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18 new giant planets found

Keck Observatory

The 18 new planets were detected using the Keck Observatory in Hawaii.

  • 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.

Artist's impression of an exoplanet

There are competing ideas about how giant planets form.

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.

Artist's impression of an exoplanet

Something stops giant planets from spiralling into their host stars.

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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Exoplanets have unearthly light shows

Artist's concept of a 'hot Jupiter' planet

Artist's concept of a 'hot Jupiter' planet with two moons and a Sun-like star. The planet is cloaked in brilliant aurorae—100-1000 times brighter than Earth's—triggered by stellar storms.

BEINGS LIVING ON ‘HOT JUPITER’ PLANETS could be treated to a dazzling nightly light show a thousand times better than Earth’s Northern and Southern Lights.

Earth’s aurorae provide a dazzling light show to people living in the polar regions, with shimmering curtains of green and red undulating across the sky like a living creature.

But new research shows that aurorae on ‘hot Jupiter’ planets closely orbiting distant stars could be 100-1000 times brighter than Earthly aurorae. They also would ripple from equator to poles (due to the planet’s proximity to any stellar eruptions), treating the entire planet to an otherworldly spectacle.

“I’d love to get a reservation on a tour to see these aurorae!” said lead author Ofer Cohen, a SHINE-NSF postdoctoral fellow at the Harvard-Smithsonian Centre for Astrophysics (CfA).

Gigantic stellar blasts

Earth’s aurorae are created when energetic particles from the Sun slam into our planet’s magnetic field. The field guides the particles toward the poles, where they smash into Earth’s atmosphere, causing air molecules to glow like a neon sign.

The same process can occur on planets orbiting distant stars, known as exoplanets.

Aurora Australis seen from the International Space Station

The Southern Lights or Aurora Australis seen from the International Space Station on July 14, 2011.

Particularly strong aurorae result when Earth is hit by a coronal mass ejection or CME—a gigantic blast that sends billions of tonnes of solar plasma (electrically charged, hot gas) into the Solar System.

A CME can disrupt Earth’s magnetosphere—the bubble of space protected by Earth’s magnetic field—causing a geomagnetic storm. In 1989, a CME hit Earth with such force that the resulting geomagnetic storm blacked out huge regions of Quebec.

Planets in the firing line

Cohen and his colleagues used computer models to study what would happen if a gas giant planet in a close orbit, just a few million kilometres from its star, were hit by a stellar eruption.

He wanted to learn the effect on the exoplanet’s atmosphere and surrounding magnetosphere.

The alien gas giant would be subjected to extreme forces. In our Solar System, a CME spreads out as it travels through space, so it’s more diffuse once it reaches us.

Aurora planet animation

In this animation, stunning aurorae (pink/purple) ripple around a 'hot Jupiter' planet.

A ‘hot Jupiter’ would feel a stronger and more focused blast, like the difference between being 100 kilometres from an erupting volcano or one kilometre away.

“The impact to the exoplanet would be completely different than what we see in our Solar System, and much more violent,” said co-author Vinay Kashyap of CfA.

Yet despite the extreme forces involved, the exoplanet’s magnetic field would shield its atmosphere from erosion.

Too close for comfort

This work has important implications for the habitability of rocky worlds orbiting distant stars. Since red dwarf stars are the most common stars in our galaxy, astronomers have suggested focusing on them in the search for Earth-like worlds.

However since a red dwarf is cooler than our Sun, a rocky planet would have to orbit very close to the star to be warm enough for water to exist as a liquid. There, it would be subjected to the sort of violent stellar eruptions Cohen and his colleagues studied.

Their future work will examine whether rocky worlds could shield themselves from such eruptions.

Adapted from information issued by the Harvard-Smithsonian Centre for Astrophysics. Images courtesy David A. Aguilar (CfA). Animation produced by Hyperspective Studios.

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Qatar-led team finds its first alien world

Artist's impression of Qatar-1b

Artist's impression of the newly discovered alien world Qatar-1b. The planet is a gas giant 20 percent larger than Jupiter in diameter and 10 percent more massive. It circles its star once every 1.4 days, meaning that its "year" is just 34 Earth hours long.

  • Planet found orbiting star 550 light-years from Earth
  • Discovered by team led by Qatari scientist
  • International effort involving Qatar, USA and UK

A QATARI ASTRONOMER has teamed with scientists at the Harvard-Smithsonian Centre for Astrophysics (CfA) in the USA and other institutions to discover a new alien world.

This “hot Jupiter” adds to the growing list of “exoplanets” orbiting distant stars. Its discovery demonstrates the power of science to transcend political boundaries and increase ties between nations.

The planet, now called Qatar-1b, orbits an orange Type K star 550 light-years away.

Qatar-1b is a gas giant 20 percent larger than Jupiter in diameter and 10 percent more massive. It belongs to the “hot Jupiter” family because it orbits just 3.5 million kilometres from its star—only six times the radius of the star—which means it is very hot.

The planet roasts at a temperature of around 1,000 degrees Celsius.

Qatar-1b circles its star once every 1.4 days, meaning that its “year” is just 34 Earth hours long. It’s expected to be tidally locked with the star, so that one side of the planet always faces the star.

As a result, the planet spins on its axis once every 34 hours—three times slower than Jupiter, which rotates once in 10 hours.

International teamwork

“The discovery of Qatar-1b is a great achievement—one that further demonstrates Qatar’s commitment to becoming a leader in innovative science and research,” said Dr Khalid Al Subai, leader of the Qatar exoplanet survey and a research director of the Qatar Foundation for Education, Science and Community Development.

The Qatar exoplanet survey hunts for stars that “wink,” dimming slightly every time an orbiting planet creates a “mini-eclipse” by crossing in front of the star as seen from Earth.

Transit searches like this must sift through thousands of stars to find the small fraction with detectable planets. The complex observations and analysis create perfect opportunities for teamwork.

To find the new world, Qatar’s wide-angle cameras (located in New Mexico) took images of the sky every clear night beginning in early 2010. The photographs then were transmitted to the UK for analysis by collaborating astronomers at St. Andrews and Leicester Universities and Qatar. That analysis narrowed the field to a few hundred candidate stars.

The Harvard-Smithsonian team, with Dr Al Subai, followed up on the most promising candidates, making spectroscopic observations with the 1.5-metre-diameter telescope at the Smithsonian’s Whipple Observatory in Arizona. They also measured the stars’ dimming more accurately with Whipple’s 1.2-metre telescope.

Adapted from information issued by Harvard-Smithsonian Centre for Astrophysics / David A. Aguilar (CfA).

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Today’s recipe: Roasted planet

Artist's illustration of the gas giant planet HD 209458b

Artist's illustration of the gas giant planet HD 209458b, as seen from a hypothetical nearby planet. The planet orbits so close to its star that its heated atmosphere is escaping into space in a comet-like tail.

  • Super-hot planet probably has a comet-like tail
  • 100 times closer to its star than Jupiter is to the Sun
  • Hubble has studied the planet’s atmosphere

As if the debate over what is and what is not a planet hasn’t gotten confusing enough, Hubble Space Telescope astronomers have now confirmed the existence of a tortured, baked object that could be called a “cometary planet.”

The gas giant planet, dubbed HD 209458b, is orbiting so close to its star that its heated atmosphere is escaping into space.

Now, observations by the new Cosmic Origins Spectrograph (COS) aboard Hubble suggest that powerful “winds” from the star are sweeping the cast-off material behind the scorched planet and shaping it into a comet-like tail.

“Since 2003 scientists have theorised that the lost mass is being pushed back into a tail and have even calculated what the tail looks like,” says astronomer Jeffrey Linsky of the University of Colorado in Boulder, leader of the COS study.

“We think we have the best observational evidence to support that theory. We have measured gas coming off the planet at specific speeds, some coming toward Earth.”

“The most likely interpretation is that we have measured the velocity of material in a tail.”

Hubble's Cosmic Origins Spectrograph

Hubble's Cosmic Origins Spectrograph can split starlight into its spectrum, enabling astronomers to detect the signatures of chemical elements.

Atmosphere escaping into space

HD 209458b has a mass slightly less than that of Jupiter, but it orbits 100 times closer to its star than Jupiter does. This means the roasted planet zips around in a mere 3.5 days. (In contrast, our Solar System’s speedster, Mercury, orbits the Sun in a leisurely 88 days.)

The planet is one of the most intensely scrutinised exoplanets (ones in other star systems) because it is one of the few known alien worlds that can be seen passing in front of, or transiting, its star. The transit causes the star’s light to dim slightly.

In fact, the gas giant is the first alien world discovered to transit its parent star. It orbits the star HD 209458, located 153 light-years from Earth.

Linsky and his team used COS to analyse the planet’s atmosphere during transiting events. During a transit, astronomers can study the structure and chemical makeup of a planet’s atmosphere by sampling the starlight that passes through it.

The dip in starlight due to the planet’s passage, excluding the planet’s atmosphere, is very small, only 1.5 percent. When the atmosphere is added, the dip jumps to 8 percent, indicating a bloated atmosphere.

COS detected the heavy elements carbon and silicon in the planet’s super-hot (1,100-degree-Celsius) atmosphere. This detection reveals that the parent star is heating the entire atmosphere, dredging up the heavier elements and allowing them to escape the planet.

The COS data also showed that the material leaving the planet was not all travelling at the same velocity.

“We found gas escaping at high velocities, with a large amount of this gas flowing toward us at [35,000 kilometres per hour],” Linsky explains.

“This large gas flow is likely gas swept up by the stellar wind to form the comet-like tail trailing the planet.”

Artist's illustration of the gas giant planet WASP-12b

Artist's illustration of the gas giant planet WASP-12b, whose atmosphere is spilling onto its parent star.

The power of Hubble

Hubble’s newest spectrograph, with its ability to probe a planet’s chemistry at ultraviolet wavelengths that are not accessible to ground-based telescopes, is proving to be an important instrument for probing the atmospheres of “hot Jupiters” like HD 209458b.

Astronomers have also used COS to sample the atmosphere of another baked planet, WASP-12b, whose puffy atmosphere is spilling onto its star.

Another Hubble instrument, the Space Telescope Imaging Spectrograph (STIS), studied HD 209458b in 2003. The STIS data showed an active, evaporating atmosphere, and a comet-tail-like structure was suggested as a possibility.

But STIS wasn’t able to obtain the spectroscopic detail necessary to show an earthward-moving component of the gas during transits.

Because of COS’s unique combination of very high ultraviolet sensitivity and good spectral resolution, the earthward moving component of the gas—the tail—could be directly detected for the first time.

Although this “extreme” planet is getting roasted by its star, it won’t be destroyed anytime soon. “It will take about a trillion years for the planet to evaporate,” Linsky says.

Adapted from information issued by the Space Telescope Science Institute / NASA, ESA, and G. Bacon (STScI).

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