Study: Stars travel more slowly at Milky Way’s edge
The findings suggest our galaxy’s core may contain less dark matter than previously estimated.
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A STUDY BY MIT PHYSICISTS SUGGEST THE MILKY WAY’S GRAVITATIONAL CORE MAY BE LIGHTER IN MASS, AND CONTAIN LESS DARK MATTER, THAN PREVIOUSLY THOUGHT.
view moreCREDIT: ESA/GAIA/DPAC
By clocking the speed of stars throughout the Milky Way galaxy, MIT physicists have found that stars further out in the galactic disk are traveling more slowly than expected compared to stars that are closer to the galaxy’s center. The findings raise a surprising possibility: The Milky Way’s gravitational core may be lighter in mass, and contain less dark matter, than previously thought.
The new results are based on the team’s analysis of data taken by the Gaia and APOGEE instruments. Gaia is an orbiting space telescope that tracks the precise location, distance, and motion of more than 1 billion stars throughout the Milky Way galaxy, while APOGEE is a ground-based survey. The physicists analyzed Gaia’s measurements of more than 33,000 stars, including some of the farthest stars in the galaxy, and determined each star’s “circular velocity,” or how fast a star is circling in the galactic disk, given the star’s distance from the galaxy’s center.
The scientists plotted each star’s velocity against its distance to generate a rotation curve — a standard graph in astronomy that represents how fast matter rotates at a given distance from the center of a galaxy. The shape of this curve can give scientists an idea of how much visible and dark matter is distributed throughout a galaxy.
“What we were really surprised to see was that this curve remained flat, flat, flat out to a certain distance, and then it started tanking,” says Lina Necib, assistant professor of physics at MIT. “This means the outer stars are rotating a little slower than expected, which is a very surprising result.”
The team translated the new rotation curve into a distribution of dark matter that could explain the outer stars’ slow-down, and found the resulting map produced a lighter galactic core than expected. That is, the center of the Milky Way may be less dense, with less dark matter, than scientists have thought.
“This puts this result in tension with other measurements,” Necib says. “There is something fishy going on somewhere, and it’s really exciting to figure out where that is, to really have a coherent picture of the Milky Way.”
The team reports its results this month in the Monthly Notices of the Royal Society Journal. The study’s MIT co-authors, including Necib, are first author Xiaowei Ou, Anna-Christina Eilers, and Anna Frebel.
“In the nothingness”
Like most galaxies in the universe, the Milky Way spins like water in a whirlpool, and its rotation is driven, in part, by all the matter that swirls within its disk. In the 1970s, astronomer Vera Rubin was the first to observe that galaxies rotate in ways that cannot be driven purely by visible matter. She and her colleagues measured the circular velocity of stars and found that the resulting rotation curves were surprisingly flat. That is, the velocity of stars remained the same throughout a galaxy, rather than dropping off with distance. They concluded that some other type of invisible matter must be acting on distant stars to give them an added push.
Rubin’s work in rotation curves was one of the first strong pieces of evidence for the existence of dark matter — an invisible, unknown entity that is estimated to outweigh all the stars and other visible matter in the universe.
Since then, astronomers have observed similar flat curves in far-off galaxies, further supporting dark matter’s presence. Only recently have astronomers attempted to chart the rotation curve in our own galaxy with stars.
“It turns out it’s harder to measure a rotation curve when you’re sitting inside a galaxy,” Ou notes.
In 2019, Anna-Christina Eilers, assistant professor of physics at MIT, worked to chart the Milky Way’s rotation curve, using an earlier batch of data released by the Gaia satellite. That data release included stars as far out as 25 kiloparsecs, or about 81,000 light years, from the galaxy’s center.
Based on these data, Eilers observed that the Milky Way’s rotation curve appeared to be flat, albeit with mild decline, similar to other far-off galaxies, and by inference, the galaxy likely bore a high density of dark matter at its core. But this view now shifted, as the telescope released a new batch of data, this time including stars as far out as 30 kiloparsecs — almost 100,000 light years from the galaxy’s core.
“At these distances, we’re right at the edge of the galaxy where stars start to peter out,” Frebel says. “No one had explored how matter moves around in this outer galaxy, where we’re really in the nothingness.”
