Carbon-Rich exoplanets

Carbon-Rich Exoplanets May Be Made of Diamonds – “Unlike Anything in Our Solar System” – SciTechDaily

Diamond Planet Rendition

Illustration of a carbon-rich planet with diamond and silica as main minerals. Water can convert a carbide planet into a diamond-rich planet. In the interior, the main minerals would be diamond and silica (a layer with crystals in the illustration). The core (dark blue) might be iron-carbon alloy. Credit: Shim/ASU/Vecteezy

As missions like NASA’s Hubble Space Telescope, TESS, and Kepler continue to provide insights into the properties of exoplanets (planets around other stars), scientists are increasingly able to piece together what these planets look like, what they are made of and if they could be habitable or even inhabited.

In a new study published recently in The Planetary Science Journal, a team of researchers from Arizona State University and the University of Chicago have determined that some carbon-rich exoplanets, given the right circumstances, could be made of diamonds and silica.

“These exoplanets are unlike anything in our solar system,” said lead author Harrison Allen-Sutter of ASU’s School of Earth and Space Exploration.

Carbon Rich Planet Slice

An unaltered carbon planet (left) transforms from a silicon carbide dominated mantle to a silica and diamond dominated mantle (right). The reaction also produces methane and hydrogen. Credit: Harrison/ASU

Diamond exoplanet formation

When stars and planets are formed, they do so from the same cloud of gas, so their bulk compositions are similar. A star with a lower carbon-to-oxygen ratio will have planets like Earth, comprised of silicates and oxides with a very small diamond content (Earth’s diamond content is about 0.001%).

But exoplanets around stars with a higher carbon-to-oxygen ratio than our sun are more likely to be carbon-rich. Allen-Sutter and co-authors Emily Garhart, Kurt Leinenweber, and Dan Shim of ASU, with Vitali Prakapenka and Eran Greenberg of the University of Chicago, hypothesized that these carbon-rich exoplanets could convert to diamond and silicate, if water (which is abundant in the universe) were present, creating a diamond-rich composition.

Diamond Anvils Aligned

In a diamond-anvil cell, two gem quality single crystal diamonds are shaped into anvils (flat top in the photo) and then faced towards each other. Samples are loaded between the culets (flat surfaces), then the sample is compressed between the anvils. Credit: Shim/ASU

Diamond-anvils and X-rays

To test this hypothesis, the research team needed to mimic the interior of carbide exoplanets using high heat and high pressure. To do so, they used high-pressure diamond-anvil cells at co-author Shim’s Lab for Earth and Planetary Materials.

First, they immersed silicon carbide in water and compressed the sample between diamonds to a very high pressure. Then, to monitor the reaction between silicon carbide and water, they conducted laser heating at the Argonne National Laboratory in Illinois, taking X-ray measurements while the laser-heated the sample at high pressures.

As they predicted, with high heat and pressure, the silicon carbide reacted with water and turned into diamonds and silica. 

Diamond Anvil Cells

The cylinder-shaped objects in this photo are diamond anvil cells. The diamond-anvil cells are mounted in copper holders and then inserted into the synchrotron X-ray/laser beam path. The photo shows diamond-anvil cells and mounts before they are aligned for X-ray/laser experiments. Credit: Shim/ASU

Habitability and inhabitability 

So far, we have not found life on other planets, but the search continues. Planetary scientists and astrobiologists are using sophisticated instruments in space and on Earth to find planets with the right properties and the right location around their stars where life could exist.

For carbon-rich planets that are the focus of this study, however, they likely do not have the properties needed for life.

While Earth is geologically active (an indicator of habitability), the results of this study show that carbon-rich planets are too hard to be geologically active and this lack of geologic activity may make atmospheric composition uninhabitable. Atmospheres are critical for life as it provides us with air to breathe, protection from the harsh environment of space and even pressure to allow for liquid water.

