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The National Aeronautics and Space Administration (NASA) is expert at garnering public favor. With competitive budgets on high alert, Congressional approbation strides front and center. That is why you hear so little about the International Space Station; its astronomical expenditures are so disproportionate to its accomplishments that even Congress is aghast. At this time of the year, NASA announcements lean to the exciting, usually the exaggerated. Some might say, the optimistic futuristic.
Such is the case with NASA’s announced discovery of “earth-like planets.”
The search for planets and the search for life on other planets has always caught the imagination of the public, so that is where the NASA give-us-more-funding-press-releases gravitate. Two years ago, it was searching for evidence for past microscopic life on Mars. Now, it is the search for planets that might nurture carbon-based life. So, what has been done? Are there other planets like earth? Is there life on other planets? What are the facts?
First, what is a planet? Astronomers now consider that a planet must satisfy three criteria,
- It must orbit around a star (like our sun).
- It must have sufficient mass to assume a mostly spherical shape.
- It must have sufficient gravity to “clear” the space around it.
Poor little Pluto could not accomplish number three, so it has been demoted. It does have four moons, though, the largest being Charon. However, Pluto and Charon orbit around each other a lot like two ice skaters clinging and slinging one-another around in circles. That leaves our solar system with eight planets, named in order of increasing distance from the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Measured to within a few degrees, all eight planets orbit the sun in the same plane, called the ecliptic. In layman’s terms, the sun and eight planets would fit into a giant celestial pancake, not very thick. All the planetary orbits lie inside the pancake.
In the 1980’s, Carl Sagan was a popular guest on talk shows. He was an astronomer whose style, if he were not an atheist, might be confused with a televangelist. Sagan was the first president of the Planetary Society. Sagan promoted SETI (Search for Extraterrestrial Intelligence). Sagan was prominent in designing the contents of the NASA time capsule sent out into space. A common Sagan “sound bite” was “billions and billions of planets out there, in space, just like our Earth that might have life.”
Here is how he came to that number. The current estimate is that there are about 250 billion stars (like our sun) in the Milky Way, alone. The Milky Way is our galaxy, a spiral; it is the bright band of light seen on a dark night.
Scientists further estimate that the universe contains about 200 billion galaxies like the Milky Way. That would mean there are fifty thousand billion, billion (50 x 1021) stars out there like our sun. Each star might have several planets, but some stars might have no planets. Even if a star had planets, those planets might be too close or too far from their star to sustain life. Maybe only 1 in every 50,000 stars has a planet orbiting it with life on the planet. (That number is just plucked out of the thin air but keep reading.) That leaves you with roughly a billion billion (1 x 1018) planets in the universe that might have the same conditions as earth. Thus, Sagan came up with the “billions and billions.” Is this a true estimate?
For reference, a person with 20/20 vision, on a dark night, standing in a dark place, can see about 3,000 stars. But, that is just on one side of the earth. So, count the other side and then realize there are about 6,000 stars visible to the unaided eye. Buy a reasonably priced pair of binoculars and you will increase the number to around 100,000 stars. A few calculations and studying the Hubble telescope pictures will convince you the number of stars in the universe is truly astronomical. How many?
Well, the number of stars in the Milky Way, 250 billion, is probably a reasonable estimate based on what we can see and estimate.
The number of galaxies in the universe, estimated to be 200 billion, is a much more “iffy” thing, but that number also is based on what can been seen on earth and with the Hubble telescope. It is the best estimate we can make until we get more data. Whatever the correct number, the number of galaxies is large beyond current measure.
How many of these galaxies have planets that might sustain life? That is anyone’s guess. In the last 20 years, evidence seems to indicate that the conditions for sustainable life may be far rarer than we originally thought. Thus, Sagan’s estimate is probably very much on the high end, but the number of possible planets that might have sustainable life is still billions or greater.
So, back to NASA, what evidence do we have that other planets even exist? In March, 2009, NASA launched the planet-hunting Kepler telescope with the specific purpose of looking for other planets.
The closest star to earth (other than the sun) is Proxima in the Centauri constellation. It is 4.3 light years away. In other words, if you travelled at the speed of light (186,000 miles per second) it would take four years and four months to get there. That is the closest star outside our solar system. The reason we can see anything at all that far away is because stars emit light. We are looking at a “sun” that far away. Planets do not emit light, they can only reflect light from their parent star. The light reflected from any planet outside our solar system is too faint to be seen using current technology. Thus, we can only detect a planet if it passes directly in front of its star, blocking some of the light and effectively “dimming” the star “momentarily.”
You will quickly reason that if someone were looking at our sun in the same manner, you could only see the planets if the edge of the pancake were turned exactly in your direction and you were viewing “edge-on.” Thus, Kepler can look at many stars, but can only detect a possible planet if the orbital plane of that planet lies directly in its line of sight.
But, once you detect a star that does periodically dim, you can tell quite a lot.
The size of the star is determined by three things, namely, 1.) the amount of light it produces, 2.) its distance (determined from parallax, red shift and other data), and 3.) the (spectral) color of its emitted radiation. All this also permits you to calculate how much heat the star produces.
The planet’s size is calculated by how long the dimming lasts. The size of the planet’s orbit is determined by how much time elapses between the “dimmings.” Finally, you can then tell the temperature on the planet by knowing the size of the star and the thus-derived orbital characteristics of the planet.
To date, Kepler has monitored about 150,000 stars. It has detected more than 2,000 planets, but those planets are so huge that their gravity would crush any kind of life. Kepler has found ONE star whose “dimmings” indicate it has two earth-SIZED planets. But, both these earth-sized planets orbit closer to their parent star than Mercury does to our sun. The possibility for sustaining carbon-based life on either of these planets is zero because they are far too hot, like Mercury. Kepler has found ONE other star whose “dimmings” indicate it has a planet orbiting in the “temperate” zone of its star, but that planet is HUGE and not a candidate for life. Forty-eight other candidates are targeted as “stars of interest.”
This is not sufficient evidence to get excited, unless you are NASA seeking Congressional support for its $600 million Kepler telescope.
Are there earth-like planets orbiting others stars with the same relative conditions as earth orbiting the sun? Are there other planets that might sustain carbon-based life? Numerical calculations using billions of stars and billions of galaxies would indicate, “Yes, there could be.” Evidence says, “We do not know at this time.” If such planets exist, do they or could they support carbon-based life? That knowledge is still in the future, but we are collecting data to try to answer that question.
At this point, we have not detected a planet with similar conditions to earth. Detecting one is on the fringe of Kepler’s capabilities. At this point, we have not detected any evidence of life anywhere else in the universe. That does not mean there are not planets and there is not life, just that we have not detected any. By the same token, it does not mean they exist, either. We are beginning to detect large planets around some stars. That is about all we can say for certain.
In the meantime, look for more announcements as NASA follows the waxing and waning of the Congressional budgets. Keep in mind that the operative words, right now, regarding earth-sized, life-sustaining planets, are the words might and could. New technologies are required to say anything definitive. By the same token, it is not hocus-pocus. It is not difficult to understand the concepts. This is an area of valid, exciting research. Just keep your scientific mind open and not shuttered by the shining and dimming of Congressional and public interest and NASA’s desire to get budget approval.