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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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Earth-sized Planets could be everywhere

Artist's impression of Earth-like planets

  • Galaxy could have more than 46 billion Earth-size planets
  • Small planets outnumber larger ones
  • Findings challenge theories of planet formation

Nearly one in four stars similar to the Sun may host planets as small as Earth, according to a new study funded by NASA and the University of California.

The study is the most extensive and sensitive planetary census of its kind. Astronomers used the W.M. Keck Observatory in Hawaii for five years to search 166 Sun-like stars near our Solar System for planets of various sizes, ranging from three to 1,000 times the mass of Earth.

All of the planets in the study orbit close to their stars. The results show more small planets than large ones, indicating small planets are more prevalent in our Milky Way galaxy.

“We studied planets of many masses—like counting boulders, rocks and pebbles in a canyon—and found more rocks than boulders, and more pebbles than rocks. Our ground-based technology can’t see the grains of sand, the Earth-size planets, but we can estimate their numbers,” said Andrew Howard of the University of California, Berkeley, lead author of the new study.

W.M. Keck Observatory

The W.M. Keck Observatory, atop Mauna Kea in Hawaii, was used to survey 166 Sun-like stars for planets of different sizes.

“Earth-size planets in our galaxy are like grains of sand sprinkled on a beach—they are everywhere.”

The study appears in the October 29 issue of the journal Science.

The research provides a tantalising clue that potentially habitable planets could also be common. These hypothesised Earth-size worlds would orbit farther away from their stars, where conditions could be favourable for life.

NASA’s Kepler spacecraft is also surveying Sun-like stars for planets and is expected to find the first true Earth-like planets in the next few years.

Small planets outnumber large ones

Howard and his planet-hunting team, which includes principal investigator Geoff Marcy, also of the University of California, Berkeley, looked for planets within 80-light-years of Earth, using the radial velocity, or “wobble,” technique.

They measured the numbers of planets falling into five groups, ranging from 1,000 times the mass of Earth, or about three times the mass of Jupiter, down to three times the mass of Earth.

The search was confined to planets orbiting close to their stars—within 0.25 astronomical units, or a quarter of the distance between our Sun and Earth.

A distinct trend jumped out of the data—smaller planets outnumber larger ones. Only 1.6 percent of stars were found to host giant planets orbiting close in. That includes the three highest-mass planet groups in the study, or planets comparable to Saturn and Jupiter.

About 6.5 percent of stars were found to have intermediate-mass planets, with 10 to 30 times the mass of Earth—planets the size of Neptune and Uranus. And 11.8 percent had the so-called “super-Earths,” weighing in at only three to 10 times the mass of Earth.

“During planet formation, small bodies similar to asteroids and comets stick together, eventually growing to Earth-size and beyond. Not all of the planets grow large enough to become giant planets like Saturn and Jupiter,” Howard said. “It’s natural for lots of these building blocks, the small planets, to be left over in this process.”

Diagram indicating numbers of different sized planets in the Galaxy

A new survey, funded by NASA and the University of California, reveals that small planets are more common than large ones.

Life in the hot zone

The astronomers extrapolated from these survey data to estimate that 23 percent of Sun-like stars in our galaxy host even smaller planets, the Earth-sized ones, orbiting in the hot zone close to a star.

“This is the statistical fruit of years of planet-hunting work,” said Marcy. “The data tell us that our galaxy, with its roughly 200 billion stars, has at least 46 billion Earth-size planets, and that’s not counting Earth-size planets that orbit farther away from their stars in the habitable zone.”

The findings challenge a key prediction of some theories of planet formation.

Models predict a planet “desert” in the hot-zone region close to stars, or a drop in the numbers of planets with masses less than 30 times that of Earth. This desert was thought to arise because most planets form in the cool, outer region of solar systems, and only the giant planets were thought to migrate in significant numbers into the hot inner region.

The new study finds a surplus of close-in, small planets where theories had predicted a scarcity.

“We are at the cusp of understanding the frequency of Earth-sized planets among planetary systems in the solar neighbourhood,” said Mario R. Perez, Keck program scientist at NASA Headquarters in Washington.

“This work is part of a key NASA science program and will stimulate new theories to explain the significance and impact of these findings.”

Adapted from information issued by NASA / JPL-Caltech / UC Berkeley / WMKO.

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“Goldilocks” planet discovered

Artist's impression of a planet orbiting Gliese 581

An artist's impression of a planet orbiting the red dwarf star Gliese 581. Astronomers have just discovered one in the star's "habitable zone", where temperatures could be right for liquid water to exist.