Weird tension
Frebel, Necib, Ou, and Eilers jumped on Gaia’s new data, looking to expand on Eilers’ initial rotation curve. To refine their analysis, the team complemented Gaia’s data with measurements by APOGEE — the Apache Point Observatory Galactic Evolution Experiment, which measures extremely detailed properties of more than 700,000 stars in the Milky Way, such as their brightness, temperature, and elemental composition.
“We feed all this information into an algorithm to try to learn connections that can then give us better estimates of a star’s distance,” Ou explains. “That’s how we can push out to farther distances.”
The team established the precise distances for more than 33,000 stars and used these measurements to generate a three-dimensional map of the stars scattered across the Milky Way out to about 30 kiloparsecs. They then incorporated this map into a model of circular velocity, to simulate how fast any one star must be traveling, given the distribution of all the other stars in the galaxy. They then plotted each star’s velocity and distance on a chart to produce an updated rotation curve of the Milky Way.
“That’s where the weirdness came in,” Necib says.
Instead of seeing a mild decline like previous rotation curves, the team observed that the new curve dipped more strongly than expected at the outer end. This unexpected downturn suggests that while stars may travel just as fast out to a certain distance, they suddenly slow down at the farthest distances. Stars at the outskirts appear to travel more slowly than expected.
When the team translated this rotation curve to the amount of dark matter that must exist throughout the galaxy, they found that the Milky Way’s core may contain less dark matter than previously estimated.
“This result is in tension with other measurements,” Necib says. “Really understanding this result will have deep repercussions. This might lead to more hidden masses just beyond the edge of the galactic disk, or a reconsideration of the state of equilibrium of our galaxy. We seek to find these answers in upcoming work, using high resolution simulations of Milky Way-like galaxies."
This research was funded, in part, by the National Science Foundation.
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Written by Jennifer Chu, MIT News
Paper: “The dark matter profile of the Milky Way inferred from its circular velocity curve”
https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stae034/7513209
JOURNAL
Monthly Notices of the Royal Astronomical Society
ARTICLE TITLE
The dark matter profile of the Milky Way inferred from its circular velocity curve”
UC Irvine-led team unravels mysteries of planet formation and evolution in distant solar system
Astronomers analyze masses, orbital properties and atmospheric features of six exoplanets
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A TEAM OF RESEARCHERS LED BY ASTRONOMERS AT UCI HAS OBTAINED HIGHLY PRECISE INFORMATION ABOUT SIX CONFIRMED EXOPLANETS ORBITING TOI-1136, A DWARF STAR ABOUT 270 LIGHT YEARS FROM EARTH.
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CREDIT: CREDIT: RAE HOLCOMB / UCI
A recently discovered solar system with six confirmed exoplanets and a possible seventh is boosting astronomers’ knowledge of planet formation and evolution. Relying on a globe-spanning arsenal of observatories and instruments, a team led by researchers at the University of California, Irvine has compiled the most precise measurements yet of the exoplanets’ masses, orbital properties and atmospheric characteristics.
In a paper published today in The Astronomical Journal, the researchers share the results of the TESS-Keck Survey, providing a thorough description of the exoplanets orbiting TOI-1136, a dwarf star in the Milky Way galaxy more than 270 light years from Earth. The study is a follow-up to the team’s initial observation of the star and exoplanets in 2019 using data from the Transiting Exoplanet Survey Satellite. That project provided the first estimate of the exoplanets’ masses by clocking transit timing variations, a measure of the gravitational pull that orbiting planets exert on one another.
For the most recent study, the researchers joined TTV data with a radial velocity analysis of the star. Using the Automated Planet Finder telescope at the Lick Observatory on California’s Mount Hamilton and the High-Resolution Echelle Spectrometer at the W.M. Keck Observatory on Hawaii’s Mauna Kea, they could detect slight variations in stellar motion via the redshift and blueshift of the Doppler effect – which helped them determine planetary mass readings of unprecedented precision.
To obtain such exact information on the planets in this solar system, the team built computer models using hundreds of radial velocity measurements layered over TTV data. Lead author Corey Beard, a UCI Ph.D. candidate in physics, said that combining these two types of readings yielded more knowledge about the system than ever before.
