New research from the Hubble Space Telescope and the ESO’s Very Large Telescope is dampening some of the enthusiasm in the search for life. Observations by both ‘scopes suggest that the raw materials necessary for life may be rare in solar systems centered around red dwarfs.
And if the raw materials aren’t there, it may mean that many of the exoplanets we’ve found in the habitable zones of other stars just aren’t habitable after-all.
The theory of how planets form has been something of an enduring mystery for scientists. While astronomers have a pretty good understanding of where planetary systems comes from – i.e. protoplanetary disks of dust and gas around new stars (aka. “Nebular Theory“) – a complete understanding of how these discs eventually become objects large enough to collapse under their own gravity has remained elusive.
But thanks to a new study by a team of researchers from France, Australia and the UK, it seems that the missing piece of the puzzle may finally have been found. Using a series of simulations, these researchers have shown how “dust traps” – i.e. regions where pebble-sized fragments could collect and stick together – are common enough to allow for the formation of planetesimals.
Until recently, the process by which protoplanetary disks of dust and gas aggregate to form peddle-sized objects, and the process by which planetesimals (objects that are one hundred meters or more in diameter) form planetary cores, have been well understood. But the process that bridges these two – where pebbles come together to form planetesimals – has remained unknown.
Part of the problem has been the fact that the Solar System, which has been our only frame of reference for centuries, formed billions of years ago. But thanks to recent discoveries (3453 confirmed exoplanets and counting), astronomers have had lots of opportunities to study other systems that are in various stages of formation. As Dr. Gonzalez explained in a Royal Astronomical Society press release:
“Until now we have struggled to explain how pebbles can come together to form planets, and yet we’ve now discovered huge numbers of planets in orbit around other stars. That set us thinking about how to solve this mystery.”
In the past, astronomers believed that “dust traps” – which are integral to planet formation – could only exist within certain environments. In these high-pressure regions, large grains of dust are slowed down to the point where they are able to come together. These regions are extremely important since they counteract the two main obstacles to planetary formation, which are drag and high-speed collisions.
Drag is caused by the effect gas has on dust grains, which causes them to slow down and eventually drift into the central star (where they are consumed). As for high-speed collisions, this is what causes large pebbles to smash into each other and break apart, thus reversing the aggregation process. Dust traps are therefore needed to ensure that dust grains are slowed down just enough so that they won’t annihilate each other when they collide.
To see just how common these dust traps were, Dr. Gonzalez and his colleagues conducted a series of computer simulations that took into account how dust in a protoplanetary disk could exert drag on the gas component – a process known as “aerodynamic drag back-reaction”. Whereas gas typically has an arresting influence on dust particles, in particularly dusty rings, the opposite can be true.
This effect has been largely ignored by astronomers up until recently, since its generally quite negligible. But as the team noted, it is an important factor in protoplanetary disks, which are known for being incredibly dusty environments. In this scenario, the effect of back-reaction is to slow inward-moving dust grains and push gas outwards where it forms high-pressure regions – i.e. “dust traps”.
Once they accounted for these effects, their simulations showed how planets form in three basic stages. In the first stage, dust grains grow in size and move inwards towards the central star. In the second, the now pebble-sized larger grains accumulate and slow down. In the third and final stage, the gas is pushed outwards by the back-reaction, creating the dust trap regions where it accumulates.
These traps then allow the pebbles to aggregate to form planetesimals, and eventually planet-sized worlds. With this model, astronomers now have a solid idea of how planetary formation goes from dusty disks to planetesimals coming together. In addition to resolving a key question as to how the Solar System came to be, this sort of research could prove vital in the study of exoplanets.
Ground-based and space-based observatories have already noted the presence of dark and bright rings that are forming in protoplanetary disks around distant stars – which are believed to be dust traps. These systems could provide astronomers with a chance to test this new model, as they watch planets slowly come together. As Dr. Gonzalez indicated:
“We were thrilled to discover that, with the right ingredients in place, dust traps can form spontaneously, in a wide range of environments. This is a simple and robust solution to a long standing problem in planet formation.”
