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Dr. Neal Sumerlin keeps us abreast of happenings in the night sky and the progress of the new Belk Astronomical Observatory.

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Pictures of Planets

Posted on 11/17/2008

Last week it was all over the news: scientists reveal first images ever of planets outside our solar system!  Well, yes that's true, but as is usually the case with scientific news reported in the popular press, there is more to the story.

First, though, here is the picture.  Taken with the Hubble Space Telescope (probably the most productive single instrument in the history of science), it shows the star Fomalhaut (pronounced "foam-a-lot") and its nearby environs.  It is hard to see this well with a low-resolution image, so in the interest of conserving bandwidth, here is the link to a pretty good one.

http://antwrp.gsfc.nasa.gov/apod/image/0811/fomalhaut_hst_lab.jpg

Planets are very difficult to see around stars, simply because the star is so much brighter.  The planet is visible only in reflected light from the star.  The problem is akin to trying to see a gnat flying around a bright searchlight!  If you could somehow block out the searchlight, you would stand a better chance of seeing the gnat.  The "coronagraph mask" in the image shows the area nearest the star where the light was occluded.  The possible planet is deep within the dust ring surrounding the planet, and is most easily visible in the close-up insert.

Fomalhaut is only 25 light years from Earth, and is a large, hot, and young star.  The dust ring that still surrounds the planet is a youthful characteristic; our own much older solar system has cleared most of that out after 4 billion years or so.  The planet we are seeing is not Earth-like, but is probably more similar to Neptune-a fairly large and gaseous denizen of the outer reaches of its star's gravitational field.

So is this the first planet ever beyond the solar system?  Hardly.  Over 300 planets have been detected around other stars, mostly by a method that detects the planet's gravitational influence on its host star.  As the planet revolves around the star, it tugs it to and fro.  This creates subtle changes in the light coming from the star, and it is those changes that provide indirect evidence of the planet's presence.

Is it the first image of an extra-solar planet, then?  That depends on how you define it.  It is the first visible-light image of another planet, yes.  But others have produced images of an object orbiting a nearby brown dwarf (a ball of gas too small to ignite nuclear fusion at its core, and therefore not officially a star), but the image was in infrared light.  This was an easier object to image, because the planet and the brown dwarf are not so different in their brightness.  This is more like finding the gnat flying around a heat lamp instead of a searchlight.  Here is that image:

Brown Dwarf with Planet

http://www.fas.org/irp/imint/docs/rst/Sect20/planet_picture_040910_02.jpg 

The brown dwarf is blue-white in this false-color image; the planet is red-orange.  My translation of the text box follows:

The length of this white line is 778 milliarcseconds, an angular measurement that corresponds to the width of a quarter as seen from 6.4 km (just under 4 miles) away.  At the brown dwarf's distance of 70 parsecs (228 light years), this corresponds to 55 astronomical units, 55 times the average Earth-Sun distance.

 And to considerably less fanfare, another infrared image was released last week of a multi-planet system around a star similar to our Sun.  It is today's Astronomy Picture of the Day: http://antwrp.gsfc.nasa.gov/apod/astropix.html.  (If you read this later, look for the picture of November 17, 2008.)

 The methods we have now can't yet detect an Earth-like planet in the "habitable zone" of a Sun-like star-the temperature region where water could exist as a liquid.  This is a prerequisite for life as we know it.  That is of course the Holy Grail for planet-searchers, and one that will be within reach with new instrumentation within a few years.

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