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As many as 70 million people in the U.S. have problems with sleep, and many are increasingly turning to melatonin supplements to help them fall asleep. People generally perceive these over-the-counter gummies, liquids, and tablets as harmless. But recent research has raised new questions about whether the popular sleep aid is as benign as people assume, especially the longer it’s used.

How does melatonin work in the body?

Melatonin is a hormone that humans naturally produce, as do all other mammals, including bears, and birds. It regulates the body’s circadian rhythm, the 24-hour biological “clock” that influences when we are sleepy or awake and alert. The body’s melatonin levels fluctuate in response to light—the brain’s control center for sleep suppresses levels during the day and boosts levels at night. As the hormone circulates in the bloodstream, it tones down the signals that keep us awake, but it doesn’t cause sleep, says Jennifer Martin, a psychologist at the Florida International University. Instead of acting like a sedative, melatonin sends the body biological signals that it’s nighttime.

What are melatonin’s short-term side effects?

Melatonin supplements are generally considered safe, but some people do report headaches, dizziness, or nausea after exposure. Taking too much at one time can disrupt sleep rather than improve it. Too much of the drug may also trigger something akin to a melatonin hangover the next day: lingering amounts in the blood can cause grogginess and tiredness.

Supplements are sold with doses that are “much, much higher than anybody is naturally producing,” says Jamie Zeitzer, a sleep specialist at Stanford University. The hormone is commonly sold in five-milligram concentrations, though some “extra strength” products say they contain up to 40 milligrams.

“More isn’t better,” Martin says. “For a lot of people, a small dose is the most beneficial.”

Getting the right amount can be tricky. While the melatonin used in supplements is chemically identical to that produced in the body, the actual amount of the hormone in these products may not match what’s on their label. Some over-the-counter products contained anywhere from 80 percent less to about 470 percent more melatonin than advertised, a 2017 study found. Melatonin is sold as a supplement, which means such products reach the market without the Food and Drug Administration first evaluating their safety or production standards. Shalini Paruthi, a sleep medicine physician at John J. Cochran Veterans Hospital in St. Louis and spokesperson for the American Academy of Sleep Medicine, says to look for U.S. Pharmacopeia–certified products, which are tested for the quantity of their listed ingredients.

How does long-term melatonin use affect health?

At a conference in early November, researchers at SUNY Downstate Health Sciences University and their colleagues presented an analysis of U.K. medical records that raised concerns about long-term melatonin use. Almost 5 percent of 65,414 adults with insomnia who took melatonin for at least a year experienced heart failure (whereas almost 3 percent of those who did not take the sleep aid did so). People who took melatonin for at least a year were also more than three times as likely to be hospitalized for heart failure and nearly twice as likely to die from any cause compared with those who had no medical record of taking melatonin.

But some experts believe the findings, which have not yet been published or peer-reviewed, might say more about the consequences of poor sleep rather than melatonin use. “I don’t see a very good reason for melatonin to be directly involved in [heart failure],” Zeitzer says. Bad sleep is more likely putting extra pressure on the cardiovascular system, he says. Chronic insomnia is already known to be related to a higher risk of cardiovascular disease, metabolic dysfunction, and issues with memory, learning, and mental health. A more likely explanation for the new study’s trends is that people with poor sleep are relying on melatonin, which might not be helping them, Martin says.

Other long-term studies haven’t found evidence that melatonin harms the heart. But there isn’t much substantial, long-term safety data for melatonin.

Is melatonin safe for children?

Parents are increasingly giving their children melatonin to help with sleep. Evidence suggests it can make a real difference for some children with neurodivergent conditions. Many autistic kids, for example, have a harder time falling—and staying—asleep. That may be because they produce melatonin differently, releasing it later in the evening or in lower amounts than usual, Paruthi says. In such cases, “we would definitely recommend a low-dose melatonin to see if that helps,” she says. “And for a lot of these kids, it really does.”

