Astronomy

In the solar system’s early years, the still-forming giant planets sidestepped, did a do-si-do and then swung one of their partners away from the sun’s gravitational grasp. Things settled, and our planetary system was in its final configuration.

What triggered that planetary shuffle has been unknown. Now, computer simulations suggest that the hot radiation of the young sun evaporating its planet-forming disk of gas and dust led to the scrambling of the giant planets’ orbits, researchers report in the April 28 Nature.

As a result, the four largest planets may have been in their final configuration within 10 million years of the solar system’s birth about 4.6 billion years ago. That’s much quicker than the 500 million years that previous work had suggested.

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The planetary-shuffling mechanism that the team uncovered in the computer simulations is very innovative, says Nelson Ndugu, an astrophysicist who studies forming planetary systems at North-West University in Potchefstroom, South Africa, and Muni University in Arua, Uganda. “It has huge potential.”

Heaps of evidence, including observations of extrasolar planetary systems forming (SN: 7/2/18), had already indicated that something in our solar system’s early history jumbled the giant planets’ orbits, which scientists call the giant-planet instability (SN: 5/25/05).

“The evidence for the giant-planet instability is really robust,” says Seth Jacobson, a planetary scientist at Michigan State University in East Lansing. “It explains many features of the outer solar system,” he says, like the large number of rocky objects beyond Neptune that make up the Kuiper Belt (SN: 12/31/09).

To figure out what triggered that instability, Jacobson and colleagues ran computer simulations of the thousands of ways that the early solar system could have developed. All started with a young star and a planet-forming disk of gas and dust surrounding the star. The team then altered the disk parameters, such as its mass, density and how fast it evolved.

The simulations also included the still-forming giant planets — five of them, in fact. Astronomers think a third ice giant, in addition to Uranus and Neptune, was originally a solar system member (SN: 4/20/12). Jupiter and Saturn round out the final tally of these massive planets.

When the sun officially became a star, that is, the moment it began burning hydrogen at its core — roughly 4.6 billion years ago — its ultraviolet emission would have hit the disk’s gas, ionizing it and heating it to tens of thousands of degrees. “This is a very well-documented process,” Jacobson says. As the gas heats, it expands and flows away from the star, beginning with the inner portion of the disk.

“The disk disperses its gas from inside out,” says Beibei Liu, an astrophysicist at Zhejiang University in Hangzhou, China. He and Jacobson collaborated with astronomer Sean Raymond of Laboratoire d’Astrophysique de Bordeaux in France in the new research.

In the team’s simulations, as the inner part of the disk dissolves, that area loses mass, so the embedded, still-forming planets feel less gravity from that region, Jacobson says. But the planets still feel the same amount of pull from the disk’s outer region. This gravitational torquing, as the team calls it, can trigger a rebound effect: “Originally, the planets migrate in, and they reach the [inner] edge of this disk, and they reverse their migration,” Liu says.

Because of Jupiter’s large mass, it’s mostly unaffected. Saturn, though, moves outward and into the region, which, in the simulations, holds the three ice giant planets. That area becomes crowded, Liu says, and close planetary interactions follow. One ice giant gets kicked out of the solar system entirely, Uranus and Neptune shift a bit farther from the sun, and “they gradually form the orbits close to our solar system’s configuration,” Liu says.

In their computer simulations, the researchers found that as the sun’s radiation evaporates the disk, a planetary reshuffle nearly always ensues. “We can’t avoid this instability,” Jacobson says.

Now that the researchers have an idea of what may have caused this solar system shuffle, the next step is to simulate how the evaporation of the disk could affect other objects.

“We’ve focused really heavily on the giant planets, because their orbits were the original motivation,” Jacobson says. “But now, we have to do the follow-up work to show how this trigger mechanism relates to the small bodies.” More

If you’re an aspiring life-form, you might want to steer clear of planets around orange dwarf stars.

Some astronomers have called these orange suns “Goldilocks stars” (SN: 11/18/09). They are dimmer and age more slowly than yellow sunlike stars, thus offering an orbiting planet a more stable climate. But they are brighter and age faster than red dwarfs, which often spew large flares. However, new observations show that orange dwarfs emit lots of ultraviolet light long after birth, potentially endangering planetary atmospheres, researchers report in a paper submitted March 29 at arXiv.org.

Using data from the Hubble Space Telescope, astronomer Tyler Richey-Yowell and her colleagues examined 39 orange dwarfs. Most are moving together through the Milky Way in two separate groups, either 40 million or 650 million years old.

