WASHINGTON — A second cancer later in life is common for childhood cancer survivors, and scientists now have a sense of the role genes play when this happens. A project that mined the genetic data of a group of survivors finds that 11.5 percent carry mutations that increase the risk of a subsequent cancer.

“We’ve always known that among survivors, a certain population will experience adverse outcomes directly related to therapy,” says epidemiologist and team member Leslie Robison of St. Jude Children’s Research Hospital in Memphis. The project sought “to find out what contribution genetics may play.” The team presented their work at the American Association of Cancer Research meeting April 3.
“This is a nice first step,” says David Malkin, a pediatric oncologist at the University of Toronto. “The results validate the thoughts of those of us who believe there is a genetic risk that increases the risk of second malignancies.”

Five-year survival rates for kids with cancer have grown to more than 80 percent. But “there are long-term consequences for having been diagnosed and treated for cancer as a child,” notes Robison. Some survivors develop a later, second cancer due to the radiation or chemotherapy that treated the first cancer (SN: 3/10/07, p. 157).

The researchers examined 3,007 survivors of pediatric cancer who routinely undergo medical evaluation at St. Jude. About a third had leukemia as children. By age 45, 29 percent of this group had developed new tumors, often in the skin, breast or thyroid.

The team cataloged each survivor’s DNA, and looked closely at 156 genes known as cancer predisposition genes. Of the survivors, 11.5 percent carried a problematic mutation in one of the 156 genes. Some genes on the list convey a higher risk than others, so the team looked further at a subset of 60 genes in which only one mutated copy in each cell is enough to cause disease. These 60 genes also have high penetrance, meaning that a mutated copy is highly likely to lead to a cancer. Nearly 6 percent of the survivors had a problematic mutation in one of these 60 genes.

The research team also separated the survivors based on whether or not they had received radiation therapy as children. Close to 17 percent of survivors not exposed to radiation therapy had a problematic mutation in the subset of 60 genes. These survivors had an increased risk for any second cancer. Those with both a mutation in one of the 60 genes and radiation in their treatment history had a higher risk for specific kinds of second cancers: breast, thyroid or sarcomas, tumors in connective tissues.
Based on the new estimates of genetic risk, the team suggests that survivors not given radiation therapy undergo genetic counseling if a second cancer develops. Counseling is also recommended “for survivors who develop a secondary breast cancer, thyroid cancer or sarcoma in a site that received prior radiation therapy,” says St. Jude epidemiologist and project team member Carmen Wilson. Counseling can provide guidance on health practices going forward, reproductive choices and the implications for immediate family members who may have inherited the mutation, notes Robison.

The extensive amount of medical and genomic information collected for the survivors could help with cancer prevention efforts in the future, Robison says. The team would like to create prediction models that consider treatment, genetics and other clinical information, in order to place survivors into different risk groups. “It’s eventually going to have clear implications for how these patients are clinically managed, and how we either prevent or ameliorate the adverse effects,” Robison says.

Malkin notes that not only “what you got for treatment, but when you got it” is another factor influencing a survivor’s risk profile for second cancers, as treatments and doses have changed over time. He also thinks the percentage of survivors at risk reported by Robison’s team is lower than expected. “Expanding the pool of genes to look at will be very informative,” he says.

A new device the size of a coffee mug can generate drinkable water from desert air using nothing but sunlight.

With this kind of device, “you can harvest the equivalent of a Coke can’s worth of water in an hour,” says cocreator Omar Yaghi, a chemist at the University of California, Berkeley. “That’s about how much water a person needs to survive in the desert.”

Though that may not sound like much, its designers say the current device is just a prototype. But the technology could be scaled up to supply fresh water to some of the most parched and remote regions of the globe, such as the Middle East and North Africa, they say.
Previous attempts at low-energy water collection struggled to function below 50 percent relative humidity (roughly the average afternoon humidity of Augusta, Ga.). Thanks to a special material, the new device pulled water from air with as low as 20 percent relative humidity, Yaghi and colleagues report online April 13 in Science. That’s like conjuring water in Las Vegas, where the average afternoon relative humidity is 21 percent.

Drinking water supplies can’t keep up with the rising demands of a growing human population, and shifts in rainfall caused by climate change are expected to exacerbate the problem. Already, two-thirds of the world’s population is experiencing water shortages (SN: 8/20/16, p. 22). One largely untapped water source is the atmosphere, which contains more than 5 billion Olympic-sized pools’ worth of moisture in the form of vapor and droplets.

Getting that moisture out is easy when the air is saturated with water. But humid regions aren’t where the water-shortage problem is, and drawing water from the drier air in parched areas is a greater challenge. Spongy materials such as silica gels can extract moisture from the air even at low relative humidity. Those materials, however, either amass water too slowly or require lots of energy to extract the collected water from the material.

The new device uses a material that avoids both problems. MIT mechanical engineer Evelyn Wang, Yaghi and colleagues repurposed an existing material composed of electrically charged metal atoms linked by organic molecules. This metal-organic framework, christened MOF-801, creates a network of microscopic, spongelike pores that can trap such gases as water vapor. At room temperature, water vapor collects in the pores. As temperatures rise, the water escapes.

The team’s prototype includes a layer of MOF-801 mixed with copper foam. Left in the shade, this layer collects water vapor from the air. When moved into direct sunlight, the layer heats up and the water vapor escapes into an underlying chamber. A condenser in the chamber cools the vapor, converting it into a potable liquid. This entire process takes around two hours.
Laboratory tests of the device harvested 2.8 liters of water per day for every kilogram of MOF-801 used. As it is now, the device could be used as a personal water source in dry regions without water-producing infrastructure, Yaghi says, or the system could be scaled up to produce enough water for a whole community.

