ALMA Captures Never-Before-Seen Details of Protoplanetary Disk

ALMA’s best image of a protoplanetary disc to date. This picture of the nearby young star TW Hydrae reveals the classic rings and gaps that signify planets are in formation in this system.
ALMA’s best image of a protoplanetary disk to date. This picture of the nearby young star TW Hydrae reveals the classic rings and gaps that signify planets are in formation in this system. Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)

TW Hydrae is a special star. Located 175 light years from Earth in the constellation Hydra the Water Snake, it sits at the center of a dense disk of gas and dust that astronomers think resembles our solar system when it was just 10 million years old. The disk is incredibly clear in images made using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, which employs 66 radio telescopes sensitive to light just beyond that of infrared.  Spread across more than 9 miles (15 kilometers), the ALMA array acts as a gigantic single telescope that can make images 10 times sharper than even the Hubble Space Telescope.

This photo of the ALMA antennas on the Chajnantor Plateau in Chile, more than 16,000 feet (5000 meters) above sea level, was taken a few days before the start of ALMA Early Science and shows only one cluster of the 66 dishes. ALMA views the sky in "submillimeter" light, a slice of the spectrum invisible to the human eye that lies between infrared and radio waves. Credit: ALMA (ESO/NAOJ/NRAO)/W. Garnier (ALMA)
This photo of the ALMA antennas on the Chajnantor Plateau in Chile, more than 16,000 feet (5000 meters) above sea level, was taken a few days before the start of ALMA Early Science and shows only one cluster of the 66 dishes. ALMA views the sky in submillimeter light, a slice of the spectrum invisible to the human eye that lies between infrared and radio waves. Credit: ALMA (ESO/NAOJ/NRAO)/W. Garnier (ALMA)

Astronomers everywhere point their telescopes at TW Hydrae because it’s the closest infant star in the sky. With an age of between 5 and 10 million years, it’s not even running on hydrogen fusion yet, the process by which stars convert hydrogen into helium to produce energy. TW Hydrae shines from the energy released as it contracts through gravity. Fusion and official stardom won’t begin until it’s dense enough and hot enough for fusion to fire up in its belly.

ALMA image of the planet-forming disk around the young, sun-like star TW Hydrae. The inset image (upper right) zooms in on the gap nearest to the star, which is at the same distance as the Earth is from the sun, and may show an infant version of our home planet emerging from the dust and gas. The additional concentric light and dark features represent other planet-forming regions farther out in the disk. Credit: S. Andrews (Harvard-Smithsonian CfA), ALMA (ESO/NAOJ/NRAO)
ALMA image of the planet-forming disk around the young, sun-like star TW Hydrae. The inset image (upper right) zooms in on the gap nearest to the star, which is at the same distance as the Earth is from the sun, and may show an infant version of our home planet emerging from the dust and gas. The additional concentric light and dark features represent other planet-forming regions farther out in the disk. Credit: S. Andrews (Harvard-Smithsonian CfA), ALMA (ESO/NAOJ/NRAO)

We see most protoplanetary disks at various angles, but TW’s has a face-on orientation as seen from Earth, giving astronomers a rare, undistorted view of the complete disk around the star. The new images show amazing detail, revealing a series of concentric bright rings of dust separated by dark gaps. There’s even indications that a planet with an Earth-like orbit has begun clearing an orbit.

“Previous studies with optical and radio telescopes confirm that TW Hydrae hosts a prominent disk with features that strongly suggest planets are beginning to coalesce,” said Sean Andrews with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, USA and lead author on a paper published today in the Astrophysical Journal Letters.