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To Richey-Yowell’s surprise, she and her team found that the ultraviolet flux didn’t drop off from the younger orange stars to the older ones — unlike the case for yellow and red stars. “I was like, `What the heck is going on?’” says Richey-Yowell, of Arizona State University in Tempe.

In a stroke of luck, another team of researchers supplied part of the answer. As yellow sunlike stars age, they spin more slowly, causing them to be less active and emit less UV radiation. But for orange dwarfs, this steady spin-down stalls when the stars are roughly a billion years old, astronomer Jason Lee Curtis at Columbia University and colleagues reported in 2019.

“[Orange] stars are just much more active for a longer time than we thought they were,” Richey-Yowell says. That means these possibly not-so-Goldilocks stars probably maintain high levels of UV light for more than a billion years.

And that puts any potential life-forms inhabiting orbiting planets on notice. Far-ultraviolet light — whose photons, or particles of light, have much more energy than the UV photons that give you vitamin D — tears molecules in a planet’s atmosphere apart. That leaves behind individual atoms and electrically charged atoms and groups of atoms known as ions. Then the star’s wind — its outflow of particles — can carry the ions away, stripping the planet of its air.

But not all hope is lost for aspiring life-forms that have an orange dwarf sun. Prolonged exposure to far-ultraviolet light can stress planets but doesn’t necessarily doom them to be barren, says Ed Guinan, an astronomer at Villanova University in Pennsylvania who was not involved in the new work. “As long as the planet has a strong magnetic field, you’re more or less OK,” he says.

Though far-ultraviolet light splits water and other molecules in a planet’s atmosphere, the star’s wind can’t remove the resulting ions if a magnetic field as strong as Earth’s protects them. “That’s why the Earth survived” as a life-bearing world, Guinan says. In contrast, Venus might never have had a magnetic field, and Mars lost its magnetic field early on and most of its air soon after.

“If the planet doesn’t have a magnetic field or has a weak one,” Guinan says, “the game is over.”

What’s needed, Richey-Yowell says, is a study of older orange dwarfs to see exactly when their UV output declines. That will be a challenge, though. The easiest way to find stars of known age is to study a cluster of stars, but most star clusters get ripped apart well before their billionth birthday (SN: 7/24/20). As a result, star clusters somewhat older than this age are rare, which means the nearest examples are distant and harder to observe. More

Fragmenting planets sweeping extremely close to their stars might be the cause of mysterious cosmic blasts of radio waves.

Milliseconds-long fast radio bursts, or FRBs, erupt from distant cosmic locales. Some of these bursts blast only once and others repeat. A new computer calculation suggests the repetitive kind could be due to a planet interacting with its magnetic host star, researchers report in the March 20 Astrophysical Journal.

FRBs are relative newcomers to astronomical research. Ever since the first was discovered in 2007, researchers have added hundreds to the tally. Scientists have theorized dozens of ways the two different types of FRBs can occur, and nearly all theories include compact, magnetic stellar remnants known as neutron stars. Some ideas include powerful radio flares from magnetars, the most magnetic neutron stars imaginable (SN: 6/4/20). Others suggest a fast-spinning neutron star, or even asteroids interacting with magnetars (SN: 2/23/22).

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“How fast radio bursts are produced is still up for debate,” says astronomer Yong-Feng Huang of Nanjing University in China.

Huang and his colleagues considered a new way to make the repeating flares: interactions between a neutron star and an orbiting planet (SN: 3/5/94). Such planets can get exceedingly close to these stars, so the team calculated what might happen to a planet in a highly elliptical orbit around a neutron star. When the planet swings very close to its star, the star’s gravity pulls more on the planet than when the planet is at its farthest orbital point, elongating and distorting it. This “tidal pull,” Huang says, will rip some small clumps off the planet. Each clump in the team’s calculation is just a few kilometers wide and maybe one-millionth the mass of the planet, he adds.

Then the fireworks start. Neutron stars spew a wind of radiation and particles, much like our own sun but more extreme. When one of these clumps passes through that stellar wind, the interaction “can produce really strong radio emissions,” Huang says. If that happens when the clump appears to pass in front of the star from Earth’s perspective, we might see it as a fast radio burst. Each burst in a repeating FRB signal could be caused by one of these clumps interacting with the neutron star’s wind during each close planet pass, he says. After that interaction, what remains of the clump drifts in orbit around the star, but away from Earth’s perspective, so we never see it again.

