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Dozens of runaway stars were caught escaping from a dense star cluster in a satellite galaxy of the Milky Way. The cluster of fast-moving stars may mean that such runaways had a greater influence on cosmic evolution than previously thought, astronomers report Oct. 9 in Nature.

Massive stars are born in new clusters, packed so close together that they can shake each other out of place. Sometimes, encounters between pairs of massive stars or neighboring supernova explosions can send a star crawling out of the cluster altogether, to seek its fate in the wider galaxy and beyond.

Astronomer Mitchel Stoop of the University of Amsterdam and his colleagues searched for runaway stars around a large cluster of massive stars called Radcliffe 136 using data from the Gaia spacecraft on the velocities and positions of billions of stars. (SN: 6/13/22). R136 is located about 170,000 light-years from Earth in the Large Magellanic Cloud, a dwarf galaxy orbiting the Milky Way.

The cluster “is an iconic object,” says astrophysicist Sally Oey of the University of Michigan in Ann Arbor, who was not involved in the new work. The view from Earth’s neighborhood is so clear, “we can see things up close and personal.”

Previous studies had found some stars escaping the cluster (SN: 5/7/10). But in a broader search, Stoop found that an astonishing 55 stars had fled at speeds greater than roughly 100,000 kilometers per hour in the past 3 million years.

“That’s an incredible number to think about,” says Stoop. The observation suggests that a third of the brightest and most massive stars born in the cluster have left home.

This means that runaway stars may be an underestimated force in the universe. These massive stars, about five to 140 times the mass of the Sun, emit ultraviolet radiation and supersonic stellar winds that can sculpt the gas and dust around them. (SN: 7/11/22). At the end of their lives, heavy stars explode as supernovae, spreading heavy elements around the galaxy. (SN: 7/7/21).

“Before, we would expect that there would probably be some escapees,” says Stoop. But because of their presumed low numbers, he says, they would be left out of studies and simulations. If each cluster loses about a third of its stars to the surrounding galaxy, or even the space between galaxies, “they could probably make a big contribution to dumping all these ultraviolet photons into the intergalactic medium.”

Such escapees may also have had a profound impact on the evolution of the early universe. Within a few hundred million years of the Big Bang, more than 13 billion years ago, a source of ultraviolet radiation stripped electrons from a diffuse cloud of hydrogen atoms, a phenomenon called reionization (SN: 11/7/19).

Astronomers think that most of the photons, or particles of light, that cleared the cosmic haze came from dwarf galaxies. (SN: 2/6/17). But simulations have revealed that only a fraction of the necessary photons can escape the environments of those galaxies. Runaway stars can help account for the change, Stoop says.

“Maybe this happened in [early universe] galaxies too, during the reionization epoch,” he says.

Oey says, “There’s no doubt that runaway stars are really important and underrated.” But, she says, there are other ways to remove ionizing radiation from galaxies, and it’s not clear how much of a difference including runaway stars would make.

The timing of the escape of stars from R136 may also throw a wrench into the broader connection of runaway stars to reionization.

Surprisingly, the stars did not all migrate in one wave. Scientists know this because they have the velocities and distances of the stars and can calculate when they began their escape. Most escapees left R136 in all directions about 1.8 million years ago, when the cluster was forming. That’s what you’d expect if they were pulled from dating other massive stars.

But 16 of the escapees left the group more recently, just 200,000 or so years ago. And everyone was running in the same direction. Stoop and his colleagues think that the escape of these stars may have been caused by a merger with another group.

“This seems like a pretty unique phenomenon,” says astrophysicist Kaitlin Kratter of the University of Arizona in Tucson. If the double ejection of R136 is unusual, then it may be difficult to extrapolate how many stars other groups lose to their cosmic environment. Finding evidence of similar waves in other clusters would help settle the question.

Astronomers have finally found an asteroid that keeps pace with Saturn in its orbit around the sun. Such objects, called Trojan asteroids, are already known for the other three giant planets.