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.
Astronomers everywhere point their telescopes at TW Hydraebecause 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.
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.
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 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.
A planetary system’s early days readily tell of turmoil. Giant planets are swept from distant birthplaces into sizzling orbits close to their host star. Others are blasted away from their star into the darkness of space. And smaller bodies, like asteroids and comets, are being traded around constantly.
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have seen the latter: swarms of Pluto-size objects turning to dust around a young star. And the image is remarkable.
“This system offers us the chance to study an intriguing time around a young, Sun-like star,” said coauthor Stuartt Corder and ALMA Deputy Director in a news release. “We are possibly looking back in time here, back to when the Sun was about 2 percent of its current age.”
The young star, HD 107146, is located roughly 90 light-years from Earth in the direction of the constellation Coma Berenices. Although the star itself is visible in any small telescope, ALMA can probe the star’s radically faint protoplanetary disk. This is the star’s dusty cocoon that coalesces into planets, comets and asteroids.
ALMA’s image revealed an unexpected bump in the number of millimeter-size dust grains far from the host star. This highly concentrated band spans roughly 30 to 150 astronomical units, the equivalent of Neptune’s orbit around the Sun to four times Pluto’s orbit.
So where is the extra dust coming from?
Typically, dust in the debris disk is simply left over material from the formation of planets. Early on, however, Pluto-size objects (otherwise known as planetesimals) will collide and blast themselves apart, also contributing to the dust. Certain models predict that this leads to a much higher concentration of dust in the most distant regions of the disk.
Although this is the case for HD 107146, “this is the opposite of what we see in younger primordial disks where the dust is denser near the star,” said lead author Luca Ricci from the Harvard-Smithsonian Center for Astrophysics. “It is possible that we caught this particular debris disk at a stage in which Pluto-size planetesimals are forming right now in the outer disk while other Pluto-size bodies have already formed closer to the star.”
Adding to this hypothesis is the fact that there’s a slight depression in the dust at 80 astronomical units, or twice Pluto’s average distance from the Sun. This could be a slight gap in the dust, where an Earth-size planet is sweeping the area clear of a debris disk.
If true, this would be the first observation of an Earth-size planet forming so far from its host star. But for now that’s a big if.
While exoplanets make the news on an almost daily basis, one of the biggest announcements occurred in 2012 when astronomers claimed the discovery of an Earth-like planet circling our nearest neighbor, Alpha Centauri B, a mere 4.3 light-years away. That’s almost close enough to touch.
Of course such a discovery has led to a heated debate over the last three years. While most astronomers remain skeptical of this planet’s presence and astronomers continue to study this system, computer simulations from 2008 actually showed the possibility of 11 Earth-like planets in the habitable zone of Alpha Centauri B.
Now, recent research suggests that five of these computer-simulated planets have a high potential for photosynthetic life.
The 2008 study calculated the likely number of planets around Alpha Centauri B by assuming an initial protoplanetary disk populated with 400 – 900 rocks, or protoplanets, roughly the size of the Moon. They then tracked the disk over the course of 200 million years through n-body simulations — models of how objects gravitationally interact with one another over time — in order to determine the total number of planets that would form from the disk.
While the number and type of exoplanets depended heavily on the initial conditions given to the protoplanetary disk, the eight computer simulations predicted the formation of 21 planets, 11 of which resided within the habitable zone of the star.
A second team of astronomers, led by Dr. Antolin Gonzalez of the Universidad Central de Las Villas in Cuba, took these computer simulations one step further by assessing the likelihood these planets are habitable or even contain photosynthetic life.
The team used multiple measures that asses the potential for life. The Earth Similarity index “is a multi-parameter first assessment of Earth-likeness for extrasolar planets,” Dr. Gonzalez told Universe Today. It predicts (on a scale from zero to one with zero meaning no similarity and one being identical to Earth) how Earth-like a planet is based on its surface temperature, escape velocity, mean radius and bulk density.
