A good look at a night sky in a remote place like a desert can be jaw-dropping. It’s hard to imagine more stars could be packed even into so vast an expanse of sky. But the view from Earth — a planet orbiting a relatively isolated sun — is nothing compared to the sight of the heavens in what’s known as a star cluster, a celestial grouping that can contain hundreds or even thousands of stars in relatively close proximity. There, astronomers say, gleaming stars fill the celestial sphere like yellow, red, and bluish-white holiday lights, some nearly as big as the moon, and a few of them bright enough to read by. Even if life does exist elsewhere in the universe, however, it’s lately seemed unlikely that anyone ever gets to enjoy such a view. Though astronomers have expected to be find plenty of planets in star clusters, the fact is they’ve so far discovered only two — and neither around sunlike stars. Their only suspicion was that the powerful crosscurrents of gravity produced by so many stars so close together was disrupting the normal flow of planet-forming material.
Now, however, that theory has come in for rethinking. In a paper just published in Astrophysical Journal Letters, astronomers announced that they’ve found two planets orbiting stars very much like our sun in what’s known as the Beehive Cluster, a grouping of about 1,000 stars located approximately 550 light years from Earth in the constellation Cancer. The existence of the two worlds both expands the canvas for all astronomers hunting for exoplanets and broadens our understanding about how some planets form in the first place.
The Beehive cluster has long been interesting to astronomers for reasons that go beyond the alien worlds it may or may not contain. All star clusters are galactic benchmarks: their stellar inhabitants are generally born at about the same time from a single, immense cloud of material. Rather than leaving the nest after birth, the stars remain close to one another, bound together by the high gravity generated from so many bodies in one region. The mere fact that the stars were born at the same time, in one great litter, makes it easy to determine all of their ages — since they’re all essentially the same. In the case of the beehive, the stars are barely out of childhood — just 600 million years old, or a fraction of the 10 billion years most sun-like stars typically live.
Samuel Quinn, a graduate student in astronomy at Georgia State University and the report’s lead author, decided to prod the beehive and see what might be residing there, at the suggestion of Russel White, the project’s principal investigator and an assistant professor of astronomy at GSU. White knew from past studies that stars loaded with metals such as iron and nickel have an increased likelihood of being home to gas-giant planets like Jupiter. What’s more, the Beehive cluster is what’s known as an open cluster — a relatively sparse grouping of relatively few stars, compared to much more densely packed globular clusters; open-cluster stars tend to be high in just the iron that would be needed, as well as carbon and silicon, all of which serve as seeds for star formation.
Quinn and his colleagues went hunting for Beehive planets by analyzing incoming light from possible host stars, looking for a telltale wobble caused by the gravitational pull of an orbiting object. When a star sways, its light waves stretch and compress, shifting from the red to the blue ends of the light spectrum. By measuring that Doppler shift, researchers can retrace the star’s wobble into a circular or oval path that mirrors a planet’s orbit and provides the information to calculate its mass and distance. It’s a precision business: in some instances those light waves shrink and expand by as little as one part in 20 million.
In this case, such painstaking work paid off: The team spotted a pair of planets, inelegantly named Pr0201b and Pr0211b, both of which are what astronomers call hot Jupiters: uninhabitable gas giants, each in a tight orbit around its own host star. Pr0201b is half the mass of Jupiter and whips around its sun-sized star in four days. Pr0211b is nearly twice Jupiter’s mass and makes a similar trip in just two days. Our Jupiter, by contrast, takes a sluggish 12 years to travel around our sun.
Tight orbits and high speed also mean high temperatures — about 1,700ºF (927ºC), simply because the planets are located so close to the solar fires. Both Pr0201b and Pr0211b are about four million miles (6.4 million km) away from their parent star compared to the 93 million miles (150 km) between Earth and the sun. If Earth were in such a cozy orbit, the sun would be a whopping 20 times larger in our sky.
The two new planets didn’t take long to arrive at their arms-length distance from their stars. Jupiter-like worlds are thought to form far away from stars, remaining cold and distant before heating up as they migrate inward through a solar system. But no one knows whether it’s the gravitational power of the solar system’s gas disk — which dissipates after several million years — that pulls them inward, or if it’s a much slower, hundreds-of-millions of years’ worth of gravitational tugging from other planets that does it. Two planets orbiting so close to such young stars suggests the shorter path. “This really opens up the field to learning how planets evolve over time,” says Quinn.
More tantalizingly than how the new bodies formed, the fact that star clusters are home to any planets at all — even if they’re scorchingly uninhabitable ones — suggests the possibility that Earth-like planets might turn up in clusters too. There may be nothing alive on Pr0201b and Pr0211b to take in the breathtaking view, but on other Beehive worlds orbiting other Beehive stars, there’s no telling what may be gazing upward up at night.
(Photos: Window on Infinity: Pictures From Space)