Posted on by Fraser Cain

Why Isn’t the Asteroid Belt a Planet?

Why Isn’t the Asteroid Belt a Planet?

It seems like there’s a strange gap in between Mars and Jupiter filled with rocky rubble. Why didn’t the asteroid belt form into a planet, like the rest of the Solar System?

Beyond the orbit of Mars lies the asteroid belt its a vast collection of rocks and ice, leftover from the formation of the solar system. It starts about 2 AU, ends around 4 AU. Objects in the asteroid belt range from tiny pebbles to Ceres at 950 km across.

Star Wars and other sci-fi has it all wrong. The objects here are hundreds of thousand of kilometers apart. There’d be absolutely no danger or tactical advantage to flying your spacecraft through it.

To begin with, there actually isn’t that much stuff in the asteroid belt. If you were to take the entire asteroid belt and form it into a single mass, it would only be about 4% of the mass of our Moon. Assuming a similar density, it would be smaller than Pluto’s moon Charon.

There’s a popular idea that perhaps there was a planet between Mars and Jupiter that exploded, or even collided with another planet. What if most of the debris was thrown out of the solar system, and the asteroid belt is what remains?

We know this isn’t the case for a few of reasons. First, any explosion or collision wouldn’t be powerful enough to throw material out of the Solar System. So if it were a former planet we’d actually see more debris.

Second, if all the asteroid belt bits came from a single planetary body, they would all be chemically similar. The chemical composition of Earth, Mars, Venus, etc are all unique because they formed in different regions of the solar system. Likewise, different asteroids have different chemical compositions, which means they must have formed in different regions of the asteroid belt.

Asteroids
Artist’s depiction of the asteroid belt between Mars and Jupiter. Credit: David Minton and Renu Malhotra

In fact, when we look at the chemical compositions of different asteroids we see that they can be grouped into different families, with each having a common origin. This gives us a clue as to why a planet didn’t form where the asteroid belt is.

If you arrange all the asteroids in order of their average distance from the Sun, you find they aren’t evenly distributed. Instead you find a bunch, then a gap, then a bunch more, then another gap, and so on. These gaps in the asteroid belt are known as Kirkwood gaps, and they occur at distances where an orbit would be in resonance with the orbit of Jupiter.

Jupiter’s gravity is so strong, that it makes asteroid orbits within the Kirkwood gaps unstable. It’s these gaps that prevented a single planetary body from forming in that region. So, because of Jupiter, asteroids formed into families of debris, rather than a single planetary body.

What do you think? What’s your favorite object in the asteroid belt. Tell us in the comments below.

Posted on by Elizabeth Howell

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

An image of TW Hydrae and the protoplanetary stuff surrounding the star. Astronomers believe a planet is forming within the gas and dust and sweeping up debris, as shown by the gap in this picture. Credit: NASA, ESA, J. Debes (STScI), H. Jang-Condell (University of Wyoming), A. Weinberger (Carnegie Institution of Washington), A. Roberge (Goddard Space Flight Center), and G. Schneider (University of Arizona/Steward Observatory)

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

Posted on by Tammy Plotner

Hubble Observes Planet-“Polluted” Dead Stars In Hyades

Artist Impression of debris around a white dwarf star. Image credit: NASA, ESA, STScI, and G. Bacon (STScI)

For those of us who practice amateur astronomy, we’re very familiar with the 150 light-year distant Hyades star cluster – one of the jewels in the Taurus crown. We’ve looked at it countless times, but now the NASA/ESA Hubble Space Telescope has taken its turn observing and spotted something astronomers weren’t expecting – the debris of Earth-like planets orbiting white dwarf stars. Are these “burn outs” being polluted by detritus similar to asteroids? According to researchers, this new observation could mean that rocky planet creation is commonplace in star clusters.

“We have identified chemical evidence for the building blocks of rocky planets,” said Jay Farihi of the University of Cambridge in England. He is lead author of a new study appearing in the Monthly Notices of the Royal Astronomical Society. “When these stars were born, they built planets, and there’s a good chance they currently retain some of them. The material we are seeing is evidence of this. The debris is at least as rocky as the most primitive terrestrial bodies in our solar system.”