Comparing the calculated bursts to two known repeaters — the first ever discovered, which repeats roughly every 160 days, and a more recent discovery that repeats every 16 days, the team found the fragmenting planet scenario could explain how often the bursts happened and how bright they were (SN: 3/2/16).

The star’s strong gravitational “tidal” pull on the planet during each close pass might change the planet’s orbit over time, says astrophysicist Wenbin Lu of Princeton University, who was not involved in this study but who investigates possible FRB scenarios. “Every orbit, there is some energy loss from the system,” he says. “Due to tidal interactions between the planet and the star, the orbit very quickly shrinks.” So it’s possible that the orbit could shrink so fast that FRB signals wouldn’t last long enough for a chance detection, he says.

But the orbit change could also give astronomers a way to check this scenario as an FRB source. Observing repeating FRBs over several years to track any changes in the time between bursts could narrow down whether this hypothesis could explain the observations, Lu says. “That may be a good clue.” More

A chance alignment may have revealed a star from the universe’s first billion years.

If confirmed, this star would be the most distant one ever seen, obliterating the previous record (SN: 7/11/17). Light from the star traveled for about 12.9 billion years on its journey toward Earth, about 4 billion years longer than the former record holder, researchers report in the March 30 Nature. Studying the object could help researchers learn more about the universe’s composition during that early, mysterious time.

“These are the sorts of things that you only hope you could discover,” says astronomer Katherine Whitaker of the University of Massachusetts Amherst, who was not part of the new study.

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The researchers found the object while analyzing Hubble Space Telescope images of dozens of clusters of galaxies nearer to Earth. These clusters are so massive that they bend and focus the light from more distant background objects, what’s known as gravitational lensing (SN: 10/6/15).

In images of one cluster, astronomer Brian Welch of Johns Hopkins University and colleagues noticed a long, thin, red arc. The team realized that the arc was a background galaxy whose light the cluster had warped and amplified.

Atop that red arc is a bright spot that is too small to be a small galaxy or a star cluster, the researchers say. “We stumbled into finding that this was a lensed star,” Welch says.

The researchers estimate that the star’s light originates from only 900 million years after the Big Bang, which took place about 13.8 billion years ago.

Welch and his colleagues think that the object, which they poetically nicknamed “Earendel” from the old English word meaning “morning star” or “rising light,” is a behemoth with at least 50 times the mass of the sun. But the researchers can’t pin down that value, or learn more about the star or even confirm that it is a star, without more detailed observations.

The researchers plan to use the recently launched James Webb Space Telescope to examine Earendel (SN: 10/6/21). The telescope, also known as JWST, will begin studying the distant universe this summer.

JWST may uncover objects from even earlier times in the universe’s history than what Hubble can see because the new telescope will be sensitive to light from more distant objects. Welch hopes that the telescope will find many more of these gravitationally lensed stars. “I’m hoping that this record won’t last very long.” More

Like two great songwriters working side by side and inspiring each other to create their best work, the Magellanic Clouds spawn new stars every time the two galaxies meet.

Visible to the naked eye but best seen from the Southern Hemisphere, the Large and Small Magellanic Clouds are by far the most luminous of the many galaxies orbiting the Milky Way. New observations reveal that on multiple occasions the two bright galaxies have minted a rash of stars simultaneously, researchers report March 25 in Monthly Notices of the Royal Astronomical Society: Letters.

Astronomer Pol Massana at the University of Surrey in England and his colleagues examined the Small Magellanic Cloud. Five peaks in the galaxy’s star formation rate — at 3 billion, 2 billion, 1.1 billion and 450 million years ago and at present — match similarly timed peaks in the Large Magellanic Cloud. That’s a sign that one galaxy triggers star formation in the other whenever the two dance close together.

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“This is the most detailed star formation history that we’ve ever had of the [Magellanic] Clouds,” says Paul Zivick, an astronomer at Texas A&M University in College Station who was not involved in the new work. “It’s painting a very compelling picture that these two have had a very intense set of interactions over the last two to three gigayears.”

Even as the two galaxies orbit the Milky Way at 160,000 and 200,000 light-years from Earth, they also orbit each other (SN: 1/9/20). Their orbit is elliptical, which means they periodically pass near each other. Just as tides from the moon’s gravity stir the seas, tides from one galaxy’s gravity slosh around the other’s gas, inducing star birth, says study coauthor Gurtina Besla, an astrophysicist at the University of Arizona in Tucson.

