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Squabbling sibling planets may have hurled space rocks when they were young.

Simulations suggest that space rocks the size of baby planets struck both the newborn Earth and Venus, but many of the rocks that only grazed Earth went on to hit — and stick — to Venus. That difference in early impacts could help explain why Earth and Venus are such different worlds today, researchers report September 23 in the Planetary Science Journal.

“The pronounced differences between Earth and Venus, in spite of their similar orbits and masses, has been one of the biggest puzzles in our solar system,” says planetary scientist Shigeru Ida of the Tokyo Institute of Technology, who was not involved in the new work. This study introduces “a new point that has not been raised before.”

Scientists have typically thought that there are two ways that collisions between baby planets can go. The objects could graze each other and each continue on its way, in a hit-and-run collision. Or two protoplanets could stick together, or accrete, making one larger planet. Planetary scientists often assume that every hit-and-run collision eventually leads to accretion. Objects that collide must have orbits that cross each other’s, so they’re bound to collide again and again, and eventually should stick.

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But previous work from planetary scientist Erik Asphaug of the University of Arizona in Tucson and others suggests that isn’t so. It takes special conditions for two planets to merge, Asphaug says, like relatively slow impact speeds, so hit-and-runs were probably much more common in the young solar system.

Asphaug and colleagues wondered what that might have meant for Earth and Venus, two apparently similar planets with vastly different climates. Both worlds are about the same size and mass, but Earth is wet and clement while Venus is a searing, acidic hellscape (SN: 2/13/18).

“If they started out on similar pathways, somehow Venus took a wrong turn,” Asphaug says.

The team ran about 4,000 computer simulations in which Mars-sized protoplanets crashed into a young Earth or Venus, assuming the two planets were at their current distances from the sun. The researchers found that about half of the time, incoming protoplanets grazed Earth without directly colliding. Of those, about half went on to collide with Venus.

Unlike Earth, Venus ended up accreting most of the objects that hit it in the simulations. Hitting Earth first slowed incoming objects down enough to let them stick to Venus later, the study suggests. “You have this imbalance where things that hit the Earth, but don’t stick, tend to end up on Venus,” Asphaug says. “We have a fundamental explanation for why Venus ended up accreting differently from the Earth.”

If that’s really what happened, it would have had a significant effect on the composition of the two worlds. Earth would have ended up with more of the outer mantle and crust material from the incoming protoplanets, while Venus would have gotten more of their iron-rich cores.

The imbalance in impacts could even explain some major Venusian mysteries, like why the planet doesn’t have a moon, why it spins so slowly and why it lacks a magnetic field — though “these are hand-waving kind of conjectures,” Asphaug says.

Ida says he hopes that future work will look into those questions more deeply. “I’m looking forward to follow-up studies to examine if the new result actually explains the Earth-Venus difference,” he says.

The idea fits into a growing debate among planetary scientists about how the solar system grew up, says planetary scientist Seth Jacobson of Michigan State University in East Lansing. Was it built violently, with lots of giant collisions, or calmly, with planets growing smoothly via pebbles sticking together?

“This paper falls on the end of lots of giant impacts,” Jacobson says.

Each rocky planet in the solar system should have very different chemistry and structure depending on which scenario is true. But scientists know the chemistry and structure of only one planet with any confidence: Earth. And Earth’s early history has been overwritten by plate tectonics and other geologic activity. “Venus is the missing link,” Jacobson says. “Learning more about Venus’ chemistry and interior structure is going to tell us more about whether it had a giant impact or not.”

Three missions to Venus are expected to launch in the late 2020s and 2030s (SN: 6/2/21). Those should help, but none are expected to take the kind of detailed composition measurements that could definitively solve the mystery. That would take a long-lived lander, or a sample return mission, both of which would be extremely difficult on hot, hostile Venus.

“I wish there was an easier way to test it,” Jacobson says. “I think that’s where we should concentrate our energy as terrestrial planet formation scientists going forward.” More

Hot blue stars kicked out of their cradles may explain a mysterious ultraviolet glow that surrounds the disks of many spiral galaxies.
A new computer simulation demonstrates that these runaway stars can populate the vast expanses beyond a galaxy’s visible disk (SN: 3/23/20). These distant regions have gas that is too warm and tenuous to make new stars, yet young stars nevertheless exist there.
“It’s a big problem for classical star formation theory,” says Eric Andersson, an astrophysicist at Lund Observatory in Sweden.
The mystery of the far-flung young stars has persisted for some time. In 2003, NASA launched the Galaxy Evolution Explorer space telescope, which surprised astronomers by discovering diffuse far-ultraviolet light in the hinterlands of nearby spiral and irregular galaxies (SN: 2/15/05). Unlike ordinary ultraviolet radiation, far-ultraviolet light has such a short wavelength that most of it doesn’t penetrate the Earth’s atmosphere.