So what makes this an uncommon occurrence? Research tells us that all stars are formed in clusters, and we know that planets form around stars. However, the equation doesn’t go hand in hand. Out of the hundreds of known exoplanets, only four are known to have homes in star clusters. As a matter of fact, that number is a meager half percent, but why? As a rule, the stars contained within a cluster are young and active. They are busy producing stellar flares and similar brilliant activity which may mask signs of emerging planets. This new research is looking to the “older” members of the cluster stars – the grandparents which may be babysitting.

To locate possible candidates, astronomers have employed Hubble’s Cosmic Origins Spectrograph and focused on two white dwarf stars. Their return showed evidence of silicon and just slight levels of carbon in their atmospheres. This observation was important because silicon is key in rocky materials – a prime ingredient on Earth’s list and other similar solid planets. This silicon signature may have come from the disintegration of asteroids as they wandered too close to the stars and were torn apart. A lack of carbon is equally exciting because, while it helps shape the properties and origins of planetary debris, it becomes scarce when rocky planets are formed. This material may have formed a torus around the defunct stars which then drew the matter towards them.

“We have identified chemical evidence for the building blocks of rocky planets,” said Farihi. “When these stars were born, they built planets, and there’s a good chance they currently retain some of them. The material we are seeing is evidence of this. The debris is at least as rocky as the most primitive terrestrial bodies in our solar system.”

Ring around the rosie? You bet. This leftover material swirling around the white dwarf stars could mean that planet formation happened almost simultaneously as the stars were born. At their collapse, the surviving gas giants may have had the gravitational “push” to relocate asteroid-like bodies into “star-grazing orbits”.

This image shows the region around the Hyades star cluster, the nearest open cluster to us. The Hyades cluster is very well-studied due to its location, but previous searches for planets have produced only one. A new study led by Jay Farihi of the University of Cambridge, UK, has now found the atmospheres of two burnt-out stars in this cluster — known as white dwarfs — to be “polluted” by rocky debris circling the star. Inset, the locations of these white dwarf stars are indicated — stars known as WD 0421+162, and WD 0431+126. Credit: NASA, ESA, STScI, and Z. Levay (STScI)
This image shows the region around the Hyades star cluster, the nearest open cluster to us. The Hyades cluster is very well-studied due to its location, but previous searches for planets have produced only one. A new study led by Jay Farihi of the University of Cambridge, UK, has now found the atmospheres of two burnt-out stars in this cluster — known as white dwarfs — to be “polluted” by rocky debris circling the star.
Inset, the locations of these white dwarf stars are indicated — stars known as WD 0421+162, and WD 0431+126. Credit: NASA, ESA, STScI, and Z. Levay (STScI)

“We have identified chemical evidence for the building blocks of rocky planets,” explains Farihi. “When these stars were born, they built planets, and there’s a good chance that they currently retain some of them. The signs of rocky debris we are seeing are evidence of this — it is at least as rocky as the most primitive terrestrial bodies in our Solar System. The one thing the white dwarf pollution technique gives us that we won’t get with any other planet detection technique is the chemistry of solid planets. Based on the silicon-to-carbon ratio in our study, for example, we can actually say that this material is basically Earth-like.”

What of future plans? According to Farihi and the research team, by continuing to observe with methods like those employed by Hubble, they can take an even deeper look at the atmospheres around white dwarf stars. They will be searching for signs of solid planet “pollution” – exploring the white dwarf chemistry and analyzing stellar composition. Right now, the two “polluted” Hyades white dwarfs are just a small segment of more than a hundred future candidates which will be studied by a team led by Boris Gansicke of the University of Warwick in England. Team member Detlev Koester of the University of Kiel in Germany is also contributing by using sophisticated computer models of white dwarf atmospheres to determine the abundances of various elements that can be traced to planets in the Hubble spectrograph data.

“Normally, white dwarfs are like blank pieces of paper, containing only the light elements hydrogen and helium,” Farihi said. “Heavy elements like silicon and carbon sink to the core. The one thing the white dwarf pollution technique gives us that we just won’t get with any other planet-detection technique is the chemistry of solid planets.”