During the last encounter, which happened 100 million to 200 million years ago, the smaller galaxy probably smashed right through the larger, Besla says, which sparked the current outbreak of star birth. The last star formation peak in the Large Magellanic Cloud occurred only in its northern section, so she says that’s probably where the collision took place.

Based on the star formation peaks, the period between Magellanic encounters has decreased from a billion to half a billion years. Besla attributes this to a process known as dynamical friction. As the Small Magellanic Cloud orbits its mate, it passes through the larger galaxy’s dark halo, attracting a wake of dark matter behind itself. The gravitational pull of this dark matter wake slows the smaller galaxy, shrinking its orbit and reducing how much time it takes to revolve around the Large Magellanic Cloud.

The future for the two galaxies may not be so starry, however. They recently came the closest they’ve ever been to the Milky Way, and its tides, Besla says, have probably yanked the pair apart. If so, the Magellanic Clouds, now separated by 75,000 light-years, may never approach each other again, putting an end to their most productive episodes of star making, just as musicians sometimes flounder after leaving bandmates to embark on solo careers. More

A new analysis of nearly a quarter million stars puts firm ages on the most momentous pages from our galaxy’s life story.

Far grander than most of its neighbors, the Milky Way arose long ago, as lesser galaxies smashed together. Its thick disk — a pancake-shaped population of old stars — originated remarkably soon after the Big Bang and well before most of the stellar halo that envelops the galaxy’s disk, astronomers report March 23 in Nature.

“We are now able to provide a very clear timeline of what happened in the earliest time of our Milky Way,” says astronomer Maosheng Xiang.

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He and Hans-Walter Rix, both at the Max Planck Institute for Astronomy in Heidelberg, Germany, studied almost 250,000 subgiants — stars that are growing larger and cooler after using up the hydrogen fuel at their centers. The temperatures and luminosities of these stars reveal their ages, letting the researchers track how different epochs in galactic history spawned stars with different chemical compositions and orbits around the Milky Way’s center.

“There’s just an incredible amount of information here,” says Rosemary Wyse, an astrophysicist at Johns Hopkins University who was not involved with the study. “We really want to understand how our galaxy came to be the way it is,” she says. “When were the chemical elements of which we are made created?”

Xiang and Rix discovered that the Milky Way’s thick disk got its start about 13 billion years ago. That’s just 800 million years after the universe’s birth. The thick disk, which measures 6,000 light-years from top to bottom in the sun’s vicinity, kept forming stars for a long time, until about 8 billion years ago.

During this period, the thick disk’s iron content shot up 30-fold as exploding stars enriched its star-forming gas, the team found. At the dawn of the thick disk era, a newborn star had only a tenth as much iron, relative to hydrogen, as the sun; by the end, 5 billion years later, a thick disk star was three times richer in iron than the sun.

Xiang and Rix also found a tight relation between a thick disk star’s age and iron content. This means gas was thoroughly mixed throughout the thick disk: As time went on, newborn stars inherited steadily higher amounts of iron, no matter whether the stars formed close to or far from the galactic center.

But that’s not all that was happening. As other researchers reported in 2018, another galaxy once hit our own, giving the Milky Way most of the stars in its halo, which engulfs the disk (SN: 11/1/18). Halo stars have little iron.

The new work revises the date of this great galactic encounter: “We found that the merger happened 11 billion years ago,” Xiang says, a billion years earlier than thought. As the intruder’s gas crashed into the Milky Way’s gas, it triggered the creation of so many new stars that our galaxy’s star formation rate reached a record high 11 billion years ago.

The merger also splashed some thick disk stars up into the halo, which Xiang and Rix identified from the stars’ higher iron abundances. These “splash” stars, the researchers found, are at least 11 billion years old, confirming the date of the merger.

The thick disk ran out of gas 8 billion years ago and stopped making stars. Fresh gas around the Milky Way then settled into a thinner disk, which has given birth to stars ever since — including the 4.6-billion-year-old sun and most of its stellar neighbors. The thin disk is about 2,000 light-years thick in our part of the galaxy.

“The Milky Way has been quite quiet for the last 8 billion years,” Xiang says, experiencing no further encounters with big galaxies. That makes it different from most of its peers.

If the thick disk really existed 13 billion years ago, Xiang says, then the new James Webb Space Telescope (SN: 1/24/22) may discern similar disks in galaxies 13 billion light-years from Earth — portraits of the Milky Way as a young galaxy. More

It’s official: The number of planets known beyond our solar system has just passed 5,000.