“It took a lot of trial and error, but we were really happy with our results after developing one of the most complicated planetary system models in exoplanet literature to date,” Beard said.
The large number of planets is one factor that inspired the astronomy team to conduct further research, according to co-author Paul Robertson, UCI associate professor of physics & astronomy.
“We viewed TOI-1136 as being highly advantageous from a research standpoint, because when a system has multiple exoplanets, we can control for the effects of planet evolution that depend on the host star, and that helps us focus on individual physical mechanisms that led to these planets having the properties that they do,” he said.
Robertson added that when astronomers try to compare planets in separate solar systems, there are many variables that can differ based on the distinct properties of the stars and their locations in disparate parts of the galaxy. He said that looking at exoplanets in the same system enables the study of planets that have experienced a similar history.
By stellar standards, TOI-1136 is young, a mere 700 million years old, another feature that has attracted exoplanet hunters. Robertson said that juvenile stars are both “difficult and special” to work with because they’re so active. Magnetism, sunspots and solar flares are more prevalent and intense during this stage of a star’s development, and the resulting radiation blasts and sculpts planets, affecting their atmospheres.
TOI-1136’s confirmed exoplanets, TOI-1136 b through TOI-1136 g, are categorized as “sub-Neptunes” by the experts. Robertson said the smallest one is more than twice the radius of Earth, and others are up to four times Earth’s radius, comparable to the sizes of Uranus and Neptune.
All these planets orbit TOI-1136 in less than the 88 days it takes Mercury to go around Earth’s sun, according to the study. “We’re packing an entire solar system into a region around the star so small that our entire planetary system here would be outside of it,” Robertson said.
“They’re weird planets to us because we don’t have anything exactly like them in our solar system,” said co-author Rae Holcomb, a UCI Ph.D. candidate in physics. “But the more we study other planet systems, it seems like they may be the most common type of planet in the galaxy.”
Another odd component to this solar system is the possible yet unconfirmed presence of a seventh planet. The researchers have detected some evidence of another resonant force in the system. Robertson explained that when planets are orbiting close to one another, they can pull on each other gravitationally.
“When you hear a chord played on a piano and it sounds good to you, it’s because there is resonance, or even spacing, between the notes that you’re hearing,” he said. “The orbital periods of these planets are spaced similarly. When the exoplanets are in resonance, the tugs are in the same direction every time. This can have a destabilizing effect, or in special cases, it can serve to make the orbits more stable.”
Robertson noted that far from answering all his team’s questions about the exoplanets in this system, the survey has made the researchers want to pursue additional knowledge, particularly about the composition of planetary atmospheres. That line of inquiry would be best approached through the advanced spectroscopy capabilities of NASA’s James Webb Space Telescope, he said.
“I am proud that both UCO’s Lick Observatory and the Keck Observatories were involved in the characterization of a really important system,” said Matthew Shetrone, deputy director of UC Observatories. “Having so many moderate-sized planets in the same system really lets us test formation scenarios. I really want to know more about these planets! Might we find a molten rock world, a water world and an ice world all in the same solar system? It almost feels like science fiction.”
Joining Robertson and Beard on this study were researchers from Spain’s Astrophysics Institute of the Canary Islands; the California Institute of Technology; Sweden’s Chalmers University of Technology; Maryland’s Johns Hopkins University; Spain’s University of La Laguna; Sweden’s Lund University; Poland’s Nicolaus Copernicus University; New Jersey’s Princeton University; Japan’s Ritsumeikan University; California’s SETI Institute; Maryland’s Space Telescope Science Institute; the University of California, Santa Cruz; the University of California, Berkeley; the University of California, Los Angeles; the University of California, Riverside; the University of Hawaii; the University of Chicago; the University of Kansas; Indiana’s University of Notre Dame; Australia’s University of Southern Queensland; and Connecticut’s Yale University. Funding was provided by the W.M. Keck Foundation, NASA and the National Science Foundation.
