physics + math
size of a fist held at arm’s length—extremely large for an astronomical telescope. When the weather deteriorates or sunrise approaches, the domes close, the telescopes stop tracking their targets, and researchers can access their data through the Internet. The system is entirely automated and built using open-source software. HATnet now monitors more than 100,000 stars every year, for a lifetime total of 700,000; HATsouth monitors 400,000 and is outpacing its predecessor in its rate of discoveries. These automated observatories were first envisioned by Bakos in 1998. At the time, he was a year undergrad in Hungary, and planned to use them not for planet hunting, but for astronomers to point telescopes at gamma-ray bursts as soon as they are discovered. “It seemed to me like a suboptimal procedure that you get a phone call from a guy in Europe, who was woken up by his cellphone, then he calls me up, then I answer the phone, then I put down the phone, then I stop my observation…just very suboptimal in the age of Ethernet and robots. The telescope should just respond to a trigger and point to the right position,” said Bakos. His focus quickly shifted from gamma rays bursts to monitoring the sky for variable stars, and by 2000, Bakos had built a working prototype. Meanwhile, he moved to Harvard’s Center for Astrophysics at a time when astrono-
mers there were extremely enthusiastic about looking for planets, convincing him to dedicate his invention to planet hunting. HAT can reliably measure 1% dips in a star’s brightness, and has discovered 50 planets to date. Most of these planets are hot Jupiters—enormous planets much heavier than Jupiter, and orbiting so close to their stars that they complete one orbit in days. As with any transit survey, large planets with short orbital periods are much easier to detect than Earth-like planets that orbit once every 365 days. In reality, smaller planets like the Earth, which orbit their stars at large distances, are overwhelmingly more common, a trend that holds to the limits of current human technology. HAT has the ability to detect super-Earths, planets a few times Earth’s mass, or 100 times less massive than Jupiter. HAT can even detect a super-Earth in the Habitable Zone of a small star. “We made simulations and calculations and yes, there is a possibility, but it would be extremely surprising,” Bakos claimed. Since its inception, HAT has detected a wide variety of planetary systems. HAT-P2b, the first supermassive Jupiter for which scientists measured an accurate radius, is a gas giant the size of Jupiter but 9 times its mass. Its orbit is so eccentric that it comes as close as 7.4 million and as far as 25 million km from its star. HAT-P-32b
is 8 times Jupiter’s volume but around the same as Jupiter mass. Its density is so low that scientists are still puzzled as to how it could have formed. HAT-P-11b, only 8% the mass and 42% the radius of Jupiter, was the first transiting “hot Neptune” discovered from the ground. Finally, the star HAT-P-13 has 2 planets: an inner planet with a 2.9day orbit and 0.8 Jupiter masses, as well as an enormous outer planet with 16 Jupiter masses and a 446-day orbit. This was the first system where a transiting exoplanet was confirmed to be accompanied by a second planet. HATnet and HATsouth are just two of many ongoing exoplanet search projects. Super-WASP employs a similar methodology, and has discovered 80 planets. The Kepler spacecraft has discovered 2700 planet candidates since its launch in 2009. It has allowed scientists to estimate that our galaxy contains 2 billion habitable Earth-like planets around Sun-like stars, of which Kepler is expected to discover at least one. Recently, spectroscopy of large extrasolar planets has become possible, allowing astronomers to determine the chemical composition of their atmospheres. In several years, the next generation of telescopes will be capable of analyzing the atmospheres of Earthlike planets to search for signs of alien life.
***HAT-P-2b shown in comparison to Jupiter
***HAT-P-11b shown in comparison to Neptune
Radius: approximately equal to Jupiter Mass: 9x that of Jupiter Characterized by an eccentric orbit
Radius: 42% that of Jupiter Mass: 8% that of Jupiter Characterized by a highly inclined orbit Known as “Hot Neptune“ (an extrasolar planet in an orbit close to its sun, with a mass similar to that of Uranus or Neptune) 23