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Stars that emit profuse amounts of this energetic radiation are hot, blue and usually much more massive than the sun. These stars don’t live long, so they must have formed recently. But the gas on the galactic outskirts isn’t cold and dense enough to collapse and create new stars.
Andersson and his colleagues propose a solution to the paradox: Many of these far-out far-ultraviolet-emitting stars weren’t born where they are now. Instead, they arose closer to the galaxy’s center and ran away from their homes.
The researchers conducted a computer simulation to model the motion of massive stars in a spiral galaxy. Some of the runaway stars in the simulation dart across thousands of light-years of space to take up residence beyond the visible edge of the galaxy’s disk, thereby explaining the far-ultraviolet light there, the researchers report online at arXiv.org on October 22.
The Milky Way has many of these runaway stars. A star can become a runaway when other massive stars fling it away through their gravity. Or, if the star orbits close to a massive star that explodes, the surviving star races away at the same speed it had been dashing around its companion. Most runaway stars are hot and blue, radiating just the type of far-ultraviolet light seen beyond the visible edges of galactic disks.
Mark Krumholz, an astronomer at the Australian National University in Canberra, calls the idea “a plausible explanation.” He also offers a way to test it: by exploiting the properties of different types of massive stars.
The rarest and most massive blue stars are so hot they ionize hydrogen gas, causing it to emit red light as electrons settle back into position around protons. But these very massive stars don’t live long, so any that reside on a galaxy’s outskirts must have been born there. After all, the stars didn’t have time to travel from elsewhere in the galaxy during their brief lives.
In contrast, less massive blue stars live longer and therefore could have reached the galactic periphery from elsewhere during their lifetimes. If the ratio of far-ultraviolet light to red light from ionized gas is much greater beyond the galaxy’s visible edge than in its disk, Krumholz says, that would suggest much of the far-ultraviolet glow in the exurbs does indeed come from runaway stars. More

A joint European and Russian mission to Mars is being postponed from July until sometime in 2022, as the coronavirus pandemic is preventing scientists from resolving a few technical difficulties, the European Space Agency said March 12. “We cannot fly in 2020,” ESA director general Jan Wörner says. “This is a disappointment for me personally, […] More

A strange, newly measured clump of stars orbiting the nearby Andromeda galaxy has the lowest level of heavy chemical elements ever seen in one of these mysterious star clusters. Named RBC EXT8, this globular cluster is also surprisingly massive, challenging theories for how such clusters and some galaxies form, astronomers report online October 15 in Science.
“It’s a very unusual object,” says astrophysicist Oleg Gnedin of the University of Michigan in Ann Arbor, who was not involved in the new discovery.
Globular clusters are crowded, spherical collections of stars that orbit a galaxy’s center, though most, including RBC EXT8, live in the galactic outskirts. The clusters are typically billions of years old, so their stars tend to be chemically pristine, meaning they formed before the universe had time to create much of any of the elements heavier than hydrogen or helium, which astronomers lump together as “metals.”
Previous observations of these clusters in the Milky Way and other galaxies had suggested that there’s a limit to how low a globular cluster’s metal content can be. The most metal-poor clusters were about 300 times less rich in heavy elements like iron than the sun, but no less.
But spectra of RBC EXT8, some 2.5 million light-years away, show that the cluster’s metal content is about 800 times less than the sun’s. The globular cluster that held the previous record for lowest “metallicity” has three times that amount.
“It was completely unexpected that we would find a globular cluster that is so metal poor,” says astronomer Søren Larsen of Radboud University in Nijmegen, the Netherlands.
The bigger, fuzzy blob in the inset image at left is RBC EXT8, a globular cluster that orbits about 88,000 light-years from the center of the galaxy Andromeda (shown at right). The cluster has surprisingly few heavy elements for its size, a new study finds.© 2020 ESASky, CFHT
What’s more, given its metal-poor status, this cluster is surprisingly massive, weighing about 1.14 million times the mass of the sun. (A mid-weight globular cluster is about 100,000 solar masses, but some clusters reach 3 million solar masses. RBC EXT8 is heavy, but not the heaviest.)
That mass makes the cluster even harder to explain because across the cosmos, the more massive a galaxy or cluster is, the more heavy elements it normally has.
There are several potential explanations for that trend, but one is simply that more massive galaxies or globular clusters have more stars. A star fuses heavy elements in its core and sprinkles them around its host cluster or galaxy as it ages. Sufficiently massive stars can explode in a supernova, spreading those metals to become part of the next generation of stars (SN: 8/9/19). So more stars means more opportunity for metals to accumulate locally.
More massive objects also have the advantage of gravity, which lets them better hold on to the metals that they do have and remain a cohesive group for billions of years. Less massive globular clusters dissolve into their host galaxies over time.
Those trends together could have explained the apparent “metallicity floor” for globular clusters — all of the less massive, more metal-poor clusters have broken apart over the eons.
RBC EXT8 turns that conventional wisdom on its head. “It’s too big to have as low metallicity as it has,” Gnedin says. “That’s the conundrum.”

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Astronomers aren’t sure how globular clusters form in general, but they probably grow within galaxies, rather than forming outside of them and being pulled in later. And so the clusters reflect the characteristics of their galaxies: small, metal-poor galaxies end up with small metal-poor globular clusters, and vice versa. But based on RBC EXT8’s metal content, it’s galactic birthplace would be less than a million solar masses, so smaller than the globular cluster itself – which is a paradox.
As a result, the cluster challenges some simplified models of galaxy formation. But it doesn’t completely break them, Gnedin says. “It’s one object, it’s not going to overturn things,” he says. “It just makes us people working on these issues have to work harder” and be more open-minded about other ways that galaxies could form.
Open-mindedness and willingness to explore is perhaps responsible for the new finding about RBC EXT8’s metals. Larsen and colleagues spotted the globular cluster at the beginning of a night of observing with the Keck telescope in Hawaii in October 2019. “It was really a serendipitous discovery,” he says. He had a spare hour before the globular clusters in galaxy M33 that his team was planning to look at rose above the horizon, so the observers picked another cluster “more or less at random” to fill the time.
“At first, I couldn’t really believe that what was coming out [in the observations] was right,” Larsen says. “But I kept working on it, and it turned out to hold up.” More