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The dismissals, while embarrassing for the White House and the Justice Department, are unlikely to be the last word on an issue of constitutional authority that many legal experts expect could be appealed to the Supreme Court. And the way Judge Currie rendered her decision left open the possibility that another prosecutor could refile the charges against both Mr. Comey and Ms. James.

Judge Currie’s orders center on Mr. Trump’s unorthodox decision to appoint Ms. Halligan to her prosecutorial position in an interim capacity, replacing his previous pick, who was also serving in a temporary role. Within days after assuming her new post, Ms. Halligan rejected the advice of the career prosecutors in her new office and moved single-handedly to indict both Mr. Comey and Ms. James, two of the president’s most reviled targets.

In her rulings on Monday, Judge Currie said that it was unlawful to appoint two interim prosecutors in succession, and dismissed the charges against Mr. Comey and Ms. James without prejudice.

The administration signaled on Monday it would appeal the judge’s ruling, rather than acquiesce to the death of two high-profile cases the president demanded they be brought.

Karoline Leavitt, the White House press secretary, told reporters that the judge “was clearly trying to shield Letitia James and James Comey from receiving accountability” and added that the Justice Department would quickly appeal “this unprecedented action.”

The dismissal of charges without prejudice meant the government could also try to refile them, whatever the outcome of the ultimate legal fight over the appointment of Ms. Halligan, a former White House aide and personal lawyer to Mr. Trump.

In a statement, a lawyer for Mr. Comey, Patrick J. Fitzgerald, said that with the dismissal of the case against his client, “an independent judiciary vindicated our system of laws not just for Mr. Comey but for all American citizens.”

Ms. James’s lawyer, Abbe Lowell, said the court ruling showed Mr. Trump “went to extreme measures to substitute one of his allies to bring these baseless charges after career prosecutors refused. This case was not about justice or the law; it was about targeting Attorney General James for what she stood for and who she challenged.”

Judge Currie’s ruling stems from a series of machinations that Mr. Trump undertook earlier this fall. Her legal rationale was based in part on the decision by another federal judge, Aileen M. Cannon, to dismiss an indictment against Mr. Trump over concerns about the appointment of Jack Smith as special counsel in that case.

In late September, he rushed to oust Ms. Halligan’s predecessor, Erik S. Siebert, the career U.S. attorney in the Eastern District of Virginia, who had expressed concern that there was not sufficient evidence to indict Mr. Comey and Ms. James. The president then replaced Mr. Siebert with Ms. Halligan, who had no previous experience as a prosecutor.

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From deep-sea hydrothermal vents to freezing glaciers, there are plenty of harsh environments on Earth. But they’re nothing compared with outer space.

There are, however, a growing list of species, such as tardigrades and certain flowering plants, that can survive in that cold vacuum. The most recent addition is a type of moss, scientists at Hokkaido University in Japan and their colleagues recently reported in iScience.

“The fact that another major group of terrestrial life can survive in space, as far as physical findings, is cool,” says University of Florida space biology expert Robert Ferl, who was not involved in the study. “Terrestrial life may not be limited to the Earth.”

Space is a tough place to survive. It lacks air and has extreme amounts of ultraviolet radiation that can damage DNA. And its temperatures range from freezing to extreme heat. But mosses are resilient. They were one of the first plants to adapt to land when such life started transitioning out of the water about 500 million years ago.

The Hokkaido University research team studied Physcomitrium patens, a species of moss that is typically found around pools of water in temperate parts of the world, including Europe, North America, and East Asia. They compared the tolerance of three different stages of the plant: the protonemata, or the moss’s juvenile stage; the brood cells, specialized cells that emerge in stressful conditions; and the plant’s reproductive spores, which are produced in a tough capsule known as the sporangium.

The researchers simulated space conditions by exposing the three tissues to UV radiation and freezing and high temperatures. For each simulation, the spores were always more resilient than the other two plant parts. “[The spores] are very strong, more than we expected,” says plant biologist and study co-author Tomomichi Fujita.

To further test the spores, they were placed on a platform outside of the International Space Station from early March to late December 2022. After they were brought back to Earth, they were grown on a petri dish, and more than 80 percent of the spores germinated.

“The next question is: Why?” Fujita says. “We don’t know the reason why [the spores] are so strong,” but it may be because they are dormant in space. Additionally, although more spores germinated than the team expected, their growth rate was delayed.

Next, the researchers want to know the genes involved in the spores’ tolerance to space to see if there was any UV-induced DNA damage.

Studying how terrestrial life, such as moss, flowers, and microorganisms, fare in space clues scientists into how future forms of life could be sustained in the stars. Though it’s a far cry from reality, knowing this could help expand human habitats beyond Earth.

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At this point, discussing the relationship kids and teens have with screens feels passé. You may have mentally muted those notifications—and experts get it. But they say it’s important for parents to continue to tune into the crisis and their kids’ behavior around screens.

“While we’ve yet to fully realize the downstream effects of this new digitized dynamic, the available data overwhelmingly points to a corrosive effect on our children’s mental health and well-being,” says Kellyn Smythe, MS, an admissions director for Pacific Quest, a residential treatment facility helping adolescents recover from social media and screen addiction.

The average teen spends nearly five hours per day on social media, according to a 2023 Gallup Poll.1 And a study from the same year suggests that habitually checking social media in early adolescence could change the brain’s sensitivity to rewards and punishments. Of course, younger kids are also using screens. A 2025 survey from the Pew Research Center found the majority of parents say their kids ages 5 to 7 and ages 2 to 4 use smartphones.

But recent research shows that screen addiction may be more important to pay attention to than screen time itself. A big reason is those who feel addicted to their devices are more at risk for mental health issues. 

Smythe has been at the forefront of this crisis. He and mental health providers discuss the signs of screen addiction and withdrawal, plus how to help your kid or teen foster a healthier relationship with their devices.

What Is Screen Addiction?

“Simply put, if your child, with any regularity, chooses screen time over in-person experiences—and seemingly can’t prevent themselves from doing so—it’s fair to deem them ‘screen-addicted,” ​​Smythe says.

Screens include smartphones, tablets, computers, and TVs. Addiction can happen because of the stimulation people get from using tech. Data shows social media, for example, triggers surges in dopamine, a neurotransmitter known as the “feel good” chemical connected to our reward-seeking behaviors.

“Individuals afflicted with screen addiction continue to gradually immerse themselves in screen time to a point where it exceeds their ability to control it,” explains Matt Glowiak, PhD, LCPC, CAADC, chief addiction specialist with Recovered.org, an organization that provides resources for mental health and addiction treatment. “They spend more time on the screen than intended or desired, even to the detriment of everything else in their lives. When not on the screen, their thoughts and emotions are nearly absent in the real-life setting while obsessing about their next use.”

It can be harder for kids and teens to pump the brakes than adults.  

“While many adults might eventually recognize problematic use but struggle to stop, with children and adolescents, considering their developmental level, problematic use is oftentimes out of their awareness,” explains Dr. Glowiak. “It becomes the ‘new norm.'”

Simply put, if your child, with any regularity, chooses screen time over in-person experiences—and seemingly can’t prevent themselves from doing so—it’s fair to deem them ‘screen-addicted.’

— Kellyn Smythe, MS

Signs Your Kid May Have Screen Addiction

When parents understand the signs of screen addiction, they can intervene. ​​When it comes to teens, Smythe encourages parents to look out for ones who:

• Habitually avoid in-person experiences, like hangouts with friends, sports, and family events, in favor of screen time
• Show irritability or have outbursts around screen time boundaries
• Attempt to or use screens as an emotional regulation tool (“For example, a teen might feel the need to engage with a digital device when attending a common social experience, such as going out to dinner with the family,” he says.)
• Miss school
• Exhibit signs of anxiety or depression
• Experience changes weight, well-being, or activity
• Avoid typical social milestones, such as attending school dances, sleepovers, family vacations, and dates

For younger kids, parents may also notice an intense preoccupation with screens, a loss of interest in other activities, frustration when they can’t use screens, and difficulty in stopping them from using them. Plus, the amount of screen time a kid wants may keep increasing.

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In August, Jemmy Jimenez Rosa and her husband, Marcel, took their three young daughters on a vacation to Cancún, Mexico. On their return to Boston Logan airport, a Customs and Border Protection officer took Ms. Rosa aside and led her to a back room where she was told she should say goodbye to her girls. “I keep thinking this is a nightmare. Is this a nightmare? Like, is this really happening?” Ms. Rosa recalled.

Ms. Rosa was placed in a detention cell at Logan. Officers gave her virtually no information and dismissed her husband’s requests that he be allowed to bring her diabetes and anxiety medication. Ms. Rosa was born in Peru and has been a lawful permanent resident of the United States since she was 9 years old; she is now 43. Just weeks before the trip to Cancún, she had renewed her green card without incident. Her husband and her daughters are American citizens.

Over the past several months, alongside a team from Opinion Video, I’ve spoken to a half-dozen people and their families who have been taken into Immigration and Customs Enforcement detention. Each was re-entering, or was already in the country legally. No one was smuggled across the border.

None of the people we spoke to had a recent criminal record. (Three had minor nonviolent brushes with the law, all in the distant past; one received a pardon.) All were treated like suspected violent criminals, forced into tiny cells, dressed in prison uniforms, manacled for transfer. Those we spoke to were held for anywhere from 10 days to over 70 days. The experience shattered their equilibrium.

Immigration and Border Patrol officers have long held extremely broad discretionary powers to welcome or reject noncitizens arriving in the United States. And this is far from the first wave of xenophobia to hit America. But something different is happening now in the breadth and ferocity of efforts to change the makeup of this country.

The videos circulating on social media are brutal and terrifying — the often violent arrests, people pulled screaming from their cars, out of day care centers, away from their children and their spouses. What should give Americans equal pause is the inhumanity happening beyond the cameras, away from the view of judges and lawyers, and the media. Due process is not a constitutional right afforded only to citizens; legal restrictions on unlawful detention apply to all people on U.S. soil.

The stories we were told call into question both the constitutionality and the morality of how the Trump administration is directing immigration policy. That immorality, once unleashed, may ultimately be aimed at others in this country, regardless of immigration status. If a woman returning from vacation with her young children can be suddenly removed from her family and her life, how can we believe that any of us will remain safe?

There was a disquieting sameness to the horror that was described to us. Those we interviewed despaired at how the detention centers were kept purposefully, horrendously cold, forcing some of them to huddle up against strangers. They spoke of lights left on 24 hours a day and of interstate transfers that came without notice. They described food that was inadequately distributed and made them unwell. Of being forced to urinate and defecate in front of fellow detainees and guards. Of being humiliated and mocked by officers. All referred to a destabilizing lack of information, the dreadful understanding that they could be held for weeks or months without anyone informing them why they were being held at all.

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Luis Manuel Diaz for The New York Times

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Before a car crash in 2008 left her paralysed from the neck down, Nancy Smith enjoyed playing the piano. Years later, Smith started making music again, thanks to an implant that recorded and analysed her brain activity. When she imagined playing an on-screen keyboard, her brain–computer interface (BCI) translated her thoughts into keystrokes — and simple melodies, such as ‘Twinkle, Twinkle, Little Star’, rang out.

But there was a twist. For Smith, it seemed as if the piano played itself. “It felt like the keys just automatically hit themselves without me thinking about it,” she said at the time. “It just seemed like it knew the tune, and it just did it on its own.”

Smith’s BCI system, implanted as part of a clinical trial, trained on her brain signals as she imagined playing the keyboard. That learning enabled the system to detect her intention to play hundreds of milliseconds before she consciously attempted to do so, says trial leader Richard Andersen, a neuroscientist at the California Institute of Technology in Pasadena.

