by Jon Pedicino
Professor of Astronomy
College of the Redwoods
Eureka, CA (SPX) Feb 21, 2011
The rush of data released earlier this month from the Kepler science team is intriguing for what it saw and didn't see in its first 4 months of observations. The relatively small Kepler space telescope is very good at what it does. It has a highly sensitive array of CCDs (light sensors) that has taken about 21 months of data.
We saw the release of a small part of that data recently. It revealed more than 1200 potential extrasolar planets. If a reasonable percentage of these worlds turn out to be independently verified, this treasure trove will yield more results in 4 months (1200+) than astronomy had found in the past 15+ years (550+ objects).
Kepler uses the transit or partial eclipse method to identify new planets. It keeps a constant vigil on 156,000 stars that are up to 3000 light-years away in a region close to the star dense galactic plane in the constellations of Cygnus and Lyra. We need to keep in mind that in order for the method to make a detection the planet has to pass in front of the star from our perspective here in the solar system.
Since other planetary systems can exist at all different angles, we would only expect that a small percentage of extrasolar systems (1-10%) would be edge on to us. That means that Kepler is really only sampling 1,500 to 15,000 systems. That makes the fact that they have found 1200 potential planets all the more impressive!
Kepler uses the drop in light arriving from the star to estimate the size and diameter of the planet. The period of repetition of the transit tells us the distance from a star of known size using a law first described by Johannes Kepler appropriately enough.
This method tells us size and orbit. We need additional data to tell us a planet's mass and therefore determine if its density or compactness is more similar to a Jupiter or Neptun- like gas giant or a denser terrestrial planet like the Earth.
The method to determine mass uses the gravitational effect that the planet has on its star and is called the radial velocity or "wobble" technique. Notice that in almost all cases, direct observations of an extrasolar planet has yet to be accomplished. It is, however, on the horizon with instruments like the Terrestrial Planet Finder that is set to launch in the next couple of years.
So what did the data show? The data returned about the 1235 possible extrasolar planets detected to date are broken down by size based on how much light the transit blocked. The most common type of objects were the 662 Neptune size objects that had a diameter about 4 times that of the Earth. The next largest group was the 288 so called "Super Earths" that have a diameter between 1.5 to 2 times that of the Earth.
Keep in mind that an object twice the size of the Earth would need to have eight times the mass of the Earth to be the same density of our planet as we are dealing with volume and therefore three dimensions (2x2x2=8). The next largest sub category were the Jupiter sized objects that are about 10 times the diameter of the Earth and numbered 165.
The Earth sized objects totaled out at 68 or 5.5% of the total. The smallest group was the Jumbo Jupiters that are probably borderline Brown Dwarfs (not full blown stars) that totaled just 19.
The transit method that Kepler uses has no bias toward large or small planets. Therefore, this group should give us our first look at the distribution of planet sizes in the broader galaxy. Result: Neptunes are abundant and perhaps dominant.
There is not enough rocky mass available for these objects to be anything other than small gas giants. A four (4) Earth diameter object would need to be 64 Earth masses to have Earth's rocky density.
Given the excitement surrounding possible rocky planets, the large abundance of Neptunes might come as a disappointment to some. Although, keep in mind that some of the best prospects for life in our solar system lie on the moons of gas giants.
The large number of Super Earths (23.3%) that may be determined to be rocky after further study, combined with the significant percentage of Earth sized bodies (5.5%) is heartening for those hoping to find a reasonable facsimile of our home world. Using this small sample as a starting point, we can assume that a sizable percentage of stars that could show evidence of Earths seem to have them.
This is big news. Earths may not be everywhere, but they aren't very rare either! Astronomers are now hard at work using the radial velocity (wobble) method to determine the mass of these bodies to flesh out their average density and therefore their general composition ( rocky versus gaseous).
The other way that this group of 1235 potential objects was broken down had to do with whether or not they were located in the so called "habitable zone."
Though I have problems with the potentially false expectations that this name implies, the definition lies in the possible surface temperature of these worlds allowing for liquid water based on their distance from their parent star. Keep in mind that stars come in a wide range of brightnesses from objects that are 1 million times the brightness of our sun (rare) to one hundred thousand times dimmer than our sun (very common).
Since the Kepler data we are looking at only references the first four months of data, this set only looks at planets that orbit within a maximum period of about 100 days.
That means that the longer period objects will start to show up as more data is released. For a shorter period planet to lie in the water-possible zone it must be circling a dimmer star than our sun. Bright stars or even sun-like stars would be too bright at that distance.
All this being said, there were 54 planets detected in the water-possible zone. Five (5) of those were Earth sized or smaller. This is where things really get interesting. These are the gems in the group.
These worlds have the potential to be more Earthlike. That is if they have an atmosphere that would need to be protected by a magnetic field similar to our own.
Of course the thickness of the atmosphere and its composition play a major role as well. Venus receives only twice the sunlight that the Earth does but has surface temperatures that are 400+ degrees Celsius (750+ degrees F) hotter than the Earth on average.
This is due to the fact that Venus' atmosphere is about 100x thicker and made almost entirely of the heating trapping gas carbon dioxide. In short, the unknowns and the particulars matter, a great deal.
Getting back to the 49 other larger planets in the group of 54, there still exists the possibility that these worlds could have interesting moons as well. Keep in mind that the moons in our solar system that have the best chance for liquid water and possibly life are moons well outside the traditional water-possible zone.
They draw their heat from internal processes stirred up by the gravity of their large host planets (e.g. Europa, Enceladus). Of course, if it's life you are looking for, we might not need a nice warm sunlit pool of water to produce it.
The nascent and blossoming field of Astrobiology has found life happily perking along in all sorts of extreme environments right here on Earth and potentially around our solar system. Either way, these five (5) smaller worlds are easily the most compelling.
So we wait for more data to be collected and released. New missions like the aforementioned Terrestrial Planet Finder will tell us even more. By using Infrared light, they might even detect these worlds directly. That light and the absorption lines therein will tell us about the presence, thickness, and most importantly composition of the atmosphere of these worlds.
Maybe they have a chemically reactive gas like oxygen that will hint at some active process replenishing it like the chemistry of life does on Earth. Combine this forthcoming data with additional data about the planet's mass and density and a clearer picture will emerge. This all happens in the next short months and couple of years.
For sure, if I was working at a SETI telescope like the Allen Telescope Array in Hat Creek , CA, I would have a set of five very interesting targets to listen to in the near future.
That group is looking for a needle in a haystack, however, the folks at Kepler just pointed to an interesting corner of the pile. For sure, that detection would answer a large number of questions really quick and change our perspective about our Earth and humanity even quicker than Kepler already has. I anxiously await the next release of results and everything that follows. What a great time to be studying the sky! We are on the cusp of something truly monumental.
Kepler at NASA
Planetquest at JPL
College of the Redwoods
Lands Beyond Beyond - extra solar planets - news and science
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