About the University of California, Irvine: Founded in 1965, UCI is a member of the prestigious Association of American Universities and is ranked among the nation’s top 10 public universities by U.S. News & World Report. The campus has produced five Nobel laureates and is known for its academic achievement, premier research, innovation and anteater mascot. Led by Chancellor Howard Gillman, UCI has more than 36,000 students and offers 224 degree programs. It’s located in one of the world’s safest and most economically vibrant communities and is Orange County’s second-largest employer, contributing $7 billion annually to the local economy and $8 billion statewide. For more on UCI, visit www.uci.edu.
Media access: Radio programs/stations may, for a fee, use an on-campus ISDN line to interview UCI faculty and experts, subject to availability and university approval. For more UCI news, visit news.uci.edu. Additional resources for journalists may be found at https://news.uci.edu/media-resources/.
JOURNAL
The Astronomical Journal
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
The TESS-Keck Survey XVII: Precise Mass Measurements in a Young, High Multiplicity Transiting Planet System using Radial Velocities and Transit Timing Variations
ARTICLE PUBLICATION DATE
29-Jan-2024
Confirmation of ancient lake on Mars builds excitement for Perseverance rover's samples
Findings reveal eons of environmental changes and offer hope that the rover’s soil and rock samples hold traces of life
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MARS PERSEVERANCE ROVER RIMFAX GROUND PENETRATING RADAR MEASUREMENTS OF THE HAWKSBILL GAP REGION OF THE JEZERO CRATER WESTERN DELTA, MARS. HAWKSBILL GAP.
view moreCREDIT: SVEIN-ERIK HAMRAN, TOR BERGER, DAVID PAIGE, UNIVERSITY OF OSLO, UCLA, CALIFORNIA INSTITUTE OF TECHNOLOGY JET PROPULSION LABORATORY, NASA
If life ever existed on Mars, the Perseverance rover’s verification of lake sediments at the base of the Jezero crater reinforces the hope that traces might be found in the crater.
In new research published in the journal Science Advances, a team led by UCLA and The University of Oslo shows that at some point, the crater filled with water, depositing layers of sediments on the crater floor. The lake subsequently shrank and sediments carried by the river that fed it formed an enormous delta. As the lake dissipated over time, the sediments in the crater were eroded, forming the geologic features visible on the surface today.
The periods of deposition and erosion took place over eons of environmental changes, the radar indicates, confirming that inferences about the Jezero crater’s geologic history based on Mars images obtained from space are accurate.
“From orbit we can see a bunch of different deposits, but we can’t tell for sure if what we’re seeing is their original state, or if we’re seeing the conclusion of a long geological story,” said David Paige, a UCLA professor of Earth, planetary and space sciences and first author of the paper. “To tell how these things formed, we need to see below the surface.”
The rover, which is about the size of a car and carries seven scientific instruments, has been exploring the 30-mile-wide crater, studying its geology and atmosphere and collecting samples since 2021. Perseverance’s soil and rock samples will be brought back to Earth by a future expedition and studied for evidence of past life.
Between May and December 2022, Perseverance drove from the crater floor onto the delta, a vast expanse of 3 billion-year-old sediments that, from orbit, resembles the river deltas on Earth.
As the rover drove onto the delta, Perseverance’s Radar Imager for Mars’ Subsurface Experiment, or RIMFAX, instrument fired radar waves downward at 10-centimeter intervals and measured pulses reflected from depths of about 20 meters below the surface. With the radar, scientists can see down to the base of the sediments to reveal the top surface of the buried crater floor.
Years of research with ground-penetrating radar and testing of RIMFAX on Earth have taught scientists how to read the structure and composition of subsurface layers from their radar reflections. The resulting subsurface image shows rock layers that can be interpreted like a highway road cut.
“Some geologists say that the ability of radar to see under the surface is kind of like cheating,” said Paige, who is RIMFAX’s deputy principal investigator.
RIMFAX imaging revealed two distinct periods of sediment deposition sandwiched between two periods of erosion. UCLA and the University of Oslo report that the crater floor below the delta is not uniformly flat, suggesting that a period of erosion occurred prior to the deposition of lake sediments. The radar images show that the sediments are regular and horizontal - just like sediments deposited in lakes on Earth. The existence of lake sediments had been suspected in previous studies, but has been confirmed by this research.