Smith is one of roughly 90 people who, over the past two decades, have had BCIs implanted to control assistive technologies, such as computers, robotic arms, or synthetic voice generators. These volunteers — paralysed by spinal-cord injuries, strokes or neuromuscular disorders, such as motor neuron disease (amyotrophic lateral sclerosis) — have demonstrated how command signals for the body’s muscles, recorded from the brain’s motor cortex as people imagine moving, can be decoded into commands for connected devices.

But Smith, who died of cancer in 2023, was among the first volunteers to have an extra interface implanted in her posterior parietal cortex, a brain region associated with reasoning, attention, and planning. Andersen and his team think that by also capturing users’ intentions and pre-motor planning, such ‘dual-implant’ BCIs will improve the performance of prosthetic devices.

Andersen’s research also illustrates the potential of BCIs that access areas outside the motor cortex. “The surprise was that when we go into the posterior parietal, we can get signals that are mixed together from a large number of areas,” says Andersen. “There’s a wide variety of things that we can decode.”

The ability of these devices to access aspects of a person’s innermost life, including preconscious thought, raises the stakes on concerns about how to keep neural data private. It also poses ethical questions about how neurotechnologies might shape people’s thoughts and actions — especially when paired with artificial intelligence.

Meanwhile, AI is enhancing the capabilities of wearable consumer products that record signals from outside the brain. Ethicists worry that, left unregulated, these devices could give technology companies access to new and more precise data about people’s internal reactions to online and other content.

Ethicists and BCI developers are now asking how previously inaccessible information should be handled and used. “Whole-brain interfacing is going to be the future,” says Tom Oxley, chief executive of Synchron, a BCI company in New York City. He predicts that the desire to treat psychiatric conditions and other brain disorders will lead to more brain regions being explored. Along the way, he says, AI will continue to improve decoding capabilities and change how these systems serve their users. “It leads you to the final question: how do we make that safe?”

Consumer concerns

Consumer neurotech products capture less-sophisticated data than implanted BCIs do. Unlike implanted BCIs, which rely on the firings of specific collections of neurons, most consumer products rely on electroencephalography (EEG). This measures ripples of electrical activity that arise from the averaged firing of huge neuronal populations and are detectable on the scalp. Rather than being created to capture the best recording possible, consumer devices are designed to be stylish (such as in sleek headbands) or unobtrusive (with electrodes hidden inside headphones or headsets for augmented or virtual reality).

Still, EEG can reveal overall brain states, such as alertness, focus, tiredness, and anxiety levels. Companies already offer headsets and software that give customers real-time scores relating to these states, with the intention of helping them to improve their sports performance, meditate more effectively, or become more productive, for example.

AI has helped to turn noisy signals from suboptimal recording systems into reliable data, explains Ramses Alcaide, chief executive of Neurable, a neurotech company in Boston, Massachusetts, that specializes in EEG signal processing and sells a headphone-based headset for this purpose. “We’ve made it so that EEG doesn’t suck as much as it used to,” Alcaide says. “Now, it can be used in real-life environments, essentially.”

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Greene announced her official departure from Congress in a video Friday. Trump first reacted to her exit by phone on that same night, telling ABC News White House correspondent Rachel Scott, “I think it’s great news for the country. It’s great.” The president started posting on social media earlier than usual Saturday, doubling down on his joy.

“Marjorie ‘Traitor’ Brown, because of PLUMMETING Poll Numbers, and not wanting to face a Primary Challenger with a strong Trump Endorsement (where she would have no chance of winning!), has decided to call it ‘quits,’” he wrote on Truth Social at 6:45 a.m.

Greene has accused the president of turning on her because of her continued call to release classified documents related to late sex trafficker and Trump friend Jeffrey Epstein. He has meanwhile accused her of going “Far Left” and claimed that she calls him too much.

“For some reason, primarily that I refused to return her never ending barrage of phone calls, Marjorie went BAD,” he wrote in his Saturday screed.