Planets with an Earth Similar index from 0.8 – 1 are considered capable of hosting life similar to Earth’s. As an example Mars has an Earth Similar index in the range of 0.6 – 0.8. It is thus too low to support life today.
However, the Earth Similarity index alone is not an objective measure of habitability, Gonzalez said. It assumes the Earth is the only planet capable of supporting life. The team also relied on the P model for biological productivity, which takes into account the planet’s surface temperature and the amount of carbon dioxide present.
At this point in time “there is no way to predict, at least approximately, the partial pressure of carbon dioxide with the known data, or the variations from a planet to another,” Gonzalez said. Instead “we assumed a constant partial pressure of carbon dioxide for all planets simplifying the model to a function of temperature.”
Gonzalez’s team found that of the 11 computer-simulated planets in the habitable zone, five planets are prone for photosynthetic life. Their Earth Similarity index values are 0.92, 0.93, 0.87, 0.91 and 0.86. If we take into account their corresponding P model values we find that two of them have better conditions than Earth for life.
According to this highly theoretical paper: if there are planets circling our nearest neighbor, they’re likely to be teeming with life. It’s important to note that while these indexes may prove to be very valuable years down the road (when we have a handful of Earth-like planets to study), we are currently only looking for life as we know it.
The paper has been published in the Cuban journal: Revista Cubana de Fisica and is available for download here. For more information on Alpha Centauri Bb please read a paper available here published in the Astrophysical Journal.
HiCIAO near-infrared image of the protoplanetary disk around PDS 70. The circular mask hides the star itself, as well as a smaller internal disk structure. (Credit: NAOJ)
Over the past couple of decades astronomers have figured out several methods for finding planets around other stars in our galaxy. Some have revealed their presence by the slight “wobble” they impart to their host stars as they orbit, while others have been discovered as they pass in front of their stars from our perspective, briefly dimming the light we see.
Now, some astronomers think they may have identified the presence of multiple planets, based on a large gap found in the disk of gas and dust surrounding a Sun-like star 460 light-years from Earth.
Using the High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) mounted on Japan’s 8.2-meter optical-infrared Subaru telescope atop Mauna Kea in Hawaii, an international team of astronomers targeted PDS 70, a young star (10 million years old) about the same mass as the Sun located 460 light-years away in the constellation Centaurus.
The near-infrared observations made by HiCIAO reveal a protoplanetary disk surrounding PDS 70. This disk is composed of gas and dust and extends billions of miles out from the star. Quite literally the stuff that planets are made of, it’s a disk much like this that our solar system likely started out as over 4.6 billion years ago.
“Thanks to the powerful combination of the Subaru Telescope and HiCIAO, we are able to probe the disks around Sun-like stars. PDS 70 shows how our solar system may have looked in its infancy. I want to continue this kind of research to understand the history of planetary formation.”
– Team Leader Jun Hashimoto (NAOJ)
Within PDS 70’s disk are several large gaps positioned at varying distances from the star itself, appearing as dark regions in the near-infrared data. These gaps — especially the largest, located about 70 AU from the star — are thought to be the result of newly-formed planets having cleared the surrounding space of dust and smaller material. It’s also believed that multiple planets may be present since, according to the team, “no single planet, regardless of how heavy or efficient it is in its formation, is sufficient to create such a giant gap.”
In addition to the large disk structure and outer gap, PDS 70 also has a smaller disk located only 1 AU away. (This disk is obscured by the HiCIAO mask in the image above.)
Further observations will be needed to locate any actual exoplanets directly, since the light from the star and scattered light within the disk makes it difficult — if not impossible with current technology — to detect the incredibly faint light reflected by planets.
Still, it’s fascinating to come across what may very well be a solar system in its infancy, giving us a glimpse back in time to our own formation.