A second period of deposition occurred when fluctuations in the lake level allowed the river to deposit a broad delta that once extended far out into the lake, but has now eroded back closer to the river’s mouth.
“The changes we see preserved in the rock record are driven by large-scale changes in the Martian environment,” Paige said. “It’s cool that we can see so much evidence of change in such a small geographic area, which allows us extend our findings to the scale of the entire crater.”
Mars Perseverance Rover RIMFAX ground penetrating radar measurements of the Hawksbill Gap region of the Jezero Crater Western Delta, Mars.
CREDIT
Svein-Erik Hamran, Tor Berger, David Paige, University of Oslo, UCLA, California Institute of Technology Jet Propulsion Laboratory, NASA
Cape_Nukshak_3D_Radar_Profile
Mars Perseverance Rover RIMFAX ground penetrating radar measurements of the Cape Nukshak region of the Jezero Crater Western Delta, Mars.
CREDIT
Svein-Erik Hamran, Tor Berger, David Paige, University of Oslo, UCLA, California Institute of Technology Jet Propulsion Laboratory, NASA
Perseverance_Sol_641_Traverse (VIDEO)
AI-interpolated video from NAVCAM images from the NASA Perseverance Rover’s as it traversed down the Jezero Western Delta from Cape Nukshak to the Crater Floor on Sol 641.
CREDIT
Lior Rubanenko, Emily Cardarelli, Justin Maki, David Paige, UCLA, California Institute of Technology Jet Propulsion Laboratory, NASA
JOURNAL
Science Advances
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Ground penetrating radar observations of the contact between the western delta and the crater floor of Jezero Crater, Mars
ARTICLE PUBLICATION DATE
26-Jan-2024
Agence France-Presse
January 26, 2024
The Hubble Space Telescope NASA/AFP/File /
The Hubble Space Telescope has observed the smallest planet outside our solar system to contain water vapor in its atmosphere, a "landmark discovery" that brings astronomy a step closer to characterizing Earth-like worlds.
At around twice Earth's diameter, planet GJ 9827d orbits a red dwarf star 97 light-years away in the constellation Pisces, NASA and the European Space Agency (ESA) said in statements on Thursday.
The team behind the finding are examining two scenarios: either the planet is a "mini-Neptune" with a hydrogen-rich atmosphere laced with water, or it's a warmer version of Jupiter's moon Europa, which contains twice as much water as Earth under its crust.
"The planet GJ 9827d could be half water, half rock," said Bjorn Benneke of the Universite de Montreal, who co-led the research. "And there would be a lot of water vapor on top of some smaller rocky body."
"Until now, we had not been able to directly detect the atmosphere of such a small planet. And we're slowly getting in this regime now," he added.
Over a period of three years, Hubble observed the planet during 11 transients, or events in which it crossed in front of its star.
During transients, starlight is filtered through the planet's atmosphere, allowing astronomers to use Hubble's instruments to analyze the patterns of colors (wavelengths), which revealed the telltale signature of water molecules.
Even if GJ 9827d has a water-rich atmosphere, its hot, Venus-like temperature of 800 degrees Fahrenheit (425 Celsius) would make it an uninhabitable steamy world.
Nevertheless, the Hubble discovery paves the way to future study of GJ 9827d and similar planets, especially by the James Webb Space Telescope, which can use its high resolution infrared images to look for more atmospheric molecules including carbon dioxide, and methane.
"Water on a planet this small is a landmark discovery," added co-leader Laura Kreidberg of Max Planck Institute for Astronomy in Germany.
"It pushes closer than ever to characterizing truly Earth-like worlds."
By AFP
Astronomers have nicknamed the stars "old smokers"
Daniel Lawler
Nearing the end of their life, they sit quietly for long periods of time, barely noticeable, before suddenly puffing out a cloud of smoke.
A mysterious new type of star nicknamed “old smoker” has been discovered hiding in the heart of our Milky Way galaxy, astronomers revealed on Friday.
The “peculiar” puffing behaviour of these stars has never been seen before in such red giants, astrophysicist Philip Lucas told AFP.