He also took aim at two Kentucky Republicans, Rep. Thomas Massie and Sen. Rand Paul. Massie filed the discharge petition to a force a vote on the release of the Epstein files, while Paul has said he was “on the side of transparency” in the matter.

Greene’s “relationship with the WORST Republican Congressman in decades, Tom Massie of Kentucky, also known as Rand Paul Jr. because he votes against the Republican Party (and really good legislation!), did not help her,” Trump wrote.

The Georgia congresswoman had broken with Trump not only over his dismissal of the Epstein “hoax,” but also over his handling of the recent government shutdown and lack of a Republican plan to help people who are losing subsidies to afford health insurance policies.

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President Donald Trump on Saturday reacted to the sudden resignation of Rep. Marjorie Taylor Greene (R-Ga.)

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Leaders of some of the world’s most powerful countries pushed back on demands that Ukraine cede territory and limit the size of its army included in President Trump’s latest proposal to end the war with Russia. But they said they believed the plan provided a basis for further negotiations, according to a joint statement released after they met in Johannesburg on Saturday.

The proposal, a 28-point plan, calls for Ukrainian concessions already largely rejected by the country’s president and allies. Mr. Trump gave President Volodymyr Zelensky of Ukraine until next week to agree to the plan, though he said Saturday the plan was “not my final offer” and suggested that the deadline could be extended should progress be made in negotiations.

It remained unclear Saturday how much wiggle room Ukraine and its European allies would have to change the proposal. Secretary of State Marco Rubio and Steve Witkoff, Mr. Trump’s special envoy, will head to Geneva on Sunday, where they are expected to meet senior Ukrainian officials to discuss Kyiv’s response to the American proposal, a U.S. official said Saturday.

Even before the latest proposal was made public last week, Ukraine’s allies in Europe and across the globe faced pressure to show they could get Ukraine the economic and military support it needs to continue battling Russia should the United States cease its contributions. Germany, which is already Ukraine’s largest military backer in Europe, this month pledged an additional $3.5 billion in assistance for next year, and France and Sweden recently pledged hundreds of new fighter jets.

Though couched in diplomatic language, public statements from European leaders about the new proposal, including at Saturday’s G20 meeting, made clear that their support for Ukraine was unwavering, whatever the pressure from the White House. In a joint statement adopted Saturday the leaders of 11 nations — including Germany, France, Britain, Japan and Canada — and the European Union said the 28-point plan included “important elements that will be essential for a just and lasting peace.”

But they also made clear that they took issue with provisions of the plan that would strip Ukraine of territory and limit the size of its armed forces.

“What is at stake is Ukrainian sovereignty and European security,” President Emmanuel Macron of France said on Saturday, adding that European countries would work with the Ukrainians over the next two days to create a plan for the way forward.

“Our problem is not the U.S.,” Mr. Macron told reporters at the G20 summit. “Our unique problem — all of us — is Russia, who started this war, who refused to take the cease-fire proposals.”

How much influence European leaders might have at this point in the war is unclear. Together with Ukraine, they have been cut out of the deliberations that produced this latest proposal. In the past though, Europe’s unified support of Kyiv together with the stubborn refusal of President Vladimir V. Putin to back down from his maximalist demands have been enough to at least preserve the status quo and keep Ukraine in the fight.

Mr. Macron said negotiating teams from the European Union, Germany, Britain and France would meet with American and Ukrainian negotiators in Geneva “to be able to bring substance back to the table and reconcile all these views.” This will be followed by a meeting of the “coalition of the willing,” a group of more than 30 countries supporting Ukraine, he said.

In a video posted to social media on Saturday, Mr. Zelensky expressed a grim view of the proposal before him and expressed doubt that it could be a starting point to negotiate peace.

“Right now, this is about much more than any points in any document,” he said. “We must ensure that nowhere in Europe or the world does the principle prevail that crimes against people and humanity, against states and nations, can ever be rewarded in any way or forgiven.”