The exoplanet census surpassed this milestone with a recent batch of 60 confirmed exoplanets. These additional worlds were found in data from NASA’s now-defunct K2 mission, the “second life” of the prolific Kepler space telescope, and confirmed with new observations, researchers report March 4 at arXiv.org.

As of March 21, these finds put NASA’s official tally of exoplanets at 5,005.

It’s been 30 years since scientists discovered the first planets orbiting another star — an unlikely pair of small worlds huddled around a pulsar (SN: 1/11/92). Today, exoplanets are so common that astronomers expect most stars host at least one (SN: 1/11/12), says astronomer Aurora Kesseli of Caltech.

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“One of the most exciting things that I think has happened in the last 30 years is that we’ve really started to be able to fill out the diversity of exoplanets,” Kesseli says

Some look like Jupiter, some look — perhaps — like Earth and some look like nothing familiar. The 5,005 confirmed exoplanets include nearly 1,500 giant gassy planets, roughly 200 that are small and rocky and almost 1,600 “super-Earths,” which are larger than our solar system’s rocky planets and smaller than Neptune (SN: 8/11/15).

Astronomers can’t say much about those worlds beyond diameters, masses and densities. But several projects, like the James Webb Space Telescope, are working on that, Kesseli says (SN: 1/24/22). “Not only are we going to find tons and tons more exoplanets, but we’re also going to start to be able to actually characterize the planets,” she says.

And the search is far from over. NASA’s newest exoplanet hunter, the TESS mission, has confirmed more than 200 planets, with thousands more yet to verify, Kesseli says (SN: 12/2/21). Ongoing searches from ground-based telescopes keep adding to the count as well.

“There’s tons of exoplanets out there,” Kesseli says, “and even more waiting to be discovered.” More

Coronal loops, well-defined hot strands of plasma that arch out into the sun’s atmosphere, are iconic to the sun’s imagery. But many of the supposed coronal loops we see might not be there at all.    

Some coronal loops might be an illusion created by “wrinkles” of greater density in a curtain of plasma dubbed the coronal veil, researchers propose March 2 in Astrophysical Journal. If true, the finding, sparked by unexpected plasma structures seen in computer simulations of the sun’s atmosphere, may change how scientists go about measuring some properties of our star.

“It’s kind of inspiring to see these detailed structures,” says Markus Aschwanden, an astrophysicist at Lockheed Martin’s Solar & Astrophysics Lab in Palo Alto, Calif., who was not involved in the study. “They are so different than what we anticipated.”

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Scientists have begun to develop a better understanding of the sun’s complex atmosphere, or corona, only in the last few years (SN: 12/19/17). Coronal loops have been used to measure many properties of the corona, including temperature and density, and they may be key to figuring out why the sun’s atmosphere is so much hotter than its surface (SN: 8/20/17). But astronomers have long wondered just how the loops appear to be so orderly when they originate in the sun’s turbulent surface (SN: 8/17/17).     

So solar physicist Anna Malanushenko and her colleagues attempted to isolate individual coronal loops in 3-D computer simulations originally developed to simulate the life cycle of a solar flare. The team expected to see neatly oriented strands of plasma, because coronal loops appear to align themselves to the sun’s magnetic field, like metal shavings around a bar magnet.

Instead, the plasma appeared as a curtainlike structure winding out from the sun’s surface that folded in on itself like a wrinkled sheet. In the simulation, many of the supposed coronal loops turned out to not be real objects. While there were structures along the magnetic fields, they were neither thin nor compact as expected. They more closely resembled clouds of smoke. As the team changed the point of view from which they looked at these wrinkles in the veil in the simulation, their shape and orientation changed. And from certain viewing angles, the wrinkles resembled coronal loops.

The observations were mind-blowing, says Malanushenko, of the National Center for Atmospheric Research in Boulder, Colo. “The traditional thought was that if we see this arching coronal loop that there is a garden hose–like strand of plasma.” The structure in the simulation was much more complex and displayed complicated boundaries and a raggedy structure.

Still, not all coronal loops are necessarily illusions within a coronal veil. “We don’t know which ones are real and which ones are not,” Malanushenko says. “And we absolutely need to be able to tell to study the solar atmosphere.”

It’s also not clear how the purported coronal veil might impact previous analyses of the solar atmosphere. “On one hand, this is depressing,” Malanushenko says of the way the new findings cast doubt on previous understandings. On the other hand, she finds the uncertainty exciting. Astronomers will need to develop a way to observe the veil and confirm its existence. “Whenever we develop new methods, we open the door for new knowledge.” More