Milky Way Could Have Over 100 Million Solar System Analogs

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Besides his Calabash pipe, Sherlock Holmes is always caricatured as having a huge magnifying glass to hunt for clues.

Astronomers attending the American Astronomical Society Meeting in Washington D.C. say that they have used nature’s magnifying glass to hunt for solar systems like ours.

Scott Gaudi of Ohio State University reported that, in a preliminary survey, he concludes that at least 15 percent of the stars in our Milky Way galaxy have a planetary architecture like that of our solar system. That is, a family of gas giant planets located as far from the star as Jupiter and Saturn are from the sun. This leaves plenty of elbowroom for undetected terrestrial planets to huddle close their star, just as Mercury, Venus, Earth and Mars do.What's more, the gas giants could irrigate parched inner terrestrial planets with ices from comets and asteroids they perturb.

This may not seem like a big number, but if the statistics hold up with further observations, it works out to several hundred million solar system analogs in the Milky Way.

A phenomenon called gravitational microlensing, where the gravity from a foreground star momentarily bends and amplifies light from a background star, was used to do a statistical search for solar systems like ours. Massive planets accompanying a star, at a wide separation from the star of a billion miles or so, will also leave a detectable lensing signature during the brief celestial alignment. 

These distant exoplanets are searched for in a worldwide collaboration called the Microlensing Follow-Up Network (MicroFUN), which monitors stars in the dense galactic bulge in the constellation Sagittarius.

Only one solar system clone was identified after 10 years of monitoring. Statistically, Gaudi maintains, six such systems should have been found if every star in the galaxy had a solar system with a similar planetary distribution.

Unlike other planet hunting techniques, microlensing works for finding big planets in large orbits. The more common exoplanet search technique, measuring stellar gravitational wobbles, would require one to two decades and longer to identify the orbital periods of planets at the distances of Jupiter and Saturn. 

Therefore, the radial velocity surveys only pick the lowest hanging fruit: Jupiters that have migrated close in to their star, and have orbital periods of literally only a few days.  A Hubble Space Telescope survey of planetary transits in the galactic bulge found that seven percent of the stars have these “hot Jupiters.”

Though over 400 exoplanets have been discovered to date, this is the first observation that offers some reassurance that our solar system is not a total weirdo in the galaxy. Score one for the chances of extraterrestrial life. Score another zero for the Rare Earth Hypothesis.