(Image credit: forplayday/iStock/Getty Images) We have learned a lot about the planets in our own backyard, and for a long time we assumed the rest of the galaxy looked roughly the same.
(Image credit: Louise Lerner)
Vast clouds of soot that form in the pressure cooker of mysterious mini-Neptune exoplanets may hold the truth about these worlds' origins."It's like you have a natural diesel engine in the deep atmosphere of a planet," lead author of a study about this research, Jeehyun Yang of the University of Chicago, said in a statement.Yang did his Ph.D. in chemical engineering, studying the exhausts of combustion engines before transitioning to study the chemistry of exoplanet atmospheres. The exhaust fumes of diesel engines are filled with black smoke made up of honeycomb-shaped particles called PAHs — polycyclic aromatic hydrocarbons. PAHs are among the most common carbon-based compounds in the cosmos, and are frequently produced whenever we burn something. (That black char on your burned toast? That's made of PAHs too.)When it comes to chemistry, some exoplanet atmospheres are more enigmatic. Take the mini-Neptunes — worlds in the size range between Earth and Neptune that are found orbiting close to their star. Despite their being the most common type of exoplanet found so far, debate continues to rage over the nature of these mid-size worlds. Are they miniature versions of hydrogen-rich gas giants like Jupiter? Are they literally smaller versions of Neptune and Uranus, rich in volatiles such as water? Or could they be habitable hycean worlds, with a dense atmosphere of hydrogen concealing a global ocean?Nobody knows for sure, and their characteristics could be varied enough that all three may apply. What is agreed upon, however, is that the mini-Neptunes did not form as close to their star as they are seen now; instead they formed farther out before migrating in. If we could answer how far out they formed, it could tell us what kind of world they are likely to be.Unfortunately, probing the chemistry of these worlds' atmospheres doesn't help much, because these atmospheres seem to be opaque, hiding the true composition of the planets. Scientific consensus is that this opaqueness is caused by hazy banks of clouds that are masking the atmospheres, but what kind of aerosol particles are in the clouds?When Yang saw the featureless spectra that the James Webb Space Telescope (JWST) was producing whenever it looked at a mini-Neptune, he noticed a distinct curve in the data that he recognized instantly as like the curve seen in the spectra of soot from a combustion engine.PAHs can form when carbon, hydrogen and oxygen react at high temperatures, often combined with high pressure, just like the conditions deep in the atmosphere of some mini-Neptunes. Yang suspects that the same reactions that take place in a combustion engine could be occurring naturally within certain mini-Neptunes, producing PAHS that amalgamate as clouds of soot that then rise higher into the atmosphere, perhaps driven upwards by thermal convection currents. What we would then see as an opaque atmosphere would in actual fact be hazy, planet-spanning clouds of soot.While the soot would explain why the JWST sees featureless spectra, it could also help solve a much more profound mystery: where did mini-Neptunes form and migrate in from?
An artist's impression of a mini-Neptune. (Image credit: NASA/ESA/CSA/D. Player (STScI))Planets form in disks of gas and dust whose properties vary with distance from their central star. Take our solar system for instance. Heavier metallic and silicate materials were found in the disk closer to the Sun, while lighter gases and frozen volatiles such as water-ice and carbon dioxide-ice were found farther out, and this is replicated in the inner planets being rocky, Jupiter and Saturn being formed of the light gases hydrogen and helium, and Uranus and Neptune being rich in frozen volatiles.Determining the ratio of carbon to oxygen in a mini-Neptune's soot could act as a measure of how far out from their star they formed, and therefore what their bulk properties are likely to be. We'd finally be able to differentiat
One of the arms of the Milky Way Galaxy seen from Patagonia, Argentina. (Image credit: Natapong Supalertsophon/Getty Images)
Our home galaxy didn't pop into existence all at once. The Milky Way was formed gradually, as smaller galaxies, or dwarf galaxies, were subsumed into our own galaxy over billions of years.It turns out that the stars leftover from these dwarf galaxies still share characteristics, and scientists are getting better at identifying them. By studying their similarities, scientists use these stars to determine their galaxies of origin. A team of astronomers say that they have identified a sample of these 20 stars that — due to their similar features — may have grown up together in a dwarf galaxy which the researchers have dubbed "Loki.""We might have detected one of the various small systems that contributed to form our Milky Way," astronomer Federico Sestito, a postdoctoral fellow at University of Hertfordshire and study coauthor, told Space.com via email.The study, published in the Monthly Notices of the Royal Astronomical Society, builds on previous work from Sestitio. He had already identified the stars that they ended up surveying for the new study. But now, Sestitio and the team have new features that they can use to identify stars' original galaxies."This work can be thought of as a sort of follow-up of previous works," Sestitio said. "In the past, we had to look at these old stars with peculiar motion; however, we lacked chemical information, which is now available with this work."Growing up togetherHelium and hydrogen were main ingredients for the early stars that were formed in our universe. Once formed, the stars fused these two elements together, which created heavier elements that made later generations of stars. This process happened again and again over many generations.Those early stars are considered "metal-poor." Because they formed so early, the stars only have traces of the heavier elements, like iron. Being metal-poor is one of the identifiers the scientists used to figure out which stars formed in the same dwarf galaxy."We think these old and metal-poor stars were formed in one small galaxy that was ingested by the forming Milky Way," Sestitio says.But it's not just that these 20 stars are metal-poor; scientists have identified many stars in our galaxy that share this feature. The stars' elemental makeup isn't sufficient for determining the galaxy. To narrow it down, the team considered other features like location and orbit."[The stars] orbital motion is peculiar as they are confined close to the Milky Way disc, which is usually populated by younger and metal-rich stars," Sestitio says.The Milky Way disc is the circular flowing whirlpool-like structure, where most of our galaxy's stars, including our own sun, are located. The 20 stars' unique positioning was another indication that they might all be related."This was possible thanks to precise orbital motion and chemical information of metal-poor and old stars," Sestitio says.While the orbital motion of these stars has been previously identified and studied, the chemical information is new, and it gave the researchers a much stronger indication for the stars' shared galaxy of origin.
