Cumberland Times-News

Bob Doyle - Astronomy

July 13, 2013

Here’s more to learn about the exoplanets

Last week’s column gave a summary of the history of the discovery of planets about other stars and the techniques used to find these alien worlds.

The most common term is “Exoplanets,” while some still use “extra solar planet.”

The great hope is that measurements will reveal an Earthlike exoplanet at the right distance from its star to allow liquid water.

Then once these candidate planets are identified, special techniques may be used to determine what gases are in these planet’s atmospheres. The presence of certain gases may indicate that the living things likely exist on the planet’s surface or in its oceans.

Here are a few terms used to characterize the variety of exoplanets: “Hot Jupiters” — giant planets with 50 or more times Earth’s mass that are too close to their stars to allow liquid water, “Super Earth” — a planet from two to 10 times the mass of the Earth, “Exo-Earth” — a planet from 1 to 10 Earth’s mass revolving about its star in a region (habitable zone) where liquid water is possible and “Rogue planet” — a planet that doesn’t orbit a star (likely formed about a star but an unfortunate encounter with another massive planet has led to its reaching a high enough speed to escape its star).

I now turn to a fine interview in August’s “Sky and Telescope” magazine on exoplanets with Sara Seager, a professor of planetary science and physics at the Massachusetts Institute of Technology. I will paraphrase some of the questions and her answers.

“How common Are Exoplanets in our galaxy?”

Based on data known presently, there should be about as many planets as stars in our galaxy. (Our galaxy is estimated to have 100 billion stars.)

“How do the big exoplanets compare in numbers with the small exoplanets?”

There are likely more small exoplanets than large exoplanets. Stars similar to our sun are likely to have close in planets, the heavy planets farther out, with medium heavy planets yet farther away.

“Are the known exoplanets around a particular star in a plane just as our planets are about our sun?”

Using the Kepler probe measurements (detection of planets by their passage across a star’s disk), it seems that many stars have a family of planets, some big and some small, all revolving in a thin plane (think pizza box carton).

“How did the ‘Hot Jupiters’ get to be in such tight orbits about their stars?”

The “Hot Jupiters” likely formed much farther away from the their stars and by magnetic processes are being drawn inward.

“How common are the rogue planets?”

There may be as many rogue planets (not in orbit about a star) as the number of exoplanets orbiting stars. Microlensing studies (light distorted gravitationally) indicate that these rogue objects may be twice as populous as stars in our galaxy.

“Can there be exoplanets in orbit about two stars, or within a double star system, orbiting about just one of the stars?”

Yes, the Kepler probe has detected a half dozen exoplanets that are orbiting about two stars. The closest star system to our sun is the triple Alpha Centauri A, B and C.

There is an exoplanet orbiting component B. A and B are sunlike stars, separated by about 20 times the Earth –Sun distance.

The Red dwarf component C (Proxima Centauri) is roughly a trillion miles from either A and B. (This star system is below the southern horizon for most of the United States, save southernmost Florida and Hawaii.)

There is another fine article on Exoplanets in the July “Scientific American” by Michael Lemonick.

SKY SIGHTS AHEAD: Tomorrow evening, the moon will be half full (like a titled letter D) in the southwestern sky. Close to the moon then will be the bright star Spica of Virgo.

On July 16 , the moon will appear below the planet Saturn. On July 18, the moon will appear in front of the stars marking the claws of the Scorpion. The moon will be full on July 22, hanging low in the sky near the tail of Capricornus.

The Cumberland Astronomy Club will meet at the LaVale Public Library on July 19 at 7:30 p.m. Members of the club are from the Tri-State area (Maryland, West Virginia and Pennsylvania). All sky gazers are welcome.

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

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Bob Doyle - Astronomy
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