Keith Cooper
Mon, October 16, 2023
An illustration of a pale mauve planet against the dark sky. The planet is very close to our vantage point, much of it is off-screen.
Thousand-mile-per-hour winds are blowing a hail of tiny quartz crystals through the silicate-enhanced, scorching hot atmosphere of a distant gas giant planet called WASP-17b, the James Webb Space Telescope (JWST) has found.
"We knew from Hubble [Space Telescope] observations that there must be aerosols — tiny particles making up clouds or haze — in WASP-17b’s atmosphere, but we didn’t expect them to be made of quartz," Daniel Grant of the University of Bristol in the UK and leader of a new study on the discovery, said in a statement.
WASP-17b is an incredible world. Orbiting every 3.7 days at a distance of just 7.8 million kilometers (4.9 million miles) from its star, which sits 1,300 light years away from Earth, WASP-17b is so close to its stellar host that its dayside temperature rises to a staggering 1,500 degrees Celsius (approximately 2,700 degrees Fahrenheit). Because the atmosphere is so hot on this exoplanet, the world has actually expanded to about 285,000 kilometers (176,892 miles) across, which is just shy of twice the diameter of Jupiter. And that's despite WASP-17b having only about half of Jupiter’s overall mass. WASP-17b is one of the "puffiest" planets known — and its bloated atmosphere makes it a great target for the James Webb Space Telescope.
Related: James Webb Space Telescope spotlights gorgeous young stars in a galaxy next door (photo)
Grant and fellow astronomers watched WASP-17b transit its star using the JWST’s Mid-Infrared Instrument (MIRI). As the exoplanet moved in front of its star from the JWST's point of view, MIRI detected starlight that was blocked by the puffy planet itself but partially absorbed by the world's atmosphere. Such measurements result in a so-called transmission spectrum, whereby certain wavelengths are blocked out by particular atmospheric molecules.
Like Jupiter, WASP-17b appeared to be mostly made from hydrogen and helium. In addition, MIRI detected carbon dioxide, water vapor and, at a wavelength of 8.6 microns, the absorption signature of pure quartz crystals. Combined with previous observations with the Hubble Space Telescope, these crystals are judged to be shaped like the same pointy, hexagonal prisms as quartz is on Earth, but just a meager 10 nanometers in size.
The transmission spectrum of WASP-17b, showing how the quartz is blocking light at a wavelength of 8.6 microns. (Image credit: NASA/ESA/CSA/Ralf Crawford (STScI))
Quartz is a form of silicate, which are minerals rich in silica and oxygen. Silicates are exceptionally common — all the rocky bodies in the solar system are made from them, and silicates have previously been detected in the atmospheres of hot Jupiter exoplanets before. However, in those cases they had been more complex, magnesium-rich crystals of olivine and pyroxene.
"We fully expected to see magnesium silicates," said Bristol’s Hannah Wakeford. "But what we’re seeing instead are likely the building blocks of those, the tiny seed particles need to form the larger silicate grains we detect in cooler exoplanets and brown dwarfs."
WASP-27b is also tidally locked, meaning it always shows the same face to its star. As winds whip around the planet, carrying along the quartz nanoparticles, they form high-altitude hazes — essentially diffuse clouds of rock crystals — at the day–night termination zone. Those hazes then venture into the dayside, and are vaporized in the heat.
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Grant explained how crystals of silicate come to be embedded in a planetary atmosphere in the first place.
"WASP-17b is extremely hot … and the pressure where the quartz crystals form high in the atmosphere is only about one-thousandth of what we experience on Earth’s surface," he said. "In these conditions, solid crystals can form directly from gas, without going through a liquid phase first."
The findings were published in October in Astrophysical Journal Letters.
The Hubble Tension Is Extremely Real—and Extremely Frustrating
Jackie Appel
Tue, October 17, 2023
JWST Has Cemented One Of The Biggest Astro PuzzlesNASA/JPL/STScI Hubble Deep Field Team
Recent measurements from JWST have confirmed the validity of the “Hubble tension.”