“Direct imaging of planets in the process of forming in protoplanetary disks would be ideal so that we can learn when, where, and how planets form,” said team leader Ruobing Dong of Princeton University.
Read more on the NAOJ website for the Subaru Observatory here.
The goal of the Strategic Exploration of Exoplanets and Disks with Subaru (SEEDS) Project is to study the disks around less massive stars like the Sun.
Inset image: Artist’s rendition of PDS 70 and its two protoplanetary disks (NAOJ)
Catching a ring – or accretion disk – around a star isn’t unusual. However, catching a sharply defined carbon-dioxide ring around a young, magnetic star that’s precisely 1 AU away with a width 0.32 AU or less might raise a few eyebrows. This isn’t just any disk, either… It’s been likened as a “rope-like structure” and there’s even more to the mystery. It’s encircling a Herbig Ae star.
Discovered with the European Southern Observatory’s Very Large Telescope, the edges of this accretion disk are uniquely crisp. Located in the constellation of Centaurus at about 700 light years distant, V1052 (HD 101412) is a parent star with an infrared excess. “HD 101412 is most unusual in having resolved, magnetically split spectral lines which reveal a surface field modulus that varies between 2.5 to 3.5 kG.” says C.R. Cowley (et al). Previous studies “have surveyed molecular emission in a variety of young stellar objects. They found the emission to be much more subdued in Herbig Ae/Be stars than their cooler congeners, the T Tauri stars. This was true for HD 101412 as well, which was among the 25 Herbig Ae/Be stars they discussed. One exception, however, was the molecule CO2, which had a very large flux in HD 101412; indeed, only one T Tauri star had a higher CO2 flux.”
It’s not unusual for carbon dioxide to be found near young stars, but it is a bit more normal for it to be distributed throughout the disk region. “It’s exciting because this is the most constrained ring we’ve ever seen, and it requires an explanation,” explains Cowley, who is professor emeritus at the University of Michigan and leader of the international research effort. “At present time, we just don’t understand what makes it a rope rather than a dish.”
Because V1052 itself is different could be the reason. It is hypothesized the magnetic fields may be holding the rings in the disk structure at a certain distance. The idea has also been forwarded that there may be “shepherding planets”, much like Saturn’s ring structure, which may be the cause. “What makes this star so special is its very strong magnetic field and the fact that it rotates extremely slow compared to other stars of the same type,” said Swetlana Hubrig, of the Leibniz Institute for Astrophysics Potsdam (AIP), Germany.
One thing that is certain is how clean and well-defined the disk lines are centered around the Earth/Sun distance. This accords well with computer modeling where “A wider disk will not fit the observations.” These observations – and the exotic parent star – have been under intense scrutiny since 2008 and the findings have been recently published on-line in Astronomy and Astrophysics. It’s work that helps deepen the understanding of the interaction between central stars, their magnetic fields, and planet-forming disks. It also allows for fact finding when it comes to diverse systems and better knowledge of how solar systems form… even unusual ones.
“Why do turbulent motions not tear the ring apart?” Cowley wondered. “How permanent is the structure? What forces might act to preserve it for times comparable to the stellar formation time itself?”
When it comes to Herbig Ae stars, they are not only rare, but present a rare opportunity for study. In this case, it gives the team something to be quite excited about.
No. It’s not a new atomic image – it’s a very unusual look at a star which could help further our understanding of stellar disk structure and planetary formation. As part of the SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru Telescope/HiCIAO) project, this image of star HR 4796 was taken with Subaru’s planet-finder camera, HiCIAO (High Contrast Instrument for the Subaru Next Generation Adaptive Optics). At only about 8-10 million years old, the feature of this stellar image is only about 240 light years away from Earth, yet fully displays its ring of dust grains which reach out about twice the distance as Pluto’s orbit from the central star. This image produced by an international group led by Motohide Tamura of NAOJ (National Astronomical Observatory of Japan) is so wonderfully detailed that an offset between its center and the star’s position can be measured. While the offset was predicted by data from the Hubble and another research group, this new photographic evidence not only confirms its presence – but shows it to be larger than expected.