Blurry as it is, the detail here is staggering. It shows a gap about 93 million miles from the central starsuggesting that a planet with a similar orbit to Earth is forming there. Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)
The model (at left) of a protoplanetary disk shows a newly forming star at the center of a saucer-shaped dust cloud. At right, a close up of TW Hydrae taken by ALMA shows a gap about 93 million miles from the central star, suggesting that a planet with a similar orbit to Earth is forming there. Credit: (Left: L. Calcada). Right: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)

Pronounced gaps that show up in the photos above are located at 1.9 and 3.7 billion miles (3-6 billion kilometers) from the central star, similar to the average distances from the sun to Uranus and Pluto in the solar system. They too are likely to be the results of particles that came together to form planets, which then swept their orbits clear of dust and gas to sculpt the remaining material into well-defined bands. ALMA picks up the faint emission of submillimeter light emitted by dust grains in the disk, revealing details as small as 93 million miles (150 million kilometers) or the distance of Earth from the sun

This image compares the size of the solar system with HL Tauri and its surrounding protoplanetary disc. Although the star is much smaller than the Sun, the disc around HL Tauri stretches out to almost three times as far from the star as Neptune is from the Sun. Credit:ALMA (ESO/NAOJ/NRAO)
This image compares the size of the solar system with HL Tauri and its surrounding protoplanetary disc. Although the star is much smaller than the Sun, the disc around HL Tauri stretches out to almost three times as far from the star as Neptune is from the Sun. Credit:ALMA (ESO/NAOJ/NRAO)

“This is the highest spatial resolution image ever of a protoplanetary disk from ALMA, and that won’t be easily beaten in the future!” said Andrews.

Earlier ALMA observations of another system, HL Tauri, show that even younger protoplanetary disks — a mere 1 million years old — look remarkably similar.  By studying the older TW Hydrae disk, astronomers hope to better understand the evolution of our own planet and the prospects for similar systems throughout the Milky Way.

Astronomers See Snow … In Space!

There’s an excellent chance of frost in this corner of the universe: astronomers have spotted a “snow line” in a baby solar system about 175 light-years away from Earth. The find is cool (literally and figuratively) in itself. More importantly, however, it could give us clues about how our own planet formed billions of years ago.

“[This] is extremely exciting because of what it tells us about the very early period in the history of our own solar system,” stated Chunhua Qi, a researcher with the Harvard-Smithsonian Center for Astrophysics who led the research.

“We can now see previously hidden details about the frozen outer reaches of another solar system, one that has much in common with our own when it was less than 10 million years old,” he added.

The real deal enhanced-color picture of TW Hydrae is below, courtesy of a newly completed telescope: the Atacama Large Millimeter/submillimeter Array in Chile. It is designed to look at grains and other debris around forming solar systems. This snow line is huge, stretching far beyond the equivalent orbit of Neptune in our own solar system. See the circle? That’s Neptune’s orbit. The green stuff is the snow line. Look just how far the green goes past the orbit.

The carbon monoxide line as seen by the Atacama Large Millimeter/submillimeter Array (ALMA) telescope. The circle represents the equivalent orbit of Neptune when comparing it to our own solar system. Credit: Karin Oberg, Harvard University/University of Virginia
The carbon monoxide line on TW Hydrae as seen by the Atacama Large Millimeter/submillimeter Array (ALMA) telescope. The circle represents the equivalent orbit of Neptune when comparing it to our own solar system. Credit: Karin Oberg, Harvard University/University of Virginia

Young stars are typically surrounded by a cloud of gas and debris that, astronomers believe, can in many cases form into planets given enough time. Snow lines form in young solar systems in areas where the heat of the star isn’t enough to melt the substance. Water is the first substance to freeze around dust grains, followed by carbon dioxide, methane and carbon monoxide.

It’s hard to spot them: “Snow lines form exclusively in the relatively narrow central plane of a protoplanetary disk. Above and below this region, stellar radiation keeps the gases warm, preventing them from forming ice,” the astronomers stated. In areas where dust and gas are more dense, the substances are insulated and can freeze — but it’s difficult to see the snow through the gas.

In this case, astronomers were able to spot the carbon monoxide snow because they looked for diazenylium, a molecule that is broken up in areas of carbon monoxide gas. Spotting it is a “proxy” for spots where the CO froze out, the astronomers said.

Here are some more of the many reasons this is exciting to astronomers:

  • Snow could help dust grains form faster into rocks and eventually, planets because it coats the grain surface into something more stickable;
  • Carbon monoxide is a requirement to create methanol, considered a building block of complex molecules and life;
  • The snow was actually spotted with only a small portion of ALMA’s 66 antennas while it was still under construction. Now that ALMA is complete, scientists are already eager to see what the telescope will turn up the next time it gazes at the system.

