.e. have a surface for life to grow on.
- 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.
- Earth has a nearly circular orbit, which results in relatively consistent solar radiation reaching the planet as it revolves around the sun.
- 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).
- Earth has a relatively consistent axial tilt due to the stabilizing effects of the Moon's gravity.
- 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.
- 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).