Wednesday, December 21, 2011

Goldilocks Planets No Fairy Tale

Throughout the twentieth century, it was not all that uncommon to hear of graduate physics students running down the halls calling out, "We found a new one!"  Such a claim usually meant that a new elementary particle had been observed in the lab, be it the muon or gluon.  The count for such particles has remained at sixteen for some time.  The large hadron collider may soon allow the long theorized but ever evasive Higgs Boson to be observed, thereby raising the tally to seventeen.  This discovery would complete the list theorized by what is commonly referred to as the standard model.

These days, when physicists are found doing their celebratory dances, it is usually because a new planet has been discovered.  And, if that planet happens to be a Goldilocks planet, they really bust a move.

Goldilocks planets lie a distance from their star such that the surface temperature can allow for water in liquid form.  Like the porridge in the famous fairy tale, the surface temperature of such planets are not too hot or too cold; they are just right.  Planets in the "Goldilocks Zone" for a given star may be conducive for the evolution of life.  And, if other conditions are met, it is possible that other communicable societies currently call the surfaces of such planets their home.  However improbable this may be, the fact that it is not impossible is one reason why the discovery of such planets is cause for celebration among physicists and astronomers, and why it is thought-provoking for us all.

The order of size of an elementary particle is O(-18) meters, while that of a typical planet is O(7) meters.  Here, the capital "O" stands for order of magnitude, and indicates ten to the power of the number in brackets.  So, a typical planet has a diameter that is roughly 10,000,000,000,000,000,000,000,000 (25 zeroes) times that of an elementary particle.  Given this comparison of size, finding planets within our galaxy should be easy, right?  Well, not quite - finding a planet in our galaxy but outside our solar system is no less of a needle in a haystack search than that for an elementary particle within an atom.

The now eight planets (sorry Pluto) orbiting our Sun were fairly easy to find compared to the those outside our solar system (known as extrasolar planets or exoplanets).  Under certain circumstances, Mars, Saturn and other planets within our solar system may be seen with the naked eye.  But, all planets inside our solar system are at most tens of AU (astronomical units) away from the Earth, leading to separations that are O(12) meters.  Our galaxy, the Milky Way, containing hundreds of billions of stars, measures tens of thousands of light years across, meaning that solar systems are separated by distances that are up to O(20) meters.  This difference in order of magnitude of eight means that exoplanets in the Milky Way are on the order of one hundred million times further from us than are planets within our solar system.  

The distance from Earth to Mars is seventy-five million kilometers - a stone's throw, cosmologically speaking.  The distance from Earth to Kepler-22b, the most intriguing Goldilocks planet discovered thus far, is six hundred light years away, or about five million billion kilometers.  Let us reduce this to terrestrial distances: if Earth to Kepler-22b is like New York City to Paris, then Earth to Mars is like the length of your thumb.

It should be noted that planets are small in cosmological terms (the diameter of the Earth is less than 1% that of the Sun).  The additional challenge with regard to finding planets of any kind is that they are not sources of light as are stars.

The discovery of exoplanets is a relatively new practice; the first such discovery occurred in 1995.  The number of discoveries has quickly ballooned to 716 thanks in part to NASA's Kepler telescope.  The telescope, launched into orbit in March 2009, surveys 150,000 stars in our galaxy seeking wobbles in star light that are indicative of the presence of a planetary body.

You might think that 150,000 stars is just a drop in the bucket, and you would be correct.  We are examining less than one millionth the total number of stars in our spiral galaxy (the closest ones).  Given the small pool of stars under surveillance and the short time of observation, the Kepler telescope has provided a significant amount of compelling data.  The newly discovered planets force cosmologists to revisit the Drake equation with new statistical information, enabling them to, with more confidence, predict the number of communicable civilizations that currently exist.

To date, of the 716 exoplanets discovered, twelve exist within the habitable zone.  Of these twelve, there are two Goldilocks planets, both discovered in 2011, worth writing home about: HD 85512b and Kepler-22b.  Kepler-22b is of particular interest because if it does indeed have an atmosphere, then its surface temperature is estimated to be around 22 degrees Celsius - "room temperature".

Now, if only I had a space ship that could travel near the speed of light.  If I did, the 600 light year journey to Kepler-22b would be feasible in my life time.  While the civilizations both on Earth and (potentially) on Kepler-22b would age by over six hundred years during my trip, the journey for me would not take much time at all.  I wonder if the hotels on Kepler-22b would accept my currency of money, and more importantly, whether or not they would have room service.

In any case, I do not have a space ship that can travel at light speed, nor do I think I would be brave enough for such a journey if I did.  Besides, I'd have to run the trip by my wife.

Returning to NASA's recent discoveries, the little bit of planetary evidence has allowed the Kepler team to extrapolate the planetary picture within our galaxy.  It is estimated that there are fifty billion planets in the Milky Way, of which more than five hundred million lay in the habitable zone of the stars they orbit.  Given the fact that such minimal efforts have allowed us to discover a planet with as much potential for fostering life as Kepler-22b, it  seems to me downright unlikely that we are the only life forms in the Milky Way, let alone the universe.

I guess the analogy that this article begins with, relating the discovery of Goldilocks planets within our galaxy to the discovery of elementary particles within an atom, is a little bit weak.  Yes, planets orbit their star, and the most famous elementary particle, electrons, orbit their nucleus, but there is one major distinction between the two searches.  The search for the building blocks of atoms appears to be nearing its end.  On the other hand, with regard to the search for habitable planets within our galaxy alone, we have just begun to scratch the surface, and there is truly no end in sight.

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