“Saturn was kind of the odd man out, if I can call it that, because even though it’s the second most massive planet in the solar system, it didn’t have any Trojans,” says Paul Wiegert, an astronomer at the University of Western Ontario in London, Canada. Like Saturn, the new asteroid takes about 30 years to orbit, but lies 60 degrees ahead of the planet in its orbit, Wiegert and colleagues report in work submitted Sept. 29 to arXiv.org.

Most of the asteroids in the solar system orbit the sun between the paths of Mars and Jupiter. However, in 1906, German astronomer Max Wolf discovered the first Trojan, which he named Achilles, orbiting the sun 60 degrees ahead of Jupiter. Since then, astronomers have found thousands of additional Trojan asteroids – some are 60 degrees ahead of Jupiter, others are 60 degrees behind. NASA’s Lucy spacecraft will visit eight of them between 2027 and 2033 (SN: 15/10/21).

Trojan asteroids also exist for Uranus and Neptune and even for Earth and Mars (SN: 2/1/22).

After a telescope image in Hawaii captured the new asteroid in 2019, an amateur astronomer in Australia, Andrew Walker, suggested the object could be a Saturnian Trojan – if it had the right orbit around the sun.

“The key to getting a good orbit for something in our solar system is to have many observations of it through different telescopes over a long period of time,” says Wiegert. So astronomer Man-To Hui at the Macau University of Science and Technology in China looked for previous images of the asteroid and also planned new observations. Measurements of the asteroid’s position – from 2015 to 2024 – confirmed its Trojan nature. Named 2019 UO₁₄the asteroid is only about 13 kilometers across, the same size as Deimos, the smaller of Mars’ two moons.

Scientists have long predicted Saturnian Trojans, says astronomer Carlos de la Fuente Marcos of the Complutense University of Madrid, who was not involved in the discovery. But all Saturnian Trojans must have unstable orbits because Saturn has giant planets on either side of it.

“Jupiter seems to be the culprit,” says de la Fuente Marcos. Jupiter’s great gravity gradually pulls a Saturnian Trojan, making its orbit around the sun increasingly elliptical. The asteroid then wanders so close to Jupiter or Uranus that one of those giant planets knocks the tiny body out of its Trojan orbit.

In fact, researchers estimate that the asteroid has been a Trojan for only about 2,000 years and will remain so for only another 1,000 years. Before its connection with the ringed planet, the asteroid was probably a centaur, an asteroid that moved around the sun between the orbits of the giant planets (SN: 11/12/77).

The asteroid may not be Saturn’s only Trojan. “I’m pretty sure there are more — maybe just a few, but this can’t be the only one,” Wiegert says.

Planetary astronomer Bonnie Buratti remembers exactly where she was the first time she heard that Jupiter’s icy moon Europa might host life.

It was the 1980s, and Buratti was a graduate student at Cornell University studying images of the planet’s moons taken during the Voyager 1 and 2 flybys in 1979. Even in those first low-resolution pictures, Europa was intriguing.

“It looked like a cracked egg,” she says.

Those cracks — in a snow-covered, icy shell — were probably filled with material that had come up from below, Buratti and colleagues had shown. That meant there had to be something under the ice.

Buratti recalls his graduate student, Steven Squyres, talking about the possibility that Europa’s ice hid a liquid salty ocean. “He said, ‘Well, there’s an ocean down there, and where there’s water, there’s life,'” she recalled. “And people laughed at him.”

They’re not laughing anymore.

Over the past four decades, Buratti has seen the search for life in the solar system go from a joke to a major mission. She is now deputy project scientist for NASA’s Europa Clipper mission, which launched on October 14 to find out if Europa is indeed a habitable world. (SN: 10/8/24).

“I’m coming home,” she says.

Space science first captured Buratti’s imagination in childhood, which coincided with the beginning of the space age. She was a child when the Soviet Union launched Sputnik and a teenager when Apollo 11 landed on the moon.

“I got a telescope when I was in third grade,” she says. She remembers discovering the constellations from her front lawn in Bethlehem, Pa. “From a young age, I was always curious.”