With new data to work from, researchers began to wonder exactly what could have caused the dust torus to run off its axis. The easiest explanation would be gravitational force – where one or more planets located inside the gap within the ring could possibly be affecting the disk. This type of action could account for an “unbalancing” which could act in a predictable manner. Current computer modeling has shown these types of “gravitational tides” can mold a dust torus in unusual ways and they cite similar data gathered from observations of bright star, Formalhaut. Since no planet candidates have yet been directly observed around HR 4796, chances are any planets present are simply too small and dim to be spotted. However, thanks to the new Suburu image, researchers feel confident their presence could be the source of the circumstellar dust ring wobble.
With image accuracy as pinpoint as the Hubble Space Telescope, the Suburu near-infrared depiction allows for extremely accurate measurements by employing its adaptive optics system. This type of advanced astrophotography also allows for angular differential imaging – by-passing the glare of the central star and enhancing the faint signature of the dust ring. Such techniques are able to establish heightened information about the relationship of the circumstellar disk and gelling planets… a process which may begin from the “left-overs” of initial star formation. As surmised, this material could either be picked up by newly formed planets or be pushed out the system via stellar winds. Either way, it is a process which eliminates the majority of the dust within a few tens of millions of years. However, there are a few stars which continue to hold on to a “secondary disk” – a collection of dust which could be attributed to the collision of planetesimals. In the case of HR 4796, this is a likely scenario and studying it may provide a better understanding of how planets could form in this alternate debris disk.
When it comes to solar systems, chances are good that we’re a lot more special than we thought. According to a German-British team led by Professor Pavel Kroupa of the University of Bonn, our orderly neighborhood of varied planet sizes quietly orbiting in a nearly circular path isn’t a standard affair. Their new models show that habitable planets might just get ejected in a violent scenario where forming solar systems mean highly inclined orbits where hot Jupiters rule.
Some 4600 million years ago, our local planetary system was surmised to have evolved from a blanket of dust surrounding a rather ordinary star. Its planets orbited the same direction as the solar spin and lined up neatly on a plane fairly close to the solar equator. We were good little children… But maybe other systems aren’t so hospitable. There could be systems where the planets cruise around in the opposite direction of their host star’s spin – and have highly inclined orbits. What could cause one protoplanetary disk to take on quiet properties while another is more radical? Try a cosmic crash.
This new study focuses on the theory of a protoplanetary disk colliding with another cloud of material… not unrealistic thinking since most stars form within a cluster. The results could mean the inclusion of up to thirty times the mass of Jupiter. This added “weight” of extra gas and dust could add a tilt to a forming system. Team member Dr Ingo Thies, also of the University of Bonn, has carried out computer simulations to test the new idea. What he has found is that adding extra material can not only incline a forming disk, but cause a reverse spin as well. It may even speed up the planetary formation, leaving the rogues in retrograde orbits. This inhospitable scenario means that smaller planets get ejected systematically, leaving only hot Jupiters to hug in close to the parent star. Thankfully our path was a bit less disturbing.
Says Dr Thies: “Like most stars, the Sun formed in a cluster, so probably did encounter another cloud of gas and dust soon after it formed. Fortunately for us, this was a gentle collision, so the effect on the disk that eventually became the planets was relatively benign. If things had been different, an unstable planetary system may have formed around the Sun, the Earth might have been ejected from the Solar System and none of us would be here to talk about it.”
Professor Kroupa sees the model as a big step forward. “We may be on the cusp of solving the mystery of why some planetary systems are tilted so much and lack places where life could thrive. The model helps to explain why our Solar System looks the way it does, with the Earth in a stable orbit and larger planets further out. Our work should help other scientists refine their search for life elsewhere in the Universe.”