The number of known multi-planetary star systems has just tripled. What’s more, the Kepler space telescope science team has just announced that they have doubled the number of confirmed exoplanetary sightings made by the observatory.

“Prior to the Kepler mission, we knew of perhaps 500 exoplanets across the whole sky,” said Doug Hudgins, Kepler program scientist at NASA Headquarters in Washington, D.C. “Now, in just two years staring at a patch of sky not much bigger than your fist, Kepler has discovered more than 60 planets and more than 2,300 planet candidates. This tells us that our galaxy is positively loaded with planets of all sizes and orbits.”

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Thursday’s announcement focuses on 11 new planetary system discoveries. 26 alien worlds orbit these 11 stars.

Although these stars may possess up to several planets each, that is where the similarities to our own solar system end. The worlds in each of these star systems have very compact orbits — the shortest orbital period (or “year”) is six days; the longest is only 143 days. As a comparison, the orbital period of Mercury is 115 days. All of these worlds have an orbital distance closer than Venus is to the sun.

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Some of the newly discovered worlds are only 1.5 times the size of Earth, while others are bigger than Jupiter. Fifteen exoplanets are between Earth and Neptune in size, but further observations will be needed to determine if any have a rocky surface like Earth, or a gaseous consistency like Neptune.

The Kepler space telescope can only detect exoplanets that pass in front of their stars as seen from Earth. Their obits are “edge on” and Kepler surveys one small region of the sky (containing around 150,000 stars) in the hope of seeing the brightness of a star briefly “dim” as the star’s exoplanet blocks some of the starlight from view. Interestingly, the gravitational effects of other planets within these star systems can be detected too.

If two exoplanets have fairly close orbits, when one passes the other, its gravitational influence may slow down or speed up the orbital speed of the other planet. It may act as a gravitational “tug.”

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This gravitational tugging can be spotted by Kepler as it makes very precise timings of an exoplanetary transit. Measuring this variation — called Transit Timing Variations (TTVs) — otherwise invisible exoplanetary buddies may be revealed.

According to NASA:

Five of the systems (Kepler-25, Kepler-27, Kepler-30, Kepler-31 and Kepler-33) contain a pair of planets where the inner planet orbits the star twice during each orbit of the outer planet. Four of the systems (Kepler-23, Kepler-24, Kepler-28 and Kepler-32) contain a pairing where the outer planet circles the star twice for every three times the inner planet orbits its star.

“These configurations help to amplify the gravitational interactions between the planets, similar to how my sons kick their legs on a swing at the right time to go higher,” said Jason Steffen, the Brinson postdoctoral fellow at Fermilab Center for Particle Astrophysics in Batavia, Ill., and lead author of a paper confirming four of the systems.

Basically, as one exoplanet zooms past another, its gravity gives its sister planet a boost that reveals itself as a TTV.

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Although other observatories are churning out a multitude of exoplanet discoveries, Kepler is opening a window into a galaxy that seems to be crammed with small worlds. In fact, it’s thought the majority of planets orbiting other stars will be more “Earth-sized” than “Jupiter-sized.” What’s more, it’s revealing a menagerie of alien worlds that are challenging our understanding about how our own solar system evolved.

It’s only a matter of time before we start seeing hints of a true “solar system-like” star system, possibly containing an Earth-sized world orbiting a sun-like star within its habitable zone. Although we’ll have no clue whether that world will have an atmosphere that could sustain life, it will show us that far from being rare, multi-planetary systems like our own are out there in abundance.

Image (top): Kepler-11, a star system crammed with 6 exoplanets — its existence defies conventional planet-forming wisdom (NASA/Tim Pyle). Image (middle): The orbital configurations of the new star systems (NASA Ames/UC Santa Cruz)