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Planet Hunting Toolkit I: Why it’s Hard March 2, 2009

Posted by CosmicThespian in Toolkit.
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This article is the first in a series called The Planet Hunting Toolkit.  Each entry in the series will introduce the methods that astronomers use to find planets.  Generally, planets must be found using indirect means.  That is, with some exceptions, we can not directly image these other worlds (although this is beginning to change as was made clear in a recent announcement).  The reason that planets are notoriously difficult to see is a result of the combined effects of contrast and angular separation.

By contrast, I am referring to the difference in the amount of light coming off the planet and the star around which it orbits.  A star is self-luminous, a planet is not. The planet’s light is reflected light from the star.   Because the planet only intercepts a tiny fraction of the star’s total light, there isn’t much to reflect.  What’s more, a planet doesn’t reflect all of the light it receives; it actually absorbs some of it.  The fraction of received light a planet reflects is known as the planet’s albedo.  Planets with lots of high cloud cover, like Venus, have high albedo.  Planets covered in predominately charcoal-colored rock, like Mercury, have low albedo.

All of this is to say that the amount of light a planet reflects into space is much, much less than how much light its host star emits.  If we restrict ourselves to visible light – that is, light that our eyes can detect – a star is roughly one billion times brighter than any planets orbiting it!  That’s a one followed by nine zeros: 1,000,000,000!  To help wrap your brain around that, one billion pennies would take the same volume as five school buses or, if stacked, would tower five times higher than the orbit of the space shuttle.

So, that’s one problem.

But the problem is compounded by considering how far apart the star and the planet appear to us.  Consider the Earth and the Sun.  The Earth is, on average, 93 million miles from the Sun – a pretty respectable distance!  If you were to travel just over 2.5 billion miles away (there’s that billion again!) – roughly to the orbit of Neptune – the apparent distance between the Earth and the Sun could be covered by your thumb (about 2 degrees).  Go one light year away (a distance of about 6 trillion miles) and the Earth and Sun would be separated by only about 3 arc seconds (arc seconds are a measure of angles.  There are 60 arc minutes to a degree and 60 arc seconds to an arc minute which gets you 3600 arc seconds in one degree).  Three arc seconds is roughly equivalent to the thickness of a dime seen from about one American football field away (300 feet).  As seen from the nearest star to our Sun, Proxima Centauri – a distance of 4.2 light years – the Earth-Sun distance now subtends less than an arc second.  In other words, take that same dime and place it just over 1000 feet away.

Perhaps now you can start to see our problem.  We’ve got two objects, one a billion times brighter than the other and separated by very small angles.  Trying to see a planet around a star 10 light years from Earth is like trying to see a candle 20 feet from a searchlight in Washington, D.C. while standing in Los Angeles.

That’s really hard!

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