Mirror Earth by Michael D. Lemonick

don’t know anything else. But the HD catalog is more widely used.” And then there are the Hipparcos and the Tycho catalogs, gathered by the European Space Agency’s Hipparcos satellite in the 1990s (the satellite’s mission was to map the positions of millions of stars; the Hipparcos catalog has very high precision, the Tycho catalog is based on somewhat less careful measurements with the same satellite).
    It doesn’t end there. Many stars are also identified by the constellation they’re part of. Alpha Centauri is the brighteststar in the southern constellation Centaurus (
alpha
is the first letter in the Greek alphabet). If a constellation has more than twenty-two stars, you run out of Greek letters, so you go to numbers. The constellation Pegasus has a star known as Alpha Pegasi, and it also has one called 1 Pegasi. Then there are stars with given names—Vega, Sirius, Aldebaran, Betelgeuse, Polaris, Arcturus, and more. These stars are so bright and prominent in the night sky that ancient Greek and Arab sky watchers thought of them as old friends, with distinct personalities. Finally, when astronomers are conducting an organized search, they’ll sometimes create their own catalogs—the Kepler catalog, the Wide Angle Search for Planets (WASP) catalog, the Hungarian Automated Telescope (HAT) catalog, and so on. According to one astronomer I spoke with, this is partly to make sure the credit for a planet’s discovery goes prominently to the search team.
    The one catalog astronomers don’t pay attention to is the one compiled by the International Star Registry. This is the company that lets you “name a star after someone.” The ads say, quite truthfully, that the names will be recorded in book form in the Library of Congress. But that’s true for any book, whether it’s an erudite volume of history or a trashy romance novel. The company makes clear on its website that astronomers don’t consult the book.
    Even when you’re talking about stars identified by their numbers in a single catalog—in this case, the Henry Draper catalog—the names begin to swim before your eyes. Unless you’re a planet hunter, that is. Then they’re as distinctive as the names of your children. “Exactly!” said Debra Fischer, aYale astronomer, when I proposed this analogy. “You don’t know HD 209458? These names are burned into my memory. Someday I will have Alzheimer’s, but I will remember these stars.” For the record, Fischer tends to rely on the Hipparcos catalog, because it has more than two million stars; because it notes their positions and distance from Earth with exquisite accuracy; and because all of the information is online, making it easy to access.
    So Latham was working on HD 114762, a star in the Henry Draper catalog. His calculations suggested a wobble with a period of about eighty-four days, about the same as Mercury’s. The magnitude of the wobble suggested an object with a mass about eleven times that of Jupiter—or rather, that was the
minimum
possible mass. This was a crucial point. Since the object pulling on the star was itself invisible, Latham couldn’t be certain its orbit was truly edge-on. If it was, the mass was eleven times that of Jupiter. If the orbit had been at right angles to Latham’s telescope, you wouldn’t see any motion at all toward or away from the telescope; it would all be side to side. But if the orbital plane was tilted somewhere in between those extremes, you’d see something between the full effect and zero. Any motion toward or away from the telescope would reflect
part
of the tugging. A much bigger object, pulling at an angle, could mimic an eleven-Jupiter-mass body seen directly edge-on.
    Latham sent an e-mail to Mazeh, with a copy to Michel Mayor, a Swiss astronomer who was also looking for wobbling stars. Mayor wasn’t looking for planets either; he was looking