  • Gliese 581g orbits in its star’s “Goldilocks” zone
  • Temperature okay for liquid water – not too hot, not too cold
  • If confirmed, will be first potentially habitable planet yet found

A team of planet-hunting astronomers, utilising the HIRES spectrometer on the W.M. Keck Observatory’s Keck I Telescope, has announced the discovery of an Earth-sized planet orbiting a nearby red dwarf star.

The new planet, known as Gliese 581g, is at a distance that places it squarely in the middle of the star’s “habitable zone” where liquid water could exist on the planet’s surface.

If confirmed, this would be the most Earth-like exoplanet and the first bona fide potentially habitable one yet discovered.

To astronomers, a “potentially habitable” planet is one that could sustain life—even the simplest of life—and not necessarily one that humans would consider a nice place to live. Habitability depends on many factors, but liquid water and an atmosphere are among the most important.

The discovery by the team, led by astronomers at the University of California, Santa Cruz, and the Carnegie Institution of Washington DC, is based on 11 years of observations made at the Keck Observatory atop Mauna Kea mountain on the Big Island of Hawaii.

“Our findings offer a very compelling case for a potentially habitable planet,” said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. “The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common.”

Diagram showing a star's habitable zone

Earth is in our Solar System's habitable or "Goldilocks" zone (blue band) where the temperature is neither too hot nor too cold for liquid water to exist on a rocky planet's surface. For hotter (whiter) or cooler (red) stars (shown at left), the zone is at a different distance.

“Advanced techniques combined with old-fashioned ground-based telescopes continue to lead the exoplanet revolution,” added Paul Butler of the Carnegie Institution.

“Our ability to find potentially habitable worlds is now limited only by our telescope time.”

Vogt and Butler lead the Lick-Carnegie Exoplanet Survey. The team’s new findings are reported in a paper to be published in the Astrophysical Journal.

Planet of perpetual night and day

The astronomers have deduced that the planet is tidally locked to the star, meaning that one side is always facing the star and basking in perpetual daylight, while the side facing away from the star is in perpetual darkness.

One effect of this is to stabilise the planet’s surface climates, according to Vogt. The most habitable zone on the planet’s surface would be the line between shadow and light (known as the “terminator”), with surface temperatures decreasing toward the dark side and increasing toward the light side.

“Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude,” Vogt said.

The researchers estimate that the average surface temperature of the planet is between -31 to -12 degrees Celsius. Actual temperatures would range from blazing hot on the side facing the star to freezing cold on the dark side.

Artist's impression of planets orbiting Gliese 581

It is now thought there are six planets circling the star Gliese 581, making it the most Solar System-like place discovered so far in the cosmos.

If Gliese 581g has a rocky composition similar to the Earth’s, its diameter would be about 1.2 to 1.4 times that of the Earth. The surface gravity would be about the same or slightly higher than Earth’s, so that a person could easily walk upright on the planet, Vogt said.

In fact, the scientists have reported the discovery of not one but two new planets circling Gliese 581. This brings to six the number of known planets around this star, the most yet discovered in a planetary system other than our own solar system.

Like our Solar System, the planets of Gliese 581 have nearly circular orbits. Gliese 581g has a mass 3 to 4 times that of the Earth and an orbital period of just under 37 days. Its mass indicates that it is probably a rocky planet with a definite surface and that it has enough gravity to hold on to an atmosphere, according to Vogt.

A difficult discovery

Although the planets themselves can’t be seen, the effect of their gravitational pull on their parent star can be measured. It shows up as a slight movement, or radial velocity change, in the star.

Multiple planets induce complex wobbles in the star’s motion, and astronomers use sophisticated analyses to distinguish the effects of the planets and determine their orbits and masses.

“It’s really hard to detect a planet like this,” Vogt said. “Every time we measure the radial velocity, that’s an evening on the telescope, and it took more than 200 observations with a precision of about 1.6 meters per second to detect this planet.”

W.M. Keck Observatory

Domes of the twin giant telescopes of the W.M. Keck Observatory on Mauna Kea mountain on the Big Island of Hawaii.

To get that many radial velocity measurements (238 in total), Vogt’s team combined their HIRES observations with published data from another group led by the Geneva Observatory (HARPS, the High Accuracy Radial velocity Planetary Search project).

In addition to the radial velocity observations, co-authors Gregory Henry and Michael Williamson of Tennessee State University made precise night-to-night brightness measurements of the star with one of Tennessee State University’s robotic telescopes.

“Our brightness measurements verify that the radial velocity variations are caused by the new orbiting planet and not by any process within the star itself,” Henry said.

How many habitable planets are out there?

Given the relatively small number of stars that have been carefully monitored by planet hunters, this discovery has come surprisingly soon.

“If these are rare, we shouldn’t have found one so quickly and so nearby,” Vogt said.

“The number of systems with potentially habitable planets is probably on the order of ten or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that’s a large number. There could be tens of billions of these systems in our galaxy.”