Source: National Radio Astronomy Observatory

 

Should This Alien World Even Exist? This Young Disk Could Challenge Planet-Formation Theories

Take a close look at the blurry image above. See that gap in the cloud? That could be a planet being born some 176 light-years away from Earth. It’s a small planet, only 6 to 28 times Earth’s mass.

That’s not even the best part.

This alien world, if we can confirm it, shouldn’t be there according to conventional planet-forming theory.

The gap in the image above — taken by the Hubble Space Telescope — probably arose when a planet under construction swept through the dust and debris in its orbit, astronomers said.

That’s not much of a surprise (at first blush) given what we think we know about planet formation. You start with a cloud of debris and gas swirling around a star, then gradually the bits and pieces start colliding, sticking together and growing bigger into small rocks, bigger ones and eventually, planets or gas giant planet cores.

But there’s something puzzling astronomers this time around: this planet is a heck of a long way from its star, TW Hydrae, about twice Pluto’s distance from the sun. Given that alien systems’ age, that world shouldn’t have formed so quickly.

An illustration of TW Hydrae's disk in comparison with that of Earth's solar system. Credit: NASA, ESA, and A. Feild (STScI)
An illustration of TW Hydrae’s disk in comparison with that of Earth’s solar system. Credit: NASA, ESA, and A. Feild (STScI)

Astronomers believe that Jupiter took about 10 million years to form at its distance away from the sun. This planet near TW Hydrae should take 200 times longer to form because the alien world is moving slower, and has less debris to pick up.

But something must be off, because TW Hydrae‘s system is believed to be only 8 million years old.

“There has not been enough time for a planet to grow through the slow accumulation of smaller debris. Complicating the story further is that TW Hydrae is only 55 percent as massive as our sun,” NASA stated, adding it’s the first time we’ve seen a gap so far away from a low-mass star.

The lead researcher put it even more bluntly: “Typically, you need pebbles before you can have a planet. So, if there is a planet and there is no dust larger than a grain of sand farther out, that would be a huge challenge to traditional planet formation models,” stated John Debes, an astronomer at the Space Telescope Science Institute in Baltimore.

Protoplanet Hypothesis
Like a raindrop forming in a cloud, a star forms in a diffuse gas cloud in deep space. As the star grows, its gravitational pull draws in dust and gas from the surrounding molecular cloud to form a swirling disk called a “protoplanetary disk.” This disk eventually further consolidates to form planets, moons, asteroids and comets. Credit: NASA/JPL-Caltech

At this point, you would suppose the astronomers are seriously investigating other theories. One alternative brought up in the press release: perhaps part of the disc collapsed due to gravitational instability. If that is the case, a planet could come to be in only a few thousand years, instead of several million.

“If we can actually confirm that there’s a planet there, we can connect its characteristics to measurements of the gap properties,” Debes stated. “That might add to planet formation theories as to how you can actually form a planet very far out.”

A rare double transit of Jupiter's moon Ganymede (top) and Io on Jan. 3, 2013. Here, the sun is shining from the left causing shadows cast by the moons to fall onto the planet's cloud tops. Credit: Damian Peach
A rare double transit of Jupiter’s moon Ganymede (top) and Io on Jan. 3, 2013. Here, the sun is shining from the left causing shadows cast by the moons to fall onto the planet’s cloud tops. Credit: Damian Peach

There’s a trick with the “direct collapse” theory, though: astronomers believe it takes a bunch of matter that is one to two times more massive than Jupiter before a collapse can occur to form a planet.

Recall that this world is no more than 28 times the mass of Earth, as best as we can figure. Well, Jupiter itself is 318 times more massive than Earth.

There are also intriguing results about the gap. Chile’s Atacama Large Millimeter/submillimeter Array (ALMA) — which is designed to look at dusty regions around young stars — found that the dust grains in this system, orbiting nearby the gap, are still smaller than the size of a grain of sand.

Astronomers plan to use ALMA and the James Webb Space Telescope, which should launch in 2018, to get a better look. In the meantime, the results will be published in the June 14 edition of the Astrophysical Journal.

Source: HubbleSite