The international team of scientists behind the discovery had not been looking for such old stars during their 10-year survey, which took in hundreds of millions of stars across the sky.
Instead they were using the VISTA telescope in the Chilean Andes to search for newborn stars — called proto-stars — which are prone to frequent, exuberant eruptions.
A newborn star, called a proto-star, observed by the VISTA telescope in the Chilean Andes. — AFP
They spotted 32 proto-stars, “the largest number anyone has ever found before in one batch”, said Lucas, a professor at the UK’s University of Hertfordshire and lead author of a new study.
But lingering in the background was a “nice surprise”, he added.
– ‘We don’t fully understand’ –
The old smokers were puffing right in the centre of the Milky Way, a densely packed and metal-rich region called the Nuclear Stellar Disc.
“What was surprising about this new discovery is that we’re seeing stars that were just sitting doing nothing at all,” Lucas said.
Then abruptly the stars would become between 40 to 100 times dimmer, sometimes so faint that the telescope’s infrared vision could barely spot them.
A couple of years later, seemingly without warning, they would return to their former brightness.
“Everything we have been able to learn about them suggests that this is a case of stars throwing off puffs of smoke — for reasons that we don’t fully understand,” Lucas said.
The “old smokers” are about 30,000 light years away, near the centre of our galaxy. — ©AFP
Those smoke puffs are thought to temporarily obscure the stars from our sight.
There are many more “heavy elements” — anything heavier than hydrogen and helium — in this region of the galaxy, which could create more dust in the star’s atmosphere, Lucas said.
Exactly what puffs that dust out remains a mystery.
But if this theory is correct, then the amount of matter being thrown off by these stars could play a significant role in how heavy elements are spread throughout our galaxy — and beyond, he said.
Lucas emphasised that these were just early best-guesses.
“We’re just sort of scrabbling around trying to see what makes the most sense,” he said.
The researchers spotted at least 21 old smokers but suspect there are many more out there.
The study was published in the Monthly Notices of the Royal Astronomical Society.
By AFP
January 19, 2024
The shape-shifting SORA-Q probe.
On board Japan’s “Moon Sniper” spacecraft is a little robot with a big mission: to pop open like a Transformer toy, wiggle across the lunar surface and beam images back to Earth.
The shape-shifting SORA-Q probe — co-developed by a major toy company — has been compared to a friendly “Star Wars” droid and a sea turtle because of the way its metal form can navigate the dusty Moonscape.
But the gadget’s chance to boldly go depends on the success of the Smart Lander for Investigating Moon (SLIM) mission, with a spacecraft dubbed the “Moon Sniper” by space agency JAXA for its precision landing capabilities.
The lightweight craft is due to begin its descent from lunar orbit at midnight on Saturday morning (1500 GMT Friday), with touchdown planned around 20 minutes later.
But success, which would make Japan the fifth nation to land on the Moon after the United States, the Soviet Union, China and India, is far from guaranteed.
Slightly bigger than a tennis ball and weighing as much as a large potato — eight centimetres (three inches) across and 250 grams (half a pound) — SORA-Q was designed by JAXA with Takara Tomy, the toy company behind the original 1984 Transformers.
Sony Group and Doshisha University in Kyoto also helped develop the device, which has a front camera on an orange panel that emerges when the its metal frame snaps open, and another on its back.
Instead of rolling on wheels, the two halves of the sphere are designed to slot out and move in tandem to propel SORA-Q along the rocky surface, a design that reduces size and weight.
“The form-shifting mechanism and ultra-compact, ultra-lightweight design have been created using the technical know-how of toy development,” the probe’s website reads.
It can move in two ways, allowing it to drive on inclines: “‘Butterfly’ driving, where both the left and right wheels move together, and ‘crawl’ driving, where they move separately,” the site explains.
Sora means “universe” in Japanese, while “Q” refers to the words “question” and “quest”, its makers say.
If the mission succeeds, the probe’s cameras will take valuable images of a crater where parts of the Moon’s mantle, usually hidden deep below its crust, are believed to be exposed.
Back on Earth, a toy version of the probe costs 21,190 yen ($140) and can roll around a living room to take pictures of cats and babies, according to its promotional video.
