protoplanetary disk – astroengine.com

This Weird Star System Is Flipping Awesome

The binary system observed by ALMA isn’t wonky, it’s the first example of a polar protoplanetary disk

Artwork of the system HD 98000. This is a binary star comprising two sun-like components, surrounded by a thick disk of material. What’s different about this system is that the plane of the stars’ orbits is inclined at almost 90 degrees to the plane of the disk. Here is a view from the surface of an imagined planet orbiting in the inner edge of the disk [University of Warwick/Mark Garlick].

Some star systems simply don’t like conforming to cosmic norms. Take HD 98000, for example: It’s a binary system consisting of two sun-like stars and it also sports a beautiful protoplanetary disk of gas and dust. So far, so good; sounds pretty “normal” to me. But that’s only part of the story.

When a star is born, it will form a disk of dust and gas — basically the leftovers of the molecular cloud the star itself formed in — creating an environment in which planets can accrete and evolve. Around a single star (like our solar system) the protoplanetary disk is fairly well behaved and will create a relatively flat disk around the star’s spin axis. For the solar system, this flat disk would have formed close to the plane of the ecliptic, an imaginary flat surface that projects out from the sun’s equator where all the planets, more or less, occupy. There are “wonky” exceptions to this rule (as, let’s face it, cosmic rules are there to be broken), but the textbook descriptions of a star system in its infancy will usually include a single star and a flat, boring disk of swirling material primed to build planets.

Cue HD 98000, a star system that has flipped this textbook description on its head, literally. As a binary, this is very different to what we’re used to with our single, lonely star. Binary stars are very common throughout the galaxy, but HD 98000 has a little something extra that made astronomers take special note. As observed by the Atacama Large Millimeter/sub-millimeter Array (ALMA), its protoplanetary disk doesn’t occupy the same plane as the binary orbit; it’s been flipped by 90 degrees over the orbital plane of the binary pair. Although such systems have been long believed to be theoretically possible, this is the first example that has been found.

“Discs rich in gas and dust are seen around nearly all young stars, and we know that at least a third of the ones orbiting single stars form planets,” said Grant M. Kennedy, of the University of Warwick and lead author of the study published today in the journal Nature Astronomy, in a statement. “Some of these planets end up being misaligned with the spin of the star, so we’ve been wondering whether a similar thing might be possible for circumbinary planets. A quirk of the dynamics means that a so-called polar misalignment should be possible, but until now we had no evidence of misaligned discs in which these planets might form.”

Artwork of the system HD 98000. This is a binary star comprising two sun-like components, surrounded by a thick disc of material [University of Warwick/Mark Garlick]

This star system makes for some rather interesting visuals, as shown in the artist’s impression at the top of the page. Should there be a planetary body orbiting the stars on the inner edge of the disk, an observer would be met with a dramatic pillar of gas and dust towering into space with the two stars either side of it in the distance. As they orbit one another, the planetary observer would see them switch positions to either side of the pillar. It goes without saying that any planet orbiting two stars would have very different seasons than Earth. It will even have two different shadows cast across the surface.

“We used to think other solar systems would form just like ours, with the planets all orbiting in the same direction around a single sun,” added co-author Daniel Price of Monash University. “But with the new images we see a swirling disc of gas and dust orbiting around two stars. It was quite surprising to also find that that disc orbits at right angles to the orbit of the two stars.”

Interestingly, the researchers note that there are another two stars orbiting beyond the disk, meaning that our hypothetical observer would have four suns of different brightnesses in the sky.

The most exciting thing to come out of this study, however, is that ALMA has detected signatures that hint at dust growth in the disk, meaning that material is in the process of clumping together. Planetary formation theories suggest that accreting dust will go on to form small asteroids and planetoids, creating a fertile enviornment in which planets can evolve.

“We take this to mean planet formation can at least get started in these polar circumbinary discs,” said Kennedy. “If the rest of the planet formation process can happen, there might be a whole population of misaligned circumbinary planets that we have yet to discover, and things like weird seasonal variations to consider.”

What was that I was saying about “cosmic norms”? When it comes to star system formation, there doesn’t appear to be any.

Reference: https://warwick.ac.uk/newsandevents/pressreleases/double_star_system
Paper: https://www.nature.com/articles/s41550-018-0667-x

Wonky Star Systems May Be Born That Way

A nearby baby star has been discovered with a warped protoplanetary disk — a feature that may reveal the true nature of the solar system’s planetary misalignments

Textbook descriptions of our solar system often give the impression that all the planets orbit the sun in well-behaved near-circular orbits. Sure, there’s a few anomalies, but, in general, we’re led to believe that everything in our interplanetary neighborhood travels around the sun around a flat orbital plane. This, however, isn’t exactly accurate.