Gliese 581, located 20 light years away from Earth, has a somewhat chequered history of habitable-planet claims. Two previously detected planets in the system lie at the edges of the habitable zone…one on the hot side (planet c) and one on the cold side (planet d).

While some astronomers still think planet d may be habitable if it has a thick atmosphere with a strong greenhouse effect to warm it up, others are sceptical. The newly discovered planet g, however, lies right in the middle of the habitable zone.

“It’s the Goldilocks planet,” Vogt said. “That’s a well-worn analogy, but in this case it fits. We had planets on both sides of the habitable zone—one too hot and one too cold—and now we have one in the middle that’s just right.”

Adapted from information issued by W.M. Keck Observatory / ESO / L. Calçada / NASA / ESA / G. Bacon (STScI).

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Far-flung planet has cloudy skies

Artist's impression of an exoplanet

A planet orbiting a nearby star is suspected to have dusty, cloudy skies, making it about 400 degrees Celsius hotter than computer models predicted. (Artist's impression)

Astronomers have tested the atmosphere of a young gas-giant planet orbiting another star, and have found that it is unlike that of any previously studied “extrasolar” planet (one that belongs to a star other than our Sun).

By obtaining a spectrum of its light, they determined the planet’s temperature. And it’s about 400 degrees Celsius higher than expected.

Extrasolar planet HR 8799 b

The young extrasolar planet HR 8799 b (green blob in the middle), isolated from the glare of its parent star.

The reason could be dust in the planet’s atmosphere.

The planet, known as HR 8799 b, is one of three gas-giant planets orbiting the star HR 8799, 130 light-years from Earth. HR 8799 b is the lightest of the three, about 7 times the mass of Jupiter.

The spectrum of a planet contains much more information than a single image—it can reveal the temperature, chemical composition, and cloud properties of the planet.

The team took the planet’s temperature by using the presence or absence of gaseous methane as a “thermometer”. They found that HR 8799 b has little or no methane.

Based on the spectrum and previously obtained images of the planet, and by comparing the observations to theoretical models of low-temperature atmospheres, they estimate the coolest temperature for the planet should be about 930 degrees Celsius.

The models, however, did a poor job of fitting all the data, predicting that HR 8799 b should be about 500 degrees, based on the age of the planet and the amount of energy it is currently emitting.

W. M. Keck Observatory

The W. M. Keck Observatory operates two 10-metre optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i.

The scientists suspect the discrepancy arises because the planet is more dusty and cloudy than the models predict. Dust would hold in the extra heat.

The HR 8799 planets are incredibly faint, about 100,000 times dimmer than their parent star. To obtain the spectrum of HR 8799 b, the team relied on the “adaptive optics” system of the giant Keck II Telescope to make an ultra-sharp image of the star for many hours. Then they used a special kind of spectrograph to precisely separate the spectrum of the planet from the light of its parent star.

“Adaptive optics systems on Keck and other large ground-based telescopes make sharper images than even the Hubble Space Telescope,” said Trent Dupuy, a University of Hawaii graduate student and co-author of the study.

Adapted from information issued by W. M. Keck Observatory / Brendan Bowler & Michael Liu (IfA, Hawaii) / Pablo McLoud / NASA / ESA / G. Bacon (STScI).

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Cosmic conundrum puzzles astronomers

Distant galaxies

Abundances of two very different types of hydrogen in certain galaxies have been found to be equal. Astronomers aren't sure why.

Researchers have uncovered a cosmological anomaly and are now trying to determine if it is an uncanny coincidence or a vital clue to understanding the origins of our Universe.

The irregularity has left the team, including researchers from Melbourne’s Swinburne University of Technology, scratching their heads.

According to Swinburne scientist Dr Michael Murphy, the research reveals a strange coincidence—or at least what appears to be a strange coincidence—occurring in distant galaxies.

The astronomers were measuring the abundance of a type of hydrogen—called deuterium-deuterated molecular hydrogen, or HD for short—in two different galaxies in the distant Universe.

“What we inadvertently discovered was that in these two galaxies the fraction of molecules which were HD was the same as the fraction of atoms which were deuterium (D), hydrogen’s doubly-heavy cousin,” says Dr Murphy.

Distant galaxies

More observations are planned of other galaxies

“We then looked at the only other two existing measurements of HD in distant galaxies and found almost exactly the same thing.”

Dr Murphy said this was extremely unusual because HD should have a far more complex life cycle than D and researchers would expect it to be produced in very different amounts.

“Because deuterium was produced just after the Big Bang and never again, measures of its abundance are extremely important in telling us about cosmology.”

A bizarre cosmic coincidence?

Measuring the abundance of deuterium is one of the few relatively precise ways of telling how many atoms there are in the Universe overall.

