My frist post on this blog space was about exoplanets.  In case you forgot, exoplanet is short of extrasolar planet which again stands for a planet in some solar system.  Given the human life span, we our presence and search for knowlede, we can fairly say until recently we could just guess such planets existed, but with advance of technology we are finding ever increasing number of those.  This blog post is about an update since my previous post.  In summary, I stated I expect to see more and more of those to be found and its importance for finding new corners of space suitable for life forms similar to ours.  Those would need to be within so called habitable zone as it is called.

 

The habitable zone may also be referred to as the life zone, Comfort Zone, Green Belt or Goldilocks Zone.  A Goldilocks planet is a planet that falls within a star's habitable zone, and the name is often specifically used for planets close to the size of Earth. The name comes from the story of Goldilocks and the Three Bears, in which a little girl chooses from sets of three items, ignoring the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right". Likewise, a planet following this Goldilocks Principle is one that is neither too close nor too far from a star to rule out liquid water on its surface and thus life (as humans understand it) on the planet. However, planets within a habitable zone that are unlikely to host life (e.g., gas giants) may also be called Goldilocks planets. The best example of a Goldilocks planet is the Earth itself.

 

Launched successfully on March 6, 2009, Kepler, a NASA Discovery mission will help scientists determine just how many Earthlike planets may exist in our galactic neighborhood.  Kepler will detect planets indirectly, using the "transit" method. A transit occurs each time a planet crosses the line-of-sight between the planet's parent star that it is orbiting and the observer. When this happens, the planet blocks some of the light from its star, resulting in a periodic dimming. This periodic signature is used to detect the planet and to determine its size and its orbit.

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Wesley A. Traub, author behind the study of data obtained by Kepler mission, has released paper by end of September 2011 with some pretty much amazing results.  In the paper, he estimates that on average 34% (+/-14%) of Sun-like stars have terrestrial planets in that Goldilocks zone.  WOW!  Let's take worst case scenario here (20%) - that would be every fifth planet.  But on average, taking statistical error into account, results show this to be every third one.  That's just beyond my best wish!

 

OK, now "not so fast moment".  This is paper based on math and current observation and some water will need to pass under the bridge before we get other people doing analysis.  Also, this result may or may not become more relevant as more data becomes available for analysis (reults from paper are based on observations done for only 136 days) and finally verified using some other observational method.  Phil Plait, "The Bad Astronomer" blogger, had a take on this paper results and I will use some of his thoughts here.

 

As paper itself states, there are couple of biases introduced in calculations and one that you must already thought of is coming from the fact that this calculations are based on 136 days data of observations (available at time of analysis).  That length of time is too short to conclusively find planets in their stars' habitable zones so Wesley was forced to look at only short-period planets (with periods of 42 days or less), much closer to their stars, and extrapolate the data from there.  He looked at stars similar to the Sun with a range from somewhat hotter to somewhat cooler, roughly F, G, and K stars.  Now this letters are coming from star classification which is sometimes referred as "Oh Be A Fine Girl (or Guy), Kiss Me!" as letters O, B, A, F, G, K, M represent spectral classes.  You can find more information here. Stars analyzed (F, G, and K) comprise very roughly a quarter of the stars in the Milky Way, or something like 50 billion stars total (rough estimate).

 

Moving on, Wesley then looked at data for all planets detected - terrestrials (or simply said Earth sized), ice giants (like Uranus and Neptune), and gas giants (like Jupiter), getting their size and orbital period.  Then he found the ratio of terrestrial planets to all the planets seen (this ratio was found for planets somewhat close in to their stars due to observational period lenght as noted before).  This has been plotted versus distance from their parent stars. He then found an equation (called a mathematical fit) that did a good job predicting the shape of the plot. Once we have this, it’s easy enough to extrapolate it out to the distance of the habitable zones of the stars.  Assuming Traub is correct, and based on rough estimate of stars (F, G and K), there could be 15 billion warm terrestrial planets in our galaxy alone!

 

Now, extrapolation is always dangerous because you can't be sure your fit behaves well outside the range in which you calculated it. As Phil in his analysis says, imagine you took a census of 1000 people ages 0 to 17, and made a fit to their height versus age. You’d find their height gets bigger with time, in general. But if you extrapolate that out to someone who is 40 years old, you might estimate they'll be 4 meters tall which doesn't make any sense as we know it.  And since we do not know very well how planets form in their solar systems (and how they move around after), this results should be taken with a grain of salt.  Nevertheless, at the end of the day, this might turn around to be on the right track and only time will tell.  Kepler mission was launched exactly for this purpose and with more data available we will know more for sure. 

 

Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry, laboratory and field research into the origins and early evolution of life on Earth, and studies of the potential for life to adapt to challenges on Earth and in outer space. Astrobiology addresses the question of whether life exists beyond Earth, and how humans can detect it if it does.  In looking for Earth-like planets around other stars, astrobiologists search for planets that can support liquid water. So these planets must have a temperature in the relatively narrow range that exists on Earth. The general thinking is that these conditions can only exist at a certain distance from the star (habitable zone).  Next study (paper), released by end of October 2011, claims habitable zone to be dramatically bigger around red dwarfs bigger than previously thought - for some 30%.

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In our Solar System, the habitable zone stretches from about 0.7 to 3 AU, approximately from the orbit of Venus to about twice the orbit of Mars. AU (astronomical unit) is a unit of length equal to about 149597870.7 km or approximately the mean Earth–Sun distance.  The size and temperature of the star are crucial, but much depends on conditions on the exoplanet itself, in particular how much light is reflected back into space, the albedo.  Albedo is the fraction of solar energy (shortwave radiation) reflected from the Earth back into space. It is a measure of the reflectivity of the earth's surface. Ice, especially with snow on top of it, has a high albedo: most sunlight hitting the surface bounces back towards space. Water is much more absorbent and less reflective. So, if there is a lot of water, more solar radiation is absorbed by the ocean than when ice dominates.

Scientists from National Centre for Atmospheric Science in Reading (UK) and NASA Ames Research Centre in new study pointed out an important new factor that dramatically extends the habitable zone around an important class of stars.  They say that the amount of light that snow and ice reflects depends on the fraction emitted at different wavelengths. The Sun produces much of its light at visible wavelengths. The albedo at these wavelengths for snow and ice is 0.8 and 05 respectively.  But the vast majority of stars are red dwarfs and these emit far more of their light at longer wavelengths.  A red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type.  They constitute the vast majority of stars and have a mass of less than half that of the Sun (down to about 0.075 solar masses, which are brown dwarfs) and a surface temperature of less than 4000 K. The albedos of ice and snow on planets orbiting M-stars are much lower than their values on Earth.albedo.jpg

Scientists have calculated the albedo for snow and ice on planets orbiting two nearby red dwarfs - Gliese 436 (just 33 light years from us) and GJ 1214 (40 light years away).  Both are known to have exoplanets, although not in the habitable zone. The wavelengths that these stars emit mean that snow and ice here have albedos of about 0.4 and 0.1 respectively.  In other words, water-bearing planets orbiting these stars ought to absorb far more energy than Earth. Therefore, this extends the radius of the potential habitable zone.  The outer edge of the habitable zone around M-stars may be 10-30% further away from the parent star than previously thought.  And not only are red dwarfs by far the most common type of star, they are also the most likely to provide us with our first view of Earth 2.0 (if we haven't seen it already). That's because they are smaller, which makes it easer to see planets orbiting close to them.  Having an extended zone makes it just that little more likely than we'll find another Earth sooner rather than later.

 

 

Credits: NASA, Wikipedia, arXiv, Phil Plait


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