That Explains a Lot. The Moon’s Largest Crater has a Chunk of Metal Embedded in it That’s 5 Times Bigger than the Big Island of Hawaii

One of the largest craters in the Solar System is on our Moon. It’s called the South Pole-Aitken (SPA) basin and it’s 2,500 km (1,600 mi) in diameter and 13 km (8.1 mi) deep. A new study says that the basin may contain an enormous chunk of metal that’s larger than Hawaii’s Big Island.

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Bizarre Double Star System Flipped its Planetary Disk on its Side

Astronomers theorize that when our Sun was still young, it was surrounded by a disc of dust and gas from which the planets eventually formed. It is further theorized that the majority of stars in our Universe are initially surrounded in this way by a “protoplanetary disk“, and that in roughly 30% of cases, these disks will go on to become a planet or system of planets.

Ordinarily, these disks are thought to orbit around the equatorial band (aka. the ecliptic) of a star or system of stars. However, new research conducted by an international group of scientists has discovered the first example of a binary star system where the orientation was flipped and the disk now orbits the stars around their poles (perpendicular to the ecliptic).

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Here are 20 Protoplanetary Disks, With Newly Forming Planets Carving Out Gaps in the Gas and Dust

The hunt for other planets in our galaxy has heated up in the past few decades, with 3869 planets being detected in 2,886 systems and another 2,898 candidates awaiting confirmation. Though the discovery of these planets has taught scientists much about the kinds of planets that exist in our galaxy, there is still much we do not know about the process of planetary formation.

To answer these questions, an international team recently used the Atacama Large Millimeter/submillimeter Array (ALMA) to conduct the first large-scale, high-resolution survey of protoplanetary disks around nearby stars. Known as the Disk Substructures at High Angular Resolution Project (DSHARP), this program yielded high-resolution images of 20 nearby systems where dust and gas was in the process of forming new planets.

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Surprising Discovery. Four Giant Planets Found Around a Very Young Star

Researchers have identified a young star with four Jupiter and Saturn-sized planets in orbit around it, the first time that so many massive planets have been detected in such a young system. Image Credit: Amanda Smith, Institute of Astronomy

What exactly is a “normal” solar system? If we thought we had some idea in the past, we definitely don’t now. And a new study led by astronomers at Cambridge University has reinforced this fact. The new study found four gas giant planets, similar to our own Jupiter and Saturn, orbiting a very young star called CI Tau. And one of the planets has an extreme orbit that takes it more than a thousand times more distant from the star than the innermost planet.

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Forming Dense Metal Planets like Mercury is Probably Pretty Difficult and Rare in the Universe

The planet Mercury, the closet planet to our Sun, is something of an exercise in extremes. Its days last longer than its years and at any given time, its sun-facing side is scorching hot while its dark side is freezing cold. It is also one of the least understood planets in our Solar System. While it is a terrestrial (i.e. rocky) planet like Earth, Venus and Mars, it has a significantly higher iron-to-rock ratio than the others.
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This Meteorite is One of the Few Remnants from a Lost Planet that was Destroyed Long Ago

What if our Solar System had another generation of planets that formed before, or alongside, the planets we have today? A new study published in Nature Communications on April 17th 2018 presents evidence that says that’s what happened. The first-generation planets, or planet, would have been destroyed during collisions in the earlier days of the Solar System and much of the debris swept up in the formation of new bodies.

This is not a new theory, but a new study brings new evidence to support it.

The evidence is in the form of a meteorite that crashed into Sudan’s Nubian Desert in 2008. The meteorite is known as 2008 TC3, or the Almahata Sitta meteorite. Inside the meteorite are tiny crystals called nanodiamonds that, according to this study, could only have formed in the high-pressure conditions within the growth of a planet. This contrasts previous thinking around these meteorites which suggests they formed as a result of powerful shockwaves created in collisions between parent bodies.

“We demonstrate that these large diamonds cannot be the result of a shock but rather of growth that has taken place within a planet.” – study co-author Philippe Gillet

Models of planetary formation show that terrestrial planets are formed by the accretion of smaller bodies into larger and larger bodies. Follow the process long enough, and you end up with planets like Earth. The smaller bodies that join together are typically between the size of the Moon and Mars. But evidence of these smaller bodies is hard to find.

One type of unique and rare meteorite, called a ureilite, could provide the evidence to back up the models, and that’s what fell to Earth in the Nubian Desert in 2008. Ureilites are thought to be the remnants of a lost planet that was formed in the first 10 million years of the Solar System, and then was destroyed in a collision.

Ureilites are different than other stony meteorites. They have a higher component of carbon than other meteorites, mostly in the form of the aforementioned nanodiamonds. Researchers from Switzerland, France and Germany examined the diamonds inside 2008 TC3 and determined that they probably formed in a small proto-planet about 4.55 billion years ago.

Philippe Gillet, one of the study’s co-authors, had this to say in an interview with Associated Press: “We demonstrate that these large diamonds cannot be the result of a shock but rather of growth that has taken place within a planet.”

According to the research presented in this paper, these nanodiamonds were formed under pressures of 200,000 bar (2.9 million psi). This means the mystery parent-planet would have to have been as big as Mercury, or even Mars.

The key to the study is the size of the nanodiamonds. The team’s results show the presence of diamond crystals as large as 100 micrometers. Though the nanodiamonds have since been segmented by a process called graphitization, the team is confident that these larger crystals are there. And they could only have been formed by static high-pressure growth in the interior of a planet. A collision shock wave couldn’t have done it.

This is what’s called a High-Angle Annular Dark-Field (HAADF) Scanning Transmission Electron Microscopy (STEM) image. The image on the left shows diamond segments with similar crystal orientations. The image on the right is a magnification of the green square area. The orange lines highlight the inclusion trails, which match between the diamond segments. But those same trails are absent from the intersecting graphite. Image: Farhang Nabiei, Philippe Gillet, et. al.

But the parent body of the ureilite meteorite in the study would have to have been subject to collisions, otherwise where is it? In the case of this meteorite, a collision and resulting shock wave still played a role.

The study goes on to say that a collision took place some time after the parent body’s formation. And this collision would have produced the shock wave that caused the graphitization of the nanodiamonds.

The key evidence is in what are called High-Angle Annular Dark-Field (HAADF) Scanning Transmission Electron Microscopy (STEM) images, as seen above. The image is two images in one, with the one on the right being a magnification of a part of the image on the left. On the left, dotted yellow lines indicate areas of diamond crystals separate from areas of graphite. On the right is a magnification of the green square.

The inclusion trails are what’s important here. On the right, the inclusion trails are highlighted with the orange lines. They clearly indicate inclusion lines that match between adjacent diamond segments. But the inclusion lines aren’t present in the intervening graphite. In the study, the researchers say this is “undeniable morphological evidence that the inclusions existed in diamond before these were broken into smaller pieces by graphitization.”

To summarize, this supports the idea that a small planet between the size of Mercury and Mars was formed in the first 10 million years of the Solar System. Inside that body, large nanodiamonds were formed by high-pressure growth. Eventually, that parent body was involved in a collision, which produced a shock wave. The shock wave then caused the graphitization of the nanodiamonds.

It’s an intriguing piece of evidence, and fits with what we know about the formation and evolution of our Solar System.


Look at This Fascinating Variety of Planet-Forming Disks Around Other Stars

The European Southern Observatory (ESO) has released a stunning collection of images of the circumstellar discs that surround young stars. The images were captured with the SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument on the ESO’s Very Large Telescope (VLT) in Chile. We’ve been looking at images of circumstellar disks for quite some time, but this collection reveals the fascinating variety of shapes an sizes that these disks can take.

New images from the SPHERE instrument on ESO’s Very Large Telescope are revealing the dusty discs surrounding nearby young stars in greater detail than previously achieved. They show a bizarre variety of shapes, sizes and structures, including the likely effects of planets still in the process of forming. Image: ESO/H. Avenhaus et al./E. Sissa et al./DARTT-S and SHINE collaborations

We have a widely-accepted model of star formation supported by ample evidence, including images like these ones from the ESO. The model starts with a cloud of gas and dust called a giant molecular cloud. Within that cloud, a pocket of gas and dust begins to coalesce. Eventually, as gravity causes material to fall inward, the pocket becomes more massive, and exerts even more gravitational pull. More gas and dust continues to be drawn in.

The material that falls in also gives some angular momentum to the pocket, which causes rotation. Once enough material is accumulated, fusion ignites and a star is born. At that point, there is a proto-star inside the cloud, with unused gas and dust remaining in a rotating ring around the proto-star. That left over rotating ring is called a circumstellar disc, out of which planets eventually form.

There are other images of circumstellar discs, but they’ve been challenging to capture. To image any amount of detail in the disks requires blocking out the light of the star at the center of the disk. That’s where SPHERE comes in.

A detailed view of the SPHERE instrument and its main subsystems. SPHERE is installed on the ESO’s VLT and saw first light in 2014. Image: ESO

SPHERE was added to the ESO’s Very Large Telescope in 2014. It’s primary job is to directly image exoplanets, but it also has the ability to capture images of circumstellar discs. To do that, it separates two types of light: polarized, and non-polarized.

Light coming directly from a star—in these images, a young star still surrounded by a circumstellar disc—is non-polarized. But once that starlight is scattered by the material in the disk itself, the light becomes polarized. SPHERE, as its name suggests, is able to separate the two types of light and isolate just the light from the disk. That is how the instrument captures such fascinating images of the disks.

An edge-on view of the disc surrounding the star GSC 07396-00759. The disc extends from the lower-left to the upper-right and the central grey region shows where the star was masked out. Credit:
ESO/E. Sissa et al.

Ever since it became clear that exoplanets are not rare, and that most stars—maybe all stars—have planets orbiting them, understanding solar system formation has become a hot topic. The problem has been that we can’t really see it happening in real time. We can look at our own Solar System, and other fully formed ones, and make guesses about how they formed. But planet formation is hidden inside those circumstellar disss. Seeing into those disks is crucial to understanding the link between the properties of the disk itself and the planets that form in the system.