The Hubble tension stems from the fact that measuring the Hubble constant—the rate of the universe’s expansion—two different ways will get you two different answers.
Having now pretty much ruled out that the Hubble telescope made a mistake, the question turns to what we don’t know that could be causing the discrepancy.
In the beginning, there was a bang. A … Big Bang, if you will. This massive explosion created the universe and filled it with the seeds of all the matter and energy currently present.
But with that kind of turbo-kickstart, growth isn’t something that ends after the initial blast. The universe continues to expand—even accelerating as it goes—at a rate referred to as the Hubble constant.
The Hubble constant is an incredibly important number to pin down in the field of cosmology, but it’s not an easy one to suss out. In fact, it’s so difficult to divine that the “Hubble tension” caused by our inability to land on a single number has become arguably as famous as the Hubble constant itself. And recent results from the James Webb Space Telescope (JWST) have just re-cemented this tension as a very real problem to overcome.
If a researcher wants to estimate the Hubble constant, they have two main avenues. There are others, of course, but the two most common are Cepheid variables and the cosmic microwave background.
The cosmic microwave background (CMB) is the leftover radiation still permeating the universe from the Big Bang. Using the CMB is often likened by researchers to using a baby picture to predict what someone will look like as an adult. To some extent, knowledge about the universe’s infancy should allow researchers to make predictions about its current state. Researchers have long used it to make inferences about the very beginnings of our universe, and have used it to predict what the Hubble constant should be. Fairly recent CMB calculations set the Hubble constant at around 68 km s⁻¹ Mpc⁻¹.
But the thing about theory is that you need to test it. And when the Hubble telescope launched, it became possible to do just that. Free of the constraints of ground-based telescopes clouded by atmosphere, the Hubble telescope was able to take extremely accurate distance measurements of far-off objects. And that’s where Cepheid variables come in.
Cepheid variables are a type of pulsating star that falls into a category of objects called “standard candles”—objects that have a measurable and consistent brightness, allowing us to fairly straightforwardly measure how far away they are. Measuring distances isn’t easy in space, but it’s crucial to being able to measure expansion.
As convenient as it would be to stop there, there’s one more step to the Cepheid method. Once their brightnesses have been divined, and we know how bright a Cepheid is that blinks at a certain rate, we use those brightnesses to calculate the brightnesses of nearby supernovae. Then, using a quality called “redshift,” we use those supernovae to calculate the brightnesses of other supernovae that are even farther away.
Because they’re much brighter, we can see supernovae at much greater distances than we can see Cepheids. And because you can see the expansion of the universe better the further away you look, we look to the far-off supernovae for the ultimate calculation. This chain of brightnesses is referred to as a “distance ladder,” and you can’t move to one rung without first calculating all the ones that come before.
Over time, the Hubble telescope clocked distances for several Cepheid variables (the first rung on that distance ladder), which in turn allowed researchers to make an experimental measurement of the Hubble constant. But instead of agreeing with the prediction, it came out totally different: 73 km s⁻¹ Mpc⁻¹.
When experiment and theory diverge, it can mean one of two things—either your measurement is wrong, or your theory is wrong. And that, in a nutshell is the Hubble tension. Which is wrong, the theory or the experiment?
This new round of measurements from the JWST seems to cement the idea that that there’s something wrong with the theory. Able to basically re-run Hubble’s observations with a higher resolution at wavelengths in which Hubble struggled to get clear pictures, JWST has pretty much confirmed that Hubble wasn’t seeing ghosts. It really is producing data that allow for an accurate calculation of the Hubble constant of the current universe.
So, what’s wrong with the theory, then? Well, if you can answer that, you’re light-years ahead of the best astrophysicists in the business. It could have something to do with dark energy, or dark matter, or gravity. Maybe we’re seeing all of the data fine, but we’re somehow wrong about the fundamental nature of Cepheid variables. Right now, we just don’t know.
But we’re certainly going to keep looking. Tension is just another word for puzzle in astrophysics.
Tue, October 17, 2023
JWST Has Cemented One Of The Biggest Astro PuzzlesNASA/JPL/STScI Hubble Deep Field Team
Recent measurements from JWST have confirmed the validity of the “Hubble tension.”
