So how many planets are there? Not in the universe, but just right here in Earth's own specific neighborhood, the Milky Way galaxy. According to an estimate to be published in February in the Astrophysical Journal, there are at least 100 billion.
In talking with the lead author on the paper, Jonathan Swift, Talking Points Memo noted this week (Jan. 2) that the number was likely a conservative estimate.
“There’s room for these numbers to really grow,” said Swift, a Caltech astronomer. “They’re not going to shrink. Our calculation is new in the sense that we are making the calculation of planets in compact systems around the most populous type of stars in the galaxy.”
The latest study, which incorporated the work of scientists from not only the California Institute of Technology in Pasadena, California (Caltech), but also the University of Notre Dame and University of Chicago, was corroborative of earlier, similar studies which made much the same estimates.
The difference, however, came in the way the latest study derived their numbers. Swift et. al., using data from the Kepler Space Telescope, targeted red dwarf stars, specifically Kepler 32. Since red dwarf stars make up approximately 75 percent of all the stars in the Milky Way, the estimate was based on these difficult to detect stars, giving the calculations more accuracy than previous studies. John Johnson, an assistant professor of astronomy at Caltech and a co-author on the paper, referred to red dwarfs as the "'Silent Majority' of the galaxy."
Red dwarfs are cooler, less massive, and longer-lived than stars such as our own Sun. Their planetary systems tend to be congregated closer to the parent star. The study's astronomers believe this to be advantageous to life having a chance to emerge on some of the planets due to the possibility of more of them being in the habitable zone of the stars (which is, of course, closer than would be the zone around a star like the Sun), allowing liquid water to form.
In fact, the closest star to the Solar System, Proxima Centauri, is a red dwarf. Although it is as yet unknown if it is a parent star, the red dwarf is part of a trinary star system, locked in a gravitational relationship with the binary system Alpha Centauri. Of those, Alpha Centauri B has been found to have a world orbiting it.
To date, more than 850 extrasolar planets have been detected since 1995, according to The Extrasolar Planets Encyclopedia. If Swift and the Caltech-led study are correct, there are literally billions upon billions of worlds yet to be discovered -- and that's just within the spiral of the Milky Way.