Pluto, for example, has an orbit around the sun that is tilted by over 17 degrees out of the plane of the ecliptic (an imaginary flat plane around which the Earth orbits the sun). Mercury has an inclination of seven degrees. Even Venus likes to misbehave and has an orbital inclination of over three degrees. If all the material that built the planets originated from the same protoplanetary disk that was — as all the artist’s impressions would have us believe — flat, what knocked all the planet’s out of alignment with the ecliptic?

Until now, it was assumed that, during the early epoch of our solar system’s planet-forming days, dynamic chaos ruled. Planets jostled for gravitational dominance, Jupiter bullied smaller worlds into other orbits (possibly chucking one or two unfortunates into deep space), and gravitational instabilities threw the rest into disorderly orbital paths. Other star systems also exhibit this orbital disorder, so perhaps it’s just an orbital consequence of a star system’s growing pains.

But there might be another contribution to the chaos: perhaps wonky star systems were just born that way.

Cue a recent observation campaign of the nearby baby star L1527. Located 450 light-years away in the direction of the Taurus Molecular Cloud, L1527 is a protostar embedded in a thick protoplanetry disk. Using the Atacama Large Millimeter/submillimeter Array (ALMA), in Chile, astronomers of the RIKEN Cluster for Pioneering Research (CPR) and Chiba University in Japan discovered that the L1527 disk is actually two disks morphed into one — both of which are out of alignment with one another. Imagine a vinyl record that has been left on a heater and you wouldn’t be far off visualizing what this baby star system looks like.

The RIKEN study, published on Jan. 1 in Nature, suggests that this warping may have been caused by jets of material emanating from the star’s birth, kicking planet-forming material into this warped configuration and, should this configuration remain stable, could result in planets with orbital planes that are significantly out of alignment.

“This observation shows that it is conceivable that the misalignment of planetary orbits can be caused by a warp structure formed in the earliest stages of planetary formation,” said team leader Nami Sakai in a RIKEN press release. “We will have to investigate more systems to find out if this is a common phenomenon or not.”

It’s interesting to think that if this protoplanetary disk warping is due to the mechanics behind the formation of the star itself, we might be able to look at mature star systems to see the ancient fingerprint of a star’s earliest outbursts or, possibly, its initial magnetic environment.

It’s possible “that irregularities in the flow of gas and dust in the protostellar cloud are still preserved and manifest themselves as the warped disk,” added Sakai. “A second possibility is that the magnetic field of the protostar is in a different plane from the rotational plane of the disk, and that the inner disk is being pulled into a different plane from the rest of the disk by the magnetic field.”

Though orbital chaos undoubtedly contributed to how our solar system looks today, with help of this research, we may be also getting a glimpse of how warped our sun’s protoplanetry disk may have been before the planets even formed.

Great Balls of ‘Space Mud’ May Have Built Earth and Delivered Life’s Ingredients

When imagining how our planet formed 4.6 billion years ago from the protoplanetary disk surrounding our sun, images of large pieces of marauding space rock slamming into the molten surface of our proto-Earth likely come to mind.

But this conventional model of planetary creation may be missing a small, yet significant, detail. Those massive space rocks may not have been the conventional solid asteroids — they might have been massive balls of space mud.

This strange detail of planetary evolution is described in a new study published in the American Association for the Advancement of Science (AAAS) journal Science Advances and it kinda makes logical sense.

Using the wonderfully-named Mars and Asteroids Global Hydrology Numerical Model (or “MAGHNUM”), planetary scientists Phil Bland (Cornell University) and Bryan Travis (Planetary Science Institute) simulated the movement of material inside primordial carbonaceous chondrite asteroids — i.e. the earliest asteroids that formed from the sun’s protoplanetary disk that eventually went on to become the building blocks for Earth.

A simulated cross section of a 200-meter wide asteroid showing its internal temperature profile and convection currents (temperatures in Celsius). Credit: PSI

It turns out that these first asteroids weren’t cold and solid lumps of rock at all. By simulating the distribution of rock grains inside these asteroids, the researchers realized that the internal heat of the objects would have melted the icy volatiles inside, which then mixed with the fine dust particles. Convection would have then dominated a large portion of these asteroids, causing continuous mixing of water and dust. Like a child squishing a puddle of dirt to create sloppy “mud pies,” this convection would have formed a ball of, you guessed it, space mud.

Travis points out that “these bodies would have accreted as a high-porosity aggregate of igneous clasts and fine-grained primordial dust, with ice filling much of the pore space. Mud would have formed when the ice melted from heat released from decay of radioactive isotopes, and the resulting water mixed with fine-grained dust.”

In other words: balls of mud held together by mutual gravity, gently convected by the heat produced by the natural decay of radioactive materials.

Should this model hold up to further scrutiny, it has obvious implications for the genesis of life on Earth and could impact the study of exoplanets and their habitable potential. The ingredients for life on Earth originated in the primordial protoplanetary soup, but until now the assumption has been that the space rocks carrying water and other chemicals were solid and frozen. If they were in fact churning away in space as dynamic mud asteroids, they could have been the “pressure cookers” that delivered those ingredients to Earth’s surface.

