### The Ultimate Alien Star System --"416 Planets in the Habitable Zone"

We astronomers are happy to have measured that up to half of the stars in the Milky Way appear to host Earth-sized planets in the habitable zone (NEarth is up to 50 percent for red dwarfs, the dominant stars in the galaxy by number). TRAPPIST-1 is a great example that goes even further and packs three planets in the habitable zone.

But I am hoping for some super- habitable systems with 10, 20, or hundreds of potentially life-bearing planets. They are sure to be low probability systems, but with five hundred billion stars in our galaxy (and sci-fi fans crossing their fingers) it’s definitely worth looking!

This system is completely stable, writes Laboratoire d'Astrophysique astronomer Sean Raymond in Nautil.us, but nature would have a tough time forming this system. If it exists, it could only have been built by a super-advanced civilization.

When Frank Drake was a boy,  growing up in 1930s Chicago, his parents, observant Baptists, enrolled him in Sunday School. By the time he was 8 years old, he suspected his religion, and others around the world, were, to some extent, environmentally determined—local chance events helped shape them. He began to think the same might be true of civilization, for humans and, perhaps, aliens as well—but he thought it better to keep these thoughts to himself.

But not for long: He would go on to found S.E.T.I., the Search for Extraterrestrial Intelligence, and laid out a simple way to estimate the number of civilizations within our galaxy that we could hope to listen-in on. It’s an equation that looks like this:

N (the number of communicable civilizations in the Milky Way)

= R (the rate at which stars form)

× NEarth (the fraction of stars with Earth-sized planets on Earth-like orbits)

× FLife (the fraction of those planets that develop life)

× FIntelligence (the fraction with intelligent life)

× FCommunication (fraction that can communicate)

× L (the average civilization’s lifetime)

In short, N = R × NEarth × FLife × FIntelligence × FCommunication × L. To determine the value of N, we just need to know the other numbers.

We know that the Milky Way makes a couple of new stars per year, so R is taken care of—but that’s it. We have no idea how common life, intelligence, or the ability to communicate are. And while we may all be rooting for the average civilization’s lifetime to be very long, we have no data.

But we are making progress on NEarth (also called “Eta-Earth”). The first Earth-sized planet orbiting another sun was discovered in 2010.

Thanks in large part to NASA’s Kepler Space Telescope, we now know of hundreds of Earth-sized worlds, and a handful as small as Mars and Mercury.

Kepler’s primary mission was to determine the abundance of Earth-sized planets orbiting at Earth-like distances around Sun-like stars. This is just NEarth for stars like our sun. But NEarth might be different for different types of stars. Unfortunately, by 2013, Kepler lost two reaction wheels—essential for pointing the telescope—and had to abandon its primary mission after acquiring about four years’ worth of data. Kepler has good statistics on planets orbiting suns on Mercury-like orbits but not Earth-like. Bummer. ( A few years ago Kepler was reincarnated in a new mode called K2, still finding planets but without hope of measuring NEarth. )

But I’m hoping for some super-habitable systems with 10, 20, or hundreds of potentially life-bearing planets.

The habitable zone is the ring around a star where the conditions are right for liquid water on a planet’s surface to exist. Those are the planets counted in NEarth. But different stars have different habitable zones: Those of red dwarf stars, cooler and fainter than our sun, are much closer; and those of brighter, hotter stars are farther out. Kepler succeeded in estimating NEarth for red dwarf stars since, for these stars, measuring Mercury-sized orbits works well enough: at least a sixth of red dwarf stars—and up to half—has an Earth-sized planet in the habitable zone. Not too shabby, red dwarfs!

Last year, astronomers announced the discovery of the spectacular TRAPPIST-1 7-planet system. Its central star is puny, just 8 percent the mass of our sun, 2,000 times fainter, and about the size of Jupiter. All seven planets are roughly Earth-sized, and they orbit extremely close to their star. The most exciting thing is, at least three (and perhaps up to four or five) live in the star’s habitable zone—if all stars had planetary systems like TRAPPIST-1, NEarth would be 3.

Another twist is that, in the TRAPPIST-1 system, life—if there is any—may naturally spread between planets: The compact orbital setup is well-suited for panspermia. If an asteroid or comet hit any of the potentially life-bearing TRAPPIST-1 planets, some of the debris would scatter to the six, raining down space-born seeds.