“Knowing this basic parameter is important if you want to know how the Universe began, the fate of the Universe and all of the steps in between,” adds Dr Murphy.

“But HD should be a completely different story,” according to Adrian Malec, a PhD student at Swinburne. “When we realised that the abundance of HD aligned with the abundance of D we were extremely surprised.

Keck Observatory

The astronomers used the huge Keck Observatory in Hawai'i

“You would expect the abundance of HD to vary dramatically from place to place in the Universe. So if it is a coincidence, then it is a one in a million,” adds Malec.

“Which means we now have to ask the question—is this is a bizarre coincidence or is it actually meaningful?”

According to Malec, the finding raises more questions that now need to be answered.

“We have four measurements of this molecule [HD] separated by very large distances, and in each case the abundance aligns with D,” he said.

The astronomers say they probably need a dozen more measurements before they can conclusively state whether this is a just strange coincidence or whether measurements of HD could potentially be used to help them understand the evolution of the universe.

The measurements were conducted using the world’s largest optical telescopes at the Keck Observatory in Hawaii.  Swinburne has an agreement with the California Institute of Technology (Caltech) that gives Swinburne astronomers access to the telescopes for up to 20 nights per year.

A paper describing the work has been accepted for publication in the journal Astrophysical Journal Letters.

Adapted from information issued by Swinburne University / STScI / Keck Observatory.

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Smallest known star duo confirmed

Artist's impression of the binary star system known as HM Cancri

About 1,600 light-years away, in a binary star system known as HM Cancri, two dense white dwarf stars orbit each other once every 5.4 minutes, based on data from the Keck Observatory. This artist's rendition shows the dance of these dead stars and the resulting gravitational waves (which would actually be invisible).

Astronomers have identified the smallest known binary star system to date. Called HM Cancri, its consists of two dead stars that revolve around each other in 5.4 minutes, by far the shortest known orbital period of any pair of stars.

The team, led by Gijs Roelofs of the Harvard-Smithsonian Center of Astrophysics, used the 10-meter Keck I telescope in Hawaii and its Low Resolution Imaging Spectrograph to study the velocity changes in the spectral lines in the light coming from HM Cancri.

They saw that as the stars orbited each other, the system’s spectral lines shifted periodically from blue to red and back, in accordance with the Doppler effect. With that velocity information, the astronomers were able to confirm the binary’s 5.4-minute period.

“When the first data from the Keck telescope arrived, and our quick analysis showed the periodic shift of the spectral lines, we knew that we had succeeded. More than ten years after its discovery, we finally had deciphered the nature of HM Cancri,” said Arne Rau of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who led the observations at Keck.

Astronomers proposed several years ago that HM Cancri was an interacting binary consisting of two dead stars and that the 5.4 minute period observed was indeed the orbital period.

The team had been trying to make precise velocity measurements to confirm the period since 2005.

X-ray evidence

HM Cancri was discovered in 1999 as a weak X-ray source in data from the German ROSAT satellite. It comprises two white dwarfs, burnt-out cinders of stars that were once similar to the Sun and contain a highly condensed form of helium, carbon and oxygen. In 2001, the X-ray, and also optical, data suggested that the two stars orbited each other in 5.4 minutes.

Another artist's conception of HM Cancri.

Another artist's conception of HM Cancri. One star is feeding the other.

But the information suggested that the binary system was roughly eight times the diameter of the Earth—equivalent to a quarter of the distance between the Earth and the Moon—or smaller. Astronomers were reluctant to accept this physical description without additional evidence. But at a distance of 16,000 light years from Earth, the binary system shines only one millionth as bright as the faintest stars visible to the naked eye, making it very hard to study. To determine with certainty the period of such a system, astronomers needed to use world’s largest telescopes to collect the additional evidence.

“This type of observation is really at the limit of what is currently possible. Not only does one need the biggest telescopes in the world, but they also have to be equipped with the best instruments available,” said team member Paul Groot of the Radboud University Nijmegen in the Netherlands.

As a result of the successful observations with Keck, astronomers now have a new cosmic laboratory to study the evolution of stars as well as general relativity.

“We know the system must have come from two normal stars that somehow spiralled together in two earlier episodes of mass transfer, but the physics of this process is very poorly understood,” said Gijs Nelemans of the Radboud University who was also part of the team.

He added that the system must be one of the most copious emitters of gravitational waves. “We hope to detect these distortions of space-time directly with the future LISA satellite. HM Cancri will now be a cornerstone system for the mission,” he said.

Adapted from information issued by Keck Observatory / NASA / Tod Strohmayer (GSFC) / Dana Berry (Chandra X-Ray Observatory) / Rob Hynes and Paul Groot, Radboud University.