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Life in the Universe VI: Space, the Desert & Exoplanets   8 comments

The Milky Way may be home to million or billions of other living planets, but there are enormous empty spaces between us.

The Milky Way may be home to million or billions of other living planets, but there are enormous empty spaces between us.

Space is on my mind here in the deserts of southern Utah.  It isn’t so much that when the sun goes down in the desert the stars shine brightly.  It is the very nature of the desert itself.  The way small clusters of people and houses seem to occur randomly with huge empty spaces between them reminds me of the scarcity of life in an immense void.

And during this time of year at least, the way the temperature drops so quickly at night and rises almost as quick in the morning reminds me of being on an airless planet where the nearby star’s light brings intense heat during the day and biting cold at night.

The landscapes of the American southwest can often be mistaken for alien ones.  On this morning I watched a couple rock climbers scale this pinnacle.

The landscapes of the American southwest can often be mistaken for alien ones. On this morning I watched a couple rock climbers scale this pinnacle.

This is an ongoing series on my blog, believe it or not.  Like space, there are long journeys involved in going from one post to the next in the series.  The last installment, Part V, began to explore the question of life outside the solar system by highlighting the indomitable Carl Sagan.  Part IV discussed the search for life within our own solar system.  This part will continue to explore the idea of life out in the universe as a whole – a challenging subject I admit I’ve been avoiding.

The question that I posed to begin, the one which underpins the meaning of this series, is explained in Part I.

The large expanses of desert are accentuated by the lack of trees, the bare rock, and the big sky.

The large expanses of desert are accentuated by the lack of trees, the bare rock, and the broad skies.

The Milky Way rises over rock formations in Canyonlands National Park, Utah.

The Milky Way rises over rock formations in Canyonlands National Park, Utah.

The Quest for Exoplanets

Humans have found over 1000 planets outside our own solar system to date, with well over 3000 potential candidates.  In typical parochial fashion, we call these extra-solar worlds exoplanets.  The Kepler space telescope is one of the finest tools we have in the quest to find exoplanets.  It explores a constellation-sized area of the Milky Way Galaxy near Cygnus, the Swan (aka the Northern Cross).

Kepler continuously monitors the brightness of more than 145,000 stars.  It looks for a slight dimming in brightness indicative of a planet crossing between earth and the star. Think of trying to detect the dimming of a bright streetlight a mile away when a moth flies in front of it and you have the idea.

To find exoplanets, astronomers have traditionally used the slight wobble of a star that occurs when an orbiting planet tugs on it.  This gives us good information on the sizes of the planets, along with how close they orbit to their host stars.  More recently the Spitzer space telescope has detected, for the first time, actual light coming from an exoplanet.  This is key.  In order to find out anything about the surfaces of these worlds we need to examine the light bouncing off them or skimming through their atmospheres.  Spitzer and some ground-based telescopes can do the former while Kepler is uniquely suited for the latter.

Turret Arch greets a rising Orion the Hunter.

Turret Arch greets a rising Orion the Hunter.

Candidates for Life

Most of what we’ve found thus far have been very massive exoplanets the size of Jupiter and larger.  Many of these “hot Jupiters” orbit very close to their stars, closer even than our own Mercury.  As our techniques get more refined and as more time goes by (allowing the wobble method to work on exoplanet candidates orbiting further from their stars), we are finding more and more planets that are close to the size of Earth.

Crucially, we are now finding planets that orbit their stars at a distance which allows liquid water to exist.  This orbital distance, which in our solar system essentially extends from Venus to Mars, is the “habitable zone”, also known as the Goldilocks Zone. Combining these two factors that are relevant to the search for earth-like life (the planet’s size and distance to its parent star), we have found to date 12 earth-like exoplanets.

The size and brightness of the host star makes a big difference in how close a planet can orbit and still be cool enough for liquid water and possible life.  We have found only one earth-sized, rocky planet thus far (Gliese 581-g), and happily this planet orbits about the same distance from its star as earth does from the sun.  But there are two problems.  First, Gliese 581 is a much smaller and cooler star than the sun.  So its habitable zone, where water may exist, is presumably much closer in.  Gliese 581-g still would orbit within it, but depending on the shape of its orbit it may get too hot for liquid water.

There’s a much bigger potential problem, however.  The very existence of Gliese 581-g is disputed by some astronomers.  Its discovery is somewhat clouded and controversial.  Confirmation of Gliese 581-g may take some time.

A survivor in Arches National Park overlooks a desolate valley at dusk.

A survivor in Arches National Park overlooks a desolate valley at dusk.

An exoplanet called Kepler 22-b is also interesting.  The Kepler space telescope caught it passing in front of its star on just the third day of the spacecraft’s operation.  Though 22-b is some 2.5 times bigger than Earth, its parent star is very similar to the Sun (G type).  Also, 22-b orbits at an average distance very similar to Earth’s, and so its year is similar to ours.  The only problem with Kepler 22-b is that we know so little about it.  For instance, we don’t know how elliptical its orbit is.  If it is highly elongated (as most explanets’ orbits are) it might spend part of its year very very close to the star and part very far away.  Earth’s orbit is nearly circular.

The closest potentially habitable exoplanet to us is Tau Ceti-e, only 12 light years away.  That is still much too far for us to visit in anything close to a human lifetime, so we need to temper our enthusiasm.  Also, Tau Ceti-e is yet another unconfirmed exoplanet.

The Milky Way Galaxy rises vertically over Canyonlands National Park.

The Milky Way Galaxy rises vertically over Canyonlands National Park as Venus sets.

Are We on the Right Track?

You might be questioning the importance of looking for exoplanets that are earth-like, orbiting sun-like stars at earth-like distances.  You might wonder why we don’t also look for life forms that aren’t anything like ours, life that perhaps does not rely on water or based on carbon.  Also you might notice that we always speak of planets.  We know from the search for life within our own solar system that the moons around planets are in some cases better candidates for life than are the planets themselves.  Finally, life in the cosmos may in some cases be decoupled from planets or moons, living instead in space, perhaps close to large energy sources (such as quasars).

You’re right to question.  Definite biases exist in the search for extraterrestrial life.  To some extent they are unavoidable.  But consider two facts: First, it is easiest to look for earth-like planets and life.  And this is not an easy enterprise to begin with.  Second, our sort of life is all that we know for certain can exist.  Again, it is hard enough to look for our type of life trillions of miles away let alone other types.  These sound like excuses for our bias, but there it is.

And so the hunt continues for exoplanets that are candidates for earth-like life.  Based on the Kepler space telescope’s findings, astronomers estimate that perhaps as many as 20% of the sun-like stars in the our galaxy have habitable planets orbiting them.  This is a stunning estimate because it suggests that there are nearly 9 billion habitable planets in the Milky Way Galaxy.  If even a tiny percentage of these planets have developed intelligent life, then we have plenty of company in our galaxy. 

Arches National Park under the winter stars.

Arches National Park under the winter stars.

 

 

 

 

 

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