Artist's conception of the Milky Way galaxy. (Image credit: NASA/JPL-Caltech/R. Hurt)Chemically uniqueThe features that the team needed to study were diverse, so they used a patchwork of methods."I think my favorite part of this research is having put together various techniques and methodologies to better understand the origin of these stars," Sestitio said.The astronomers used high-resolution spectroscopy, orbital motion, and even theoretical simulations to interpret the stars' chemical and orbital characteristics."We are providing a complete picture, as much as we can, of the properties of these stars," Sestitio said.The team compared the chemical properties in the stars to those of stars in the galactic halo, dwarf galaxies, as well as simulated populat
(Image credit: forplayday/iStock/Getty Images)
We have learned a lot about the planets in our own backyard, and for a long time we assumed the rest of the galaxy looked roughly the same. A rocky planet meant a clear-cut structure: a dense metallic core, a silicate mantle, and a thin atmosphere on top. That picture works fine for Earth.But according to a new paper submitted to the Astrophysical Journal, it might not work for most of the rocky planets in the universe. By far the most common type of planet we have found around other stars is about a class of worlds called sub-Neptunes: planets larger than Earth but smaller than Neptune. Their close cousins, the super-Earths, are slightly smaller and likely lost most of their hydrogen long ago. The textbook story has these planets forming in essentially the same way Earth did, just with different amounts of leftover gas piled on top. Iron sinks to the middle, silicate rock floats above it, hydrogen sits on top of that.But here is the wrinkle. At the pressures and temperatures inside a sub-Neptune, hydrogen, silicate, and iron don’t actually behave like they do near the surface of Earth. Above about 4,000 degrees Kelvin, hydrogen and molten silicate become fully miscible. They stop being oil and water. They become one fluid. The authors behind a new study submitted to the Astrophysical Journal and currently available on arXiv worked out what that means for the structure of these planets, and the answer is surprising.If a planet accretes less than about one percent of its mass in hydrogen, it follows the familiar script and forms a discrete metallic core just like Earth. But if it picks up more hydrogen than that, the whole inside of the planet becomes a single, mixed, churning fluid of iron, silicate, and hydrogen. No core. No mantle. Just a homogeneous blend all the way down to within a few thousand kilometers of the center.That is a significant departure from how we usually draw these worlds in cross-section. The internal structure determines how a planet cools, how it holds onto its atmosphere, and how its radius evolves over time. The authors find that this miscibility framework can reproduce a number of features we already see in the exoplanet population that the old layered-cake models struggled to explain.One of those features is the radius gap, the curious deficit of planets right between super-Earth and sub-Neptune sizes that the James Webb Space Telescope and Kepler Space Telescope have mapped out.Another is the way planet radii depend on orbital period. Both fall out naturally if you assume that young sub-Neptunes store a substantial fraction of their hydrogen inside this miscible interior, then slowly release it into the outer envelope as the planet cools and the miscibility region shrinks. The hydrogen literally bubbles out of the rock over hundreds of millions of years.
Artist's impression of a sub-Neptune exoplanet. (Image credit: Pablo Carlos Budassi/Stocktrek Images/Getty Images)There is a testable consequence here, and that is what makes this paper more than a thought experiment. If hydrogen is gradually exsolving from the interior into the atmosphere, then young sub-Neptunes should contract more slowly than standard models predict.They should look slightly puffier than they should be for their age. We are now starting to find sub-Neptunes around very young stars (cosmic toddlers, only tens of millions of years old) where this signature could actually be measured. JWST and the next generation of transit surveys are going to put numbers on it.The caveats are real. The model rests on theoretical extrapolations of how hydrogen, silicate, and iron behave at conditions we can’t yet reproduce in a laboratory, although high-pressure experiments are starting to catch up. The internal heat budgets of these planets are still uncertain, and small errors in those parameters propagate into the predictions. And the inverse modeling approach the authors use (start with the obs
Alien Life Could Look Nothing Like What We Expect. Here's How Microbes Beyond Earth Might Live Without Liquid Water Smithsonian Magazine
Astronomers detect a solar system they say should not be possible CNN
Discussion (0)