The device’s ability to produce water at low relative humidity is a breakthrough, says Krista Walton, a chemical engineer at Georgia Tech in Atlanta. “No one else is using MOFs like this today,” she says.

As for the cost of scaling up, the ingredients used in the device’s metal-organic framework “aren’t exotic,” Walton says. Producing large amounts of the material “would definitely be possible if the demand were there.”

A recent upsurge in planet-warming methane may not be caused by increasing emissions, as previously thought, but by methane lingering longer in the atmosphere.

That’s the conclusion of two independent studies that indirectly tracked concentrations of hydroxyl, a highly reactive chemical that rips methane molecules apart. Hydroxyl levels in the atmosphere decreased roughly 7 or 8 percent starting in the early 2000s, the studies estimate.

The two teams propose that the hydroxyl decline slowed the breakdown of atmospheric methane, boosting levels of the greenhouse gas. Concentrations in the atmosphere have crept up since 2007, but during the same period, methane emissions from human activities and natural sources have remained stable or even fallen slightly, both studies suggest. The research groups report their findings online April 17 in Proceedings of the National Academy of Sciences.
“If hydroxyl were to decline long-term, then it would be bad news,” says Matt Rigby, an atmospheric scientist at the University of Bristol in England who coauthored one of the studies. Less methane would be removed from the atmosphere, he says, so the gas would hang around longer and cause more warming.

The stability of methane emissions might also vindicate previous studies that found no rise in emissions. The Environmental Protection Agency, for instance, has reported that U.S. emissions remained largely unchanged from 2004 to 2014 (SN Online: 4/14/16).

Methane enters the atmosphere from a range of sources, from decomposing biological material in wetlands to leaks in natural gas pipelines. Ton for ton, that methane causes 28 to 36 times as much warming as carbon dioxide over a century.

Since the start of the Industrial Revolution, atmospheric methane concentrations have more than doubled. By the early 2000s, though, levels of the greenhouse gas inexplicably flatlined. In 2007, methane levels just as mysteriously began rising again. The lull and subsequent upswing puzzled scientists, with explanations ranging from the abundance of methane-producing microbes to the collapse of the Soviet Union.

Those proposals didn’t account for what happens once methane enters the atmosphere. Most methane molecules in the air last around a decade before being broken apart during chemical reactions with hydroxyl. Monitoring methane-destroying hydroxyl is tricky, though, because the molecules are so reactive that they survive for less than a second after formation before undergoing a chemical reaction.
Neither study can show conclusively that hydroxyl levels changed, notes Stefan Schwietzke, an atmospheric scientist at the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colo. The papers nevertheless add a new twist in explaining the mysterious methane rise, he says. “Basically these studies are opening a new can of worms, and there was no shortage of worms.”

Despite being conducted by two separate teams — one headed by Rigby and the other by atmospheric scientist Alex Turner of Harvard University — the new studies used the same roundabout approach to tracking hydroxyl concentrations over time.

Both teams followed methyl chloroform, an ozone-depleting substance used as a solvent before being banned by the Montreal Protocol. Like methane, methyl chloroform also breaks apart in reactions with hydroxyl. Unlike methane, though, emission rates of methyl chloroform are fairly easy to track because the chemical is entirely human-made.

Examining methyl chloroform measurements gathered since the 1980s revealed that hydroxyl concentrations have probably wobbled over time, contributing to the odd pause and rise in atmospheric methane concentrations. But to know for sure whether hydroxyl levels varied or remained steady, scientists will need to take a more detailed look at regional emissions of methane and methyl chloroform, Rigby says.

Why hydroxyl levels might have fallen also remains unclear. Turner and colleagues note that the ban on ozone-depleting substances like methyl chloroform might be the cause. The now-recovering ozone layer (SN: 12/24/16, p. 28) blocks some ultraviolet light, an important ingredient in the formation of hydroxyl. Identifying the cause of the hydroxyl changes could help climate scientists better predict how methane levels will behave in the future.

NEW ORLEANS — A relatively small brain can pack a big evolutionary punch. Consider Homo naledi, a famously puzzling fossil species in the human genus. Despite having a brain only slightly larger than a chimpanzee’s, H. naledi displays key humanlike neural features, two anthropologists reported April 20 at the annual meeting of the American Association of Physical Anthropologists.

Those brain characteristics include a region corresponding to Broca’s area, which spans parts of the right and left sides of the brain in present-day people. The left side is typically involved in speech and language.
“It looks like Homo naledi’s brain evolved a huge amount of shape change that supported social emotions and advanced communication of some type,” said Shawn Hurst of Indiana University Bloomington, who presented the new findings. “We can’t say for sure whether that included language.” Frontal brain locations near Broca’s area contribute to social emotions such as empathy, pride and shame. As interactions within groups became more complex in ancient Homo species, neural capacities for experiencing social emotions and communicating verbally blossomed, Hurst suspects.

Scientists don’t know how long ago H. naledi inhabited Africa’s southern tip. If H. naledi lived 2 million or even 900,000 years ago, as some researchers have suggested (SN: 8/6/16, p. 12), humanlike brains with a language-related area would be shocking. A capacity for language is thought to have emerged in Homo over the last few hundred thousand years at most.

Discoverers of H. naledi, led by anthropologist Lee Berger of the University of the Witwatersrand in Johannesburg, will announce an estimated age for the species and describe new fossil finds within the next few weeks, Hurst said.

Hurst and Ralph Holloway of Columbia University led a team that laser scanned the inside surfaces of several partial H. naledi skulls to create virtual casts, or endocasts, of brain surfaces. An endocast reproduces the shape and, