Are the Gaps in These Disks Caused by Planets?

Astronomers like observing distant young stars as they form. Stars are born out of a molecular cloud, and once enough of the matter in that cloud clumps together, fusion ignites and a star begins its life. The leftover material from the formation of the star is called a circumstellar disk.

As the material in the circumstellar disk swirls around the now-rotating star, it clumps up into individual planets. As planets form in it, they leave gaps in that disk. Or so we think.

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Mars Was Hit By a Lot of Protoplanets Early in its History, Taking Longer to Form than Previously Thought.

There are around 61,000 meteorites on Earth, or at least that’s how many have been found. Out of those, about 200 of them are very special: they came from Mars. And those 200 meteorites have been important clues to how Mars formed in the early Solar System.

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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.

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