Planetary science attracted him to the biggest personalities in the field. In graduate school, she worked with famous scientists, including Frank Drake and Carl Sagan, who were at the forefront of efforts to take the search for extraterrestrial life seriously (SN: 1/11/09; SN: 11/7/14). This gave her a sense that the universe might teem with life, but not the support she needed to get through her Ph.D. She ended up working with the less famous but equally charismatic astronomer Joe Veverka. It was Veverka who gave her the Voyager images.

Buratti joined NASA’s Jet Propulsion Laboratory in Pasadena, California, in 1985 and has been there ever since. But while the Galileo spacecraft was finding evidence of Europa’s subsurface ocean in the 1990s, Buratti was busy exploring Saturn with the Cassini mission (SN: 18.2.02).

Saturn’s moons were full of surprises, including ghostly hydrocarbon lakes on Titan, water plumes from Enceladus, and a mysterious ridge that makes Iapetus look like a nut (SN: 15.4.19; SN: 8/4/14; SN: 21.4.14). “It was just one thing after another,” says Buratti.

These discoveries helped advance the notion that subsurface oceans in the solar system might not be so strange after all. Hints of oceans have since appeared as far from the sun as Pluto, Buratt’s favorite planet — and yes, she still calls it a planet (SN: 27.3.20). There may also be oceanic worlds orbiting other stars.

So when the Europa Clipper reaches Jupiter in 2030, scientists are looking at this moon as an example of worlds that may be common in the universe. Clipper will orbit Jupiter and make at least 49 flybys of Europa to limit the amount of time the spacecraft spends in Jupiter’s punishing radiation belts. Measurements of the moon’s surface composition, gravity and internal structure will be needed to assess how suitable the small world is for life.

Buratti joined the Clipper mission in 2022, as one of the people charged with making sure the team gets as much science out of the mission as they can. “We’ve always felt that our role is to improve the science, to get the best science out of the mission,” she says. She and the scientific community at large are confident they will find something good.

“We are very confident that there is a habitable environment,” she says. Echoing that grad school speech from decades ago, she adds, “On Earth, wherever you see water, you see life. So I think it’s a really good place to look.”

This exoplanet’s atmosphere is going full steam ahead.

A planet beyond our solar system called GJ 9827d has an atmosphere composed almost entirely of hot water molecules, astronomers report in Astrophysical Journal Letters October 4.

“We’re using the term ‘steam world,'” says astronomer Ryan MacDonald at the University of Michigan in Ann Arbor.

GJ 9827d was discovered in 2017 orbiting a star about 100 light years from Earth. About twice the size of Earth and three times the mass of Earth, it is a type of planet called sub-Neptune. (SN: 8/8/22). Worlds like this are the most common in the galaxy, although our solar system has none.

But just knowing the size and mass of the planet is not enough to deduce what it is made of. To probe exoplanet skies, astronomers analyze the starlight that filters through the planet’s atmosphere as it passes in front of its parent star. (SN: 6/7/24).

MacDonald and colleagues used the James Webb Space Telescope to observe two such transits or transits of GJ 9827d in November 2023. The Hubble Space Telescope had made similar observations and seen signs of water molecules in the planet’s atmosphere, astronomers reported last year . But it wasn’t enough to tell if the atmosphere had just a little water in it, or if it was an entire watery world.

Combining the views of the two telescopes made it abundantly clear that the atmosphere was almost all water. The temperature of the planet is about 340 ° Celsius, so all that water must be steam.

Such steamy worlds “have been predicted, but this is the first observational evidence that they really exist,” says Macdonald. “I feel like a Star Trek explorer.”

There may not be a solid rocky surface beneath the planet’s steamy sky. Deep in the atmosphere, the pressure from all that water must increase enough to force the water molecules into strange and exotic forms of matter, like supercritical fluids or hot, high-pressure ice, MacDonald says.

This makes GJ 9827d an unlikely place to find life. But studying its atmosphere is good practice for observing potentially habitable planets.

“It’s proof of principle that we can detect heavier atmospheres,” says MacDonald. “We’re on the right track to where we want to be, astrobiologically.”