- 18 new planets found orbiting “retired” stars
- 50 per cent increase in this class of planets
- Competing ideas for how giant planets form
DISCOVERIES OF NEW PLANETS just keep coming and coming. Take, for instance, the 18 recently found by a team of astronomers led by scientists at the California Institute of Technology (Caltech).
“It’s the largest single announcement of planets in orbit around stars more massive than the Sun, aside from the discoveries made by the Kepler [space telescope] mission,” says John Johnson, assistant professor of astronomy at Caltech.
Using the Keck Observatory in Hawaii—with follow-up observations using the McDonald and Fairborn Observatories in Texas and Arizona, respectively—the researchers surveyed about 300 stars.
They focused on “retired” A-type stars that are more than 1.5 times more massive than the Sun. These stars are just past the main stage of their life—hence, “retired”—and are now puffing up into what’s called sub-giant stars.
The astronomers searched for stars of this type that wobble, which could be caused by the gravitational tug of an orbiting planet.
By searching the stars’ spectra for Doppler shifts—the lengthening and contracting of wavelengths due to motion away from and toward the observer—the team detected 18 planets with masses similar to Jupiter’s.
This marks a 50 percent increase in the number of known planets orbiting massive stars.
Competing planet formation concepts
The researchers say the findings also lend further support to the idea that planets grow from seed particles that accumulate gas and dust in a cloud surrounding a newborn star.
In this concept, tiny particles start to clump together, eventually snowballing into a planet. If this is correct, the characteristics of the resulting planetary system—such as the number and size of the planets, or their orbital shapes—will depend on the mass of the star.
In another theory, planets form when large amounts of gas and dust in the cloud spontaneously collapse into big, dense clumps that then become planets. But in this picture, it turns out that the mass of the host star doesn’t affect the kinds of planets that are produced.
So far, as the number of discovered planets has grown, astronomers are finding that stellar mass does seem to be important in determining the prevalence of giant planets. The newly discovered planets further support this pattern—and are therefore consistent with the first theory, the one stating that planets are born from seed particles.
Nature vs nurture?
There’s another interesting twist, Johnson adds: “Not only do we find Jupiter-like planets more frequently around massive stars, but we find them in wider orbits.” If you took a sample of 18 planets around Sun-like stars, he explains, half of them would orbit close to their stars. But in the cases of the new planets, all are farther away.
In systems with Sun-like stars, gas giants like Jupiter acquire close orbits when they migrate toward their stars. According to theories of planet formation, gas giants could only have formed far from their stars, where it’s cold enough for their constituent gases and ices to exist.
So for gas giants to orbit nearer to their stars, gravitational interactions have to have taken place to pull the planets in. Then, some other mechanism—perhaps the star’s magnetic field—has to kick in to stop them from spiralling into a fiery death.
The question, Johnson says, is why this doesn’t seem to happen with so-called “hot Jupiters” orbiting massive stars, and whether that dearth is due to nature or nurture.
In the nature explanation, Jupiter-like planets that orbit massive stars just wouldn’t ever migrate inward. In the nurture interpretation, the planets would move in, but there would be nothing to prevent them from plunging into their stars. Or perhaps the stars evolve and swell up, consuming their planets.
Adapted from information issued by Caltech. Images courtesy Rick Peterson / W.M. Keck Observatory / Gbacon / STScI / AVL.
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