The Hubble tension stems from the fact that measuring the Hubble constant—the rate of the universe’s expansion—two different ways will get you two different answers.
Having now pretty much ruled out that the Hubble telescope made a mistake, the question turns to what we don’t know that could be causing the discrepancy.
In the beginning, there was a bang. A … Big Bang, if you will. This massive explosion created the universe and filled it with the seeds of all the matter and energy currently present.
But with that kind of turbo-kickstart, growth isn’t something that ends after the initial blast. The universe continues to expand—even accelerating as it goes—at a rate referred to as the Hubble constant.
The Hubble constant is an incredibly important number to pin down in the field of cosmology, but it’s not an easy one to suss out. In fact, it’s so difficult to divine that the “Hubble tension” caused by our inability to land on a single number has become arguably as famous as the Hubble constant itself. And recent results from the James Webb Space Telescope (JWST) have just re-cemented this tension as a very real problem to overcome.
If a researcher wants to estimate the Hubble constant, they have two main avenues. There are others, of course, but the two most common are Cepheid variables and the cosmic microwave background.
The cosmic microwave background (CMB) is the leftover radiation still permeating the universe from the Big Bang. Using the CMB is often likened by researchers to using a baby picture to predict what someone will look like as an adult. To some extent, knowledge about the universe’s infancy should allow researchers to make predictions about its current state. Researchers have long used it to make inferences about the very beginnings of our universe, and have used it to predict what the Hubble constant should be. Fairly recent CMB calculations set the Hubble constant at around 68 km s⁻¹ Mpc⁻¹.
But the thing about theory is that you need to test it. And when the Hubble telescope launched, it became possible to do just that. Free of the constraints of ground-based telescopes clouded by atmosphere, the Hubble telescope was able to take extremely accurate distance measurements of far-off objects. And that’s where Cepheid variables come in.
Cepheid variables are a type of pulsating star that falls into a category of objects called “standard candles”—objects that have a measurable and consistent brightness, allowing us to fairly straightforwardly measure how far away they are. Measuring distances isn’t easy in space, but it’s crucial to being able to measure expansion.
As convenient as it would be to stop there, there’s one more step to the Cepheid method. Once their brightnesses have been divined, and we know how bright a Cepheid is that blinks at a certain rate, we use those brightnesses to calculate the brightnesses of nearby supernovae. Then, using a quality called “redshift,” we use those supernovae to calculate the brightnesses of other supernovae that are even farther away.
Because they’re much brighter, we can see supernovae at much greater distances than we can see Cepheids. And because you can see the expansion of the universe better the further away you look, we look to the far-off supernovae for the ultimate calculation. This chain of brightnesses is referred to as a “distance ladder,” and you can’t move to one rung without first calculating all the ones that come before.
Over time, the Hubble telescope clocked distances for several Cepheid variables (the first rung on that distance ladder), which in turn allowed researchers to make an experimental measurement of the Hubble constant. But instead of agreeing with the prediction, it came out totally different: 73 km s⁻¹ Mpc⁻¹.
When experiment and theory diverge, it can mean one of two things—either your measurement is wrong, or your theory is wrong. And that, in a nutshell is the Hubble tension. Which is wrong, the theory or the experiment?
This new round of measurements from the JWST seems to cement the idea that that there’s something wrong with the theory. Able to basically re-run Hubble’s observations with a higher resolution at wavelengths in which Hubble struggled to get clear pictures, JWST has pretty much confirmed that Hubble wasn’t seeing ghosts. It really is producing data that allow for an accurate calculation of the Hubble constant of the current universe.
So, what’s wrong with the theory, then? Well, if you can answer that, you’re light-years ahead of the best astrophysicists in the business. It could have something to do with dark energy, or dark matter, or gravity. Maybe we’re seeing all of the data fine, but we’re somehow wrong about the fundamental nature of Cepheid variables. Right now, we just don’t know.
But we’re certainly going to keep looking. Tension is just another word for puzzle in astrophysics.
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