The team also plans to look deeper into the stellar composition as well. “The beauty of this technique is that whatever the Universe is doing, we’ll be able to measure it,” Farihi said. “We have been using the Solar System as a kind of map, but we don’t know what the rest of the Universe does. Hopefully with Hubble and its powerful ultraviolet-light spectrograph COS, and with the upcoming ground-based 30- and 40-metre telescopes, we’ll be able to tell more of the story.”

And we’ll be listening…

Original Story Source: Hubble News Release.

Posted on by Nancy Atkinson

Herschel Observatory Detects ‘Oceans’ of Water Around Distant Star

Detection of water vapour in the spectrum of TW Hydrae's protoplanetary disc. Credits: ESA/NASA/JPL-Caltech/M. Hogerheijde (Leiden Observatory)

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There’s enough water in a planet-forming disk around a distant star to fill several thousand Earth oceans, according to new observations with the Herschel space observatory. Astronomers have found evidence of water vapor originating from ice on dust grains in the disc around a young star, TW Hydrae. The star is between 5-10 million years old, so is in its final stages of formation.

“The detection of water sticking to dust grains throughout the disc would be similar to events in our own Solar System’s evolution, where over millions of years, similar dust grains then coalesced to form comets,” said Michiel Hogerheijde of Leiden University in the Netherlands, who led the study. “These comets we believe became a contributing source of water for the planets.”

Herschel has found water around other stars, such as an old red giant star CW Leonis, and other telescopes like Spitzer have also observed abundant water in nascent planet forming regions around other stars.

But scientists say this latest research from Herschel breaks new ground in understanding water’s role in planet-forming discs and gives scientists a new testing ground for looking at how water came to our own planet.

“With Herschel we can follow the trail of water through all the steps of star and planet formation,” said Göran Pilbratt, Herschel Project Scientist at ESA.

Scientists think the water vapor signature is produced when the ice coated dust grains are warmed by interstellar UV radiation.

Read more on this discovery at the ESA Herschel website.

Posted on by Tammy Plotner

Coming To A Solar System Near You… Super-Earth!

Planetary system of HR 8799 imaged by Marois et al (2010). The central star is of spectral type A with a mass of 1.5 solar-masses at a distance of 128 light-years from the Sun. The planets have the masses of Mb = 7MJ , Mc = Md = 10MJ , and Me = (7?10)MJ , with semimajor axes of 68, 38, 24, and 14.5 AU, respectively. Figure with the permission of NPG.

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It is our general understanding of solar system composition that planets fall into two categories: gas giants like Jupiter, Saturn, Neptune and Uranus… and rocky bodies that support some type of atmosphere like Earth, Mars and Venus. However, as we reach further into space we’re beginning to realize the Solar System is pretty unique because it doesn’t have a planetary structure which meets in the middle. But just because we don’t have one doesn’t mean they don’t exist. As a matter of fact, astronomers have found more than 30 of them and they call this new class of planet a “Super-Earth”.

“Super-Earths, a class of planetary bodies with masses ranging from a few Earth-masses to slightly smaller than Uranus, have recently found a special place in the exoplanetary science.” says Nader Haghighipour of the Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii. “Being slightly larger than a typical terrestrial planet, super-Earths may have physical and dynamical characteristics similar to those of Earth whereas unlike terrestrial planets, they are relatively easier to detect.”

Having a super-Earth in the neighborhood opens the avenue towards habitability. Chances are planets of this type have a dynamic core and are able to maintain a type of atmosphere. When combined with being within the habitable zone of a host star, this raises the bar towards possible life on other planets.

“It is important to note that the notion of habitability is defined based on the life as we know it. Since Earth is the only habitable planet known to humankind, the orbital and physical characteristics of Earth are used to define a habitable planet.” says Haghighipour. “In other words, habitability is the characteristic of an environment which has similar properties as those of Earth, and the capability of developing and sustaining Earthly life.”

But being a super-Earth means that there is a lot more going on than just being in the zone. To qualify it must meet three requirements: its composition, the manifestation of plate tectonics, and the presence of a magnetic field. For the first, the presence of liquid water is a high priority. In order to determine this possibility the values of its mass and radius have to be known. To date, two super-Earth planets for which these values have been determined – CoRoT-7b and GJ 1214b – have given us fascinating numerical modeling to help us better understand their composition. Plate tectonics also plays a role through geophysical evolution – just as the presence of a magnetic field has been considered essential for habitability.