Saturday’s meeting took place on the sidelines of the Group of 20 heads of state summit, an annual gathering that brings together leaders of the world’s largest economies to try to agree on shared ambitions on the most pressing global issues. The United States did not send a delegation; Mr. Trump said he was boycotting it because of his belief that South Africa was persecuting its white minority.

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António Costa, president of the European Council, left, and Ursula von der Leyen, president of the European Commission, right, with President Cyril Ramaphosa of South Africa, in Johannesburg on Thursday.Credit…Joao Silva/The New York Times

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Roughly four and a half billion years ago, the planet Theia slammed into Earth, destroying Theia, melting large fractions of Earth’s mantle, and ejecting a huge debris disk that later formed the moon. Scientists have long wondered what Theia was made of and where it came from. Now they have evidence that it formed very close to home.

The original giant impact model of the moon’s creation, proposed in the 1970s, predicted the moon was made mostly of Theia’s material. This scenario implied there should be differences in the chemical composition of the moon and Earth, but research has found that the two are nearly identical—far more similar than two independent planetary bodies should be. A new study, published today in Science, took a close look at other things Theia gave us beside the moon: additional molybdenum and iron left behind in the collision.

Ancient Earth would have had these heavy elements accumulate in its core but not in the rocky mantle closer to the surface, so any iron present now in Earth’s mantle likely came from Theia and can tell us about that planet’s composition, says study co-author Thorsten Kleine, director of the Max Planck Institute for Solar System Research in Göttingen, Germany.

Kleine and his colleagues analyzed 15 terrestrial rocks and six lunar samples brought to Earth by Apollo missions. They focused first on iron isotopes: variations of the element with different numbers of neutrons. Rocks and planets in the Solar System share nearly identical distributions of these isotopes, but in the last few years, Kleine and some of the new paper’s co-authors discovered that certain very tiny deviations from the standard iron isotope ratio can reveal the sample’s place of origin. “The discovery of iron isotope anomalies is relatively recent, which I guess is why no one has done it for the moon so far,” Kleine says. “These analyses are difficult and the variations are small, so it is not an easy experiment to do.” The team combined the data on iron with isotope distributions of molybdenum and zirconium found in the same samples to reverse engineer Theia’s likely size and composition. The researchers also compared the measurements to those of samples from 20 meteorites that originated in both the inner and outer Solar System to determine Theia’s place of origin.

The new study reveals Theia as a rocky planet with a metallic core that most likely contained five to ten percent the mass of Earth and formed in the inner Solar System, closer to the sun than Earth is. This picture lines up with previous hypotheses for why the bodies were so similar, Kleine says; what we didn’t know was where it formed exactly.

Back in 2020, Kleine and other scientists demonstrated that celestial bodies that formed closer to the sun are richer in heavy elements such as molybdenum. Following this principle, Kleine and the new study’s co-authors estimated that Earth has a bit more molybdenum and zirconium than it should and figured these additional heavy elements must have been brought here by Theia. They combined that data with what they’d learned about the iron.

“The authors make new iron isotope measurements at exceptional levels of precision,” says planetary scientist Sara Russell, leader of the Planetary Materials Group at the Natural History Museum, London, who was not involved in the new study. For her, the study’s implications go beyond just the origins of Theia—they help us understand what eventually shaped the Earth-moon system into a cradle of life. “This careful work and insightful modeling help us better understand our origins,” she adds.

The team hasn’t yet run its proposed scenario through simulations of the giant impact, Kleine says, but he looks forward to running those simulations, as well as analyzing lunar samples to look for other element isotopes.

Russell hopes future sample-return missions can boost this type of analysis. “I find it amazing we are still learning new things about the moon and Earth over 50 years since the Apollo astronauts collected these rocks from the lunar surface,” Russell says. “Collecting samples in space and bringing them to curate on Earth means we can make much more detailed measurements than are possible in space and preserves them for future generations to make their own discoveries.”

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