test 1 Astronomy - May 2018 USA | Page 24

N NOT LONG AGO, THE ORIGIN OF THE SOLAR SYSTEM SEEMED AN ORDERLY AFFAIR. A cloud of dust and gas coalesced around a nascent Sun. Close to the newborn star, volatile substances that vaporize at relatively low temperatures, like water and other ices, turned into gas and left behind the rocky planets. Farther out, gas giants formed beyond the “snow line,” where these volatiles condensed. Earth became the most massive rocky planet by happenstance, and Jupiter the accidental ruler among the giants. Shortly after the planets had formed, a 300 million-year pummeling known as the Late Heavy Bombardment gave us the pockmarked faces of the Moon, Mercury, and other airless worlds. When that ended some 3.8 billion years ago, all was quiet save for the odd comet or asteroid that might sail through the inner system and hit Earth — like the one that killed the dinosaurs 66 million years ago. Planetary scientists thought other solar systems would look roughly like ours; after all, the Copernican principle tells us we’re not spe- cial, and our system likely is average. Astronomers now know that these for- mation scenarios are spectacularly wrong. Planets probably bounced around our Sun like billiard balls before settling into their current stately dance. And two decades’ worth of observing planets around other stars shows that our solar system actually is quite the oddball. But that leaves the ques- tion: How did it get this way? It’s not you, it’s me Exoplanets were among the first signs that something was amiss with our solar sys- tem. Much of this evidence came from NASA’s Kepler spacecraft, which fixes on stars and looks for planets transiting across their faces. Such transits reveal a planet’s size and period. When Kepler launched in 2009, the num- ber of confirmed exoplanet systems was in the low hun- dreds. Kepler has since pushed that number into the thousands. If you plot the periods of confirmed planets against their radii, most fall between one and 10 times Earth’s radius with peri- ods of 120 days or less. Multiplanet systems tend to have lots of “super-Earths” — rocky planets bigger than our own, but no more than about 17 Earth masses (approximately the mass of Neptune). Super-Earths are the most common species among the more than 3,700 exoplanets discovered so far — and yet, none exist in our solar system. Another type of planet that shows up consistently, though much less often, is the hot Jupiter. These are gas giants that lie close to their stars. When astronomers dis- covered 51 Pegasi b — the first planet found around a Sun-like star — in 1995, it turned out to be a hot Jupiter. Now known as Dimidium, the planet is half Jupiter’s mass and circles its star in 4.2 days at an average distance of 4.8 million miles (7.8 million kilometers). In contrast, Mercury takes 88 days to orbit the Sun and gets no closer than 29 million miles (46 million km). Then-current models had no sensible way to make a Jupiter-sized planet so close to its star. Dimidium must have moved. “The standard model, the one everyone had in their heads, was that the inner disk was dry and volatile-poor, with planets like Mercury, which is iron rich and dense. In the outer system, you’d get colder, and have lower densi- ties,” says David Minton of Purdue University, who studies planet bombard- ments and formation. “The Kepler mission turned that on its head.” It’s possible that detection bias has played a role in these results. Planet-finding methods favor worlds with short periods and high Planets probably bounced around our Sun like billiard balls before settling into their current stately dance. 24 A ST R O N O M Y • MAY 2018 Jupiter largely controlled the solar system’s early evolution. Its inward migration limited the amount of material available to form terrestrial planets. Its subsequent outward migration moved the giant worlds. NASA/JPL/SSI masses. Scientists have discovered most exoplanets via Kepler’s transit method, or by using spectroscopy to observe small changes in a star’s velocity along the line of sight. But an alien astronomer looking at our solar system with a Kepler-like tele- scope would have to wait at least 24 years to see two Jupiter orbits, the minimum needed to confirm its existence. Still, even accounting for this bias, the existence of super-Earths and hot Jupiters showed that models of planet formation at the very least needed work. Missing mass and Mars Besides the exoplanets, other clues showed up closer to home. When planetary scien- tists tried building solar systems with increasingly sophisticated computer simulations, they kept getting a Mars that was five to 10 times as massive as the one we have. In addition, models of the disk from which the planets formed assume it had a relatively uniform density. Astronomer Alessandro Morbidelli at the Observatoire de la Côte d’Azur in Nice, France, has made a career of modeling planet dynam- ics. He says a uniform density would point to at least an Earth’s mass of material in the current asteroid belt. But the actual mass of the whole thing is only a few