The discs imaged in this collection are mostly from a study called the DARTTS-S (Discs ARound T Tauri Stars with SPHERE) survey. T Tauri stars are young stars less than 10 million years old. At that age, planets are still in the process of forming. The stars range from 230 to 550 light-years away from Earth. In astronomical terms, that’s pretty close. But the blinding bright light of the stars still makes it very difficult to capture the faint light of the discs.

One of the images is not a T Tauri star and is not from the DARTTS-S study. The disc around the star GSC 07396-00759, in the image above, is actually from the SHINE (SpHere INfrared survey for Exoplanets) survey, though the images itself was captured with SPHERE. GSC 07396-00759 is a red star that’s part of a multiple star system that was part of the DARTTS-S study. The puzzling thing is that red star is the same age as the T TAURI star in the same system, but the ring around the red star is much more evolved. Why the two discs around two stars the same age are so different from each other in terms of time-scale and evolution is a puzzle, and is one of the reasons why astronomers want to study these discs much more closely.

We can study our own Solar System, and look at the positions and characteristics of the planets and the asteroid belt and Kuiper Belt. From that we can try to guess how it all formed, but our only chance to understand how it all came together is to look at other younger solar systems as they form.

The SPHERE instrument, and other future instruments like the James Webb Space Telescope, will allow us to look into the circumstellar discs around other stars, and to tease out the details of planetary formation. These new images from SPHERE are a tantalizing taste of the detail and variety we can expect to see.

Hubble Sees a Huge Dust Cloud Around a Newly Forming Star

Younger stars have a cloud of dusty debris encircling them, called a circumstellar disk. This disk is material left over from the star’s formation, and it’s out of this material that planets form. But scientists using the Hubble have been studying an enormous dust structure some 150 billion miles across. Called an exo-ring, this newly imaged structure is much larger than a circumstellar disk, and the vast structure envelops the young star HR 4796A and its inner circumstellar disk.

Discovering a dust structure around a young star is not new, and the star in this new paper from Glenn Schneider of the University of Arizona is probably our most (and best) studied exoplanetary debris system. But Schneider’s paper, along with capturing this new enormous dust structure, seems to have uncovered some of the interplay between the bodies in the system that has previously been hidden.

Schneider used the Space Telescope Imaging Spectrograph (STIS) on the Hubble to study the system. The system’s inner disk was already well-known, but studying the larger structure has revealed more complexity.

The Hubble Space Telescope has imaged a vast, complex dust structure surrounding the star HR 4769A. The bright, inner ring is well-known to astronomers, but the huge dust structure surrounding the whole system is a new discovery. Image: NASA/ESA/G. Schneider (Univ. of Arizona)

The origin of this vast structure of dusty debris is likely collisions between newly forming planets within the smaller inner ring. Outward pressure from the star HR 4769A then propelled the dust outward into space. The star is 23 times more luminous than our Sun, so it has the necessary energy to send the dust such a great distance.

A press release from NASA describes this vast exo-ring structure as a “donut-shaped inner tube that got hit by a truck.” It extends much further in one direction than the other, and looks squashed on one side. The paper presents a couple possible causes for this asymmetric extension.

It could be a bow wave caused by the host star travelling through the interstellar medium. Or it could be under the gravitational influence of the star’s binary companion (HR 4796B), a red dwarf star located 54 billion miles from the primary star.

“The dust distribution is a telltale sign of how dynamically interactive the inner system containing the ring is'” – Glenn Schneider, University of Arizona, Tucson.

The asymmetrical nature of the vast exo-structure points to complex interactions between all of the stars and planets in the system. We’re accustomed to seeing the radiation pressure from the host star shape the gas and dust in a circumstellar disk, but this study presents us with a new level of complexity to account for. And studying this system may open a new window into how solar systems form over time.

Artist’s impression of circumstellar disk of debris around a distant star. These disk are common around younger stars, but the star in this study has a massive dust cloud that envelops and dwarfs the smaller, inner ring. Credit: NASA/JPL

“We cannot treat exoplanetary debris systems as simply being in isolation. Environmental effects, such as interactions with the interstellar medium and forces due to stellar companions, may have long-term implications for the evolution of such systems. The gross asymmetries of the outer dust field are telling us there are a lot of forces in play (beyond just host-star radiation pressure) that are moving the material around. We’ve seen effects like this in a few other systems, but here’s a case where we see a bunch of things going on at once,” Schneider further explained.

The paper suggests that the location and brightness of smaller rings within the larger dust structure places constraints on the masses and orbits of planets within the system, even when the planets themselves can’t be seen. But that will require more work to determine with any specificity.

This paper represents a refinement and advancement of the Hubble’s imaging capabilities. The paper’s author is hopeful that the same methods using in this study can be used on other similar systems to better understand these larger dust structures, how they form, and what role they play.

As he says in the paper’s conclusion, “With many, if not most, technical challenges now understood and addressed, this capability should be used to its fullest, prior to the end of the HST mission, to establish a legacy of the most robust images of high-priority exoplanetary debris systems as an enabling foundation for future investigations in exoplanetary systems science.”