The twin telescopes at Keck Observatory in Hawaii. The astronomers used Keck to discover 18 new Jupiter-like planets orbiting massive stars. Photo: Rick Peterson/W.M. Keck Observatory
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
aside from the discoveries made by the Kepler mission,” says John Johnson,
assistant professor of astronomy at Caltech and the first author on the team’s
paper, which was published in The
Astrophysical Journal Supplement Series. The Kepler mission is a
space telescope that has so far identified more than 1,200 possible planets,
though the majority of those have not yet been confirmed.
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 those
dubbed “retired” A-type stars that are more than one and a half 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 a subgiant star.
To look for planets, 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 wobbly stars’ spectra for Doppler shifts—the
lengthening and contracting of wavelengths due to motion away from and toward
the observer—the team found 18 planets with masses similar to Jupiter’s.
This new bounty marks a 50% increase in the number of
known planets orbiting massive stars and, according to Johnson, provides an
invaluable population of planetary systems for understanding how planets—and
our own solar system—might form. The researchers say that the findings also
lend further support to the theory that planets grow from seed particles that
accumulate gas and dust in a disk surrounding a newborn star.
According to this theory, tiny particles start to clump
together, eventually snowballing into a planet. If this is the true sequence of
events, 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. For instance, a more massive star would mean a bigger disk, which
in turn would mean more material to produce a greater number of giant planets.
In another theory, planets form when large amounts of gas
and dust in the disk spontaneously collapse into big, dense clumps that then
become planets. But in this picture, it turns out that the mass of the 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.
“It’s nice to see all these converging lines of
evidence pointing toward one class of formation mechanisms,” Johnson says.
There’s another interesting twist, he 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
sunlike stars, he explains, half of them would orbit close to their stars. But
in the cases of the new planets, all are farther away, at least 0.7
astronomical units from their stars. (One astronomical unit, or AU, is the
distance from Earth to the sun.)
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, certain gravitational
interactions have to take place to pull these planets in. Then, some other
mechanism—perhaps the star’s magnetic field—has to kick in to stop them from
spiraling 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. Which is the case? According to Johnson,
subgiants like the A stars they were looking at in this paper simply don’t
expand enough to gobble up hot Jupiters. So unless A stars have some unique
characteristic that would prevent them from stopping migrating planets—such as
a lack of a magnetic field early in their lives—it looks like the nature
explanation is the more plausible one.
The new batch of planets have yet another interesting
pattern: their orbits are mainly circular, while planets around sunlike stars
span a wide range of circular to elliptical paths. Johnson says he’s now trying
to find an explanation.
For Johnson, these discoveries
have been a long time coming. This latest find, for instance, comes from an
astronomical survey that he started while a graduate student; because these
planets have wide orbits, they can take a couple of years to make a single
revolution, meaning that it can also take quite a few years before their stars’
periodic wobbles become apparent to an observer. Now, the discoveries are finally
coming in. “I liken it to a garden—you plant the seeds and put a lot of
work into it,” he says. “Then, a decade in, your garden is big and
flourishing. That’s where I am right now. My garden is full of these big,
bright, juicy tomatoes—these Jupiter-sized planets.”