“Regardless of habitability, this is one additional step in helping us understand and characterize our ever-increasing and improving observations of exoplanets,” said Allen-Sutter. “The more we learn, the better we’ll be able to interpret new data from upcoming future missions like the James Webb Space Telescope and the Nancy Grace Roman Space Telescope to understand the worlds beyond our own solar system.”

Reference: “Oxidation of the Interiors of Carbide Exoplanets” by H. Allen-Sutter, E. Garhart, K. Leinenweber, V. Prakapenka, E. Greenberg and S.-H. Shim, 26 August 2020, The Planetary Science Journal.

DOI: 10.3847/PSJ/abaa3e

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exoplanets giant

Giant exoplanets directly observed orbiting Sun-like star – Physics World –

Image of exoplanets orbiting a Sun-like Star
Caught on camera: This first-ever image of a multi-planet system around a Sun-like star shows the two planets (centre and bottom right) orbiting TYC 8998-760-1 (Courtesy: ESO/Bohn et al.)

The first ever direct image of a young Sun-like star accompanied by two giant exoplanets has been captured by astronomers using the Very Large Telescope (VLT) in Chile. The two planets are orbiting the star TYC 8998-760-1, which lies 300 light-years away from Earth. The star has an almost identical mass to our Sun, but is just 17 million years old compared with 4.6 billion years for our Sun.

Perhaps the most striking feature of this system is the large mass of the planets and their giant orbits. The inner planet has 14 times Jupiter’s mass and the outer one six times. They orbit at distances of roughly 160 and 320 times the Earth–Sun distance respectively, which is more than 30 times larger than the orbits of Jupiter and Saturn.

This discovery is a snapshot of an environment that is very similar to our solar system, but at a much earlier stage of its evolution.

Alexander Bohn, Leiden University

“This discovery is a snapshot of an environment that is very similar to our solar system, but at a much earlier stage of its evolution,” says Alexander Bohn from at Leiden University in the Netherlands, who led the new research, published in Astrophysical Journal Letters.

A turbulent past

Bohn told Physics World that this system might represent the lower mass end of multiple star formation. In other words, had a bit more stuff accreted from the protostellar cloud, then this could have resulted in a binary star system, rather than one star and two huge planets.

Alternatively, these giant planets could be the result of several small planetessimals clustering together into cores, which eventually gained enough gravitational pull to accrete gas from the circumstellar disc. This is the favoured scenario for the formation of the largest planets in our own solar system.

“To explain the large separations of our detected planets, however, some kind of outward migration is required,” says Bohn. “This can be performed by gravitationally scattering off of each other or with another, so far undetected, third object in the system.”

To date, more than 4000 exoplanets have been detected. However, the vast majority of these have been spotted using indirect methods, such as observing the dip in starlight as a planet transits between its parent star and our line of sight.

Only a tiny fraction of these planets has been directly observed, and the direct imaging of two or more exoplanets around the same star is even rarer. Only two such systems have been directly observed so far, both around stars markedly different from our Sun.

Hot young planets

These latest images were possible due to the SPHERE instrument on the VLT at the European Southern Observatory (ESO) in Chile, which uses a coronagraph to block bright starlight, allowing much fainter planets to be seen. While older planets, such as those seen in our solar system, are too cool to be found with this technique, young planets are hotter, and so glow brighter in infrared light.

This discovery still tells us that there is not just one way to form a planetary system.

Carlo Felice Manara, European Southern Observatory

“This discovery still tells us that there is not just one way to form a planetary system, and the outcome of the process can be very different,” says Carlo Felice Manara, an astronomer based at ESO headquarters in Germany, who was not involved in this latest research. “Why in our solar system we only have planets with mass of Jupiter, or less, and no more massive planets is still an open question.”

Further observations of TYC 8998-760-1 system will enable astronomers to better understand its dynamics. ESO’s Extremely Large Telescope – scheduled to capture its first astronomical images in 2025 – could even detect further planets with Neptune or Saturn masses. Unfortunately, even that instrument is  unlikely to detect rocky planets similar to Earth, as these are just too faint.

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