So the next question would be: how did these exotic asteroids shape life on Earth?

Mysterious Fomalhaut b Might Not Be an Exoplanet After All

The famous exoplanet was the first to be directly imaged by Hubble in 2008 but many mysteries surround its identity — so astronomers are testing the possibility that it might actually be an exotic neutron star.

Located 25 light-years from Earth, the bright star Fomalhaut is quite the celebrity. As part of a triple star system (its distant, yet gravitationally bound siblings are orange dwarf TW Piscis Austrini and M-type red dwarf LP 876-10) Fomalhaut is filled with an impressive field of debris, sharing a likeness with the Lord Of The Rings’ “Eye of Sauron.” And, in 2008, the eerie star system shot to fame as the host of the first ever directly-imaged exoplanet.

At the time, the Hubble Space Telescope spotted a mere speck in Fomalhaut’s “eye,” but in the years that followed the exoplanet was confirmed — it was a massive exoplanet approximately the size of Jupiter orbiting the star at a distance of around 100 AU (astronomical units, where 1 AU is the average distance the Earth orbits the sun). It was designated Fomalhaut b.

This was a big deal. Not only was it the first direct observation of a world orbiting another star, Hubble was the aging space telescope that found it. Although the exoplanet was confirmed in 2013 and the International Astronomical Union (IAU) officially named the exoplanet “Dagon” after a public vote in 2015, controversy surrounding the exoplanet was never far away, however.

Astronomers continue to pick at Fomalhaut’s mysteries and, in new research to be published in the journal Monthly Notices of the Royal Astronomical Society, Fomalhaut b’s identity has been thrown into doubt yet again.

“It has been hypothesized to be a planet, however there are issues with the observed colors of the object that do not fit planetary models,” the researchers write. “An alternative hypothesis is that the object is a neutron star in the near fore- or background of Fomalhaut’s disk.” The research team is lead by Katja Poppenhaeger, of Queen’s University, Belfast, and a preprint of their paper (“A Test of the Neutron Star Hypothesis for Fomalhaut b”) can be found via arXiv.org.

Fomalhaut b was detected in visible and near-infrared wavelengths, but followup studies in other wavelengths revealed some peculiarities. For starters, the object is very bright in blue wavelengths, something that doesn’t quite fit with exoplanetary formation models. To account for this, theorists pointed to a possible planetary accretion disk like a system of rings. This may be the reason for the blue excess; the debris is reflecting more starlight than would be expected to be reflected by the planet alone. However, when other studies revealed the object is orbiting outside the star system’s orbital plane, this explanation wasn’t fully consistent with what astronomers were seeing.

Other explanations were put forward — could it be a small, warm world with lots of planetesimals surrounding it? Or is it just a clump of loosely-bound material and not a planet at all? — but none seem to quite fit the bill.

In this new research, Poppenhaeger’s team pondered the idea that Fomalhaut b might actually be a neutron star either in front or behind the Fomalhaut debris disk and, although their work hasn’t proven whether Fomalhaut b is an exoplanet or not, they’ve managed to put some limits on the neutron star hypothesis.

Neutron stars are the left-overs of massive stars that have run out of fuel and gone supernova. They are exotic objects that are extremely dense and small and, from our perspective, may produce emissions in visible and infrared wavelengths that resemble a planetary body. Cool and old neutron stars will even generate bluer light, which could explain the strange Fomalhaut b spectra.

Neutron stars also produce ultraviolet light and X-rays and, although it is hard to separate the UV light coming from the exoplanet and the UV light coming from the star, X-ray emissions should be resolvable.

Artist’s impression of a magnetar, an extreme example of a neutron star (ESO/L.Calçada)

So, using observations from NASA’s Chandra X-ray Observatory, the researchers looked at Fomalhaut b in soft X-rays and were able to put some pretty strong constraints on whether or not this object really could be a neutron star. As it turned out, Chandra didn’t detect X-rays (within its capabilities). This doesn’t necessarily mean that it isn’t a neutron star, it constrains what kind of neutron star it could be. Interestingly, it also reveals how far away this object could be.

Assuming it is a neutron star with a typical radius of 10 kilometers, and as no X-ray emissions within Chandra’s wavelength range were detected, this object would be a neutron star with a surface temperature cooler than 90,000 Kelvin — revealing that it is over 10 million years old. For this hypothesis to hold, the neutron star would actually lie behind the Fomalhaut system, around 44 light-years (13.5 parsecs) from Earth.

Further studies are obviously needed and, although the researchers point out that Fomalhaut b is still most likely an exoplanet with an extensive ring system (just with some strange and as-yet unexplained characteristics), it’s interesting to think that it could also be a neutron star that isn’t actually in the Fomalhaut system at all. In fact, it could be the closest neutron star to Earth, providing a wonderful opportunity for astronomical studies of these strange and exotic objects.