“Whether and how magnetic fields are developed around super-Earths is an active topic of research.” notes Haghighipour. “In general, in order for a magnetic field to be in place around an Earth-like planet, a dynamo action has to exist in the planet’s core.”

Last, but not least, comes an atmosphere – the “presence of which has profound effects on its capability in developing and maintaining life.” From its chemical properties we can derive the “planet’s possible biosignatures” as well as the chemicals which formed it. Atmosphere means environment and all of this leads back to being within a habitable zone and of sufficient gravity to keep atmospheric molecules from escaping. Says Haghighipour, “It would not be unrealistic to assume that super-Earths carry gaseous envelopes. Around low-mass stars, some of such atmosphere-bearing super-Earths may even have stable orbits in the habitable zones of their host stars.”

Has a super-Earth been detected? You betcha’… and studied right down to its spectral signature. “The recently detected super-Earth GL 581 g with its possible atmospheric circulation in the habitable zone of its host star may in fact be one of such planets.” says Haghighipour. “More advanced telescopes are needed to identify the biosignatures of these bodies and the physical and compositional characteristics of their atmospheres.”

Further Reading: Super-Earths: A New Class of Planetary Bodies.

Posted on by Ryan Anderson

Mysterious Alien Dust Hints at Violent Planet Formation

Image credit: Lynette Cook for Gemini Observatory/AURA
An artist's depiction of two colliding rocky bodies. Such a collision is the most likely source for the warm dust in the HD 131488 system. Image credit: Lynette Cook for Gemini Observatory/AURA

An artist’s rendition of colliding planets, the most likely explanation for the warm dust observed around HD 131488. Image credit: Lynette Cook for Gemini Observatory/AURA

Five-hundred light years away, worlds are colliding, and they’re made of nothing we’ve ever seen.

Last week at the 215th American Astronomical Society meeting, UCLA astronomers announced that they had found warm dust – evidence for the violent collision of rocky planets – around a star called HD 131488. The strange thing is, the composition of the dust has little in common with the composition of rocky bodies in any other known system.

“Typically, dust debris around other stars, or our own Sun, is of the olivine, pyroxene, or silica variety, minerals commonly found on Earth,” said Dr. Carl Melis, who led the research as a graduate student at UCLA. “The material orbiting HD 131488 is not one of these dust types. We have yet to identify what species it is – it really appears to be a completely alien type of dust.”

The warm dust in the HD 131488 system is concentrated in an area close to the star, where temperatures are similar to those on Earth. The researchers concluded that the most likely source for dust in that part of the system would be the collision of two rocky planetary bodies. Only five other stars like HD 131488 with dust in their terrestrial planet zone are known. “Interestingly, all five of these stars have ages in the range of 10-30 million years,” Melis said. “This finding indicates that the epoch of final catastrophic mass accretion for terrestrial planets, the likes of which could have resulted in the formation of the Earth-Moon system in our own Solar System, occurs in this narrow age range for stars somewhat more massive than the Sun.”

The team also discovered a unique second dusty region in the outer reaches of the HD 131488 system, comparable to the location of Pluto and other Kuiper Belt objects in our own solar system.
Image Credit: Lynette Cook for Gemini Observatory/AURA

Top: Illustration depicting the location of the warm and cold dust rings in the HD131488 system. Bottom: Comparable regions in our own solar system, with the orbits of the outer planets for scale. Image Credit: Lynette Cook for Gemini Observatory/AURA

“The hot dust almost certainly came from a recent catastrophic collision between two large rocky bodies in HD 131488’s inner planetary system,” Melis said. “The cooler dust, however, is unlikely to have been produced in a catastrophic collision and is probably left over from planet formation that took place farther away from HD 131488.”

“…for some reason stars that have large amounts of orbiting warm dust do not also show evidence for the presence of cold dust. HD 131488 dramatically breaks this pattern,” said Dr. Benjamin Zuckerman, a co-author on the paper and a professor of physics and astronomy at UCLA.

With its unusual dust composition and unique combination of warm and cold dust regions, the HD 131488 system is now under intense scrutiny. Melis and colleagues plan to continue trying to determine the composition of the dust, and will search for other stars with the dusty evidence for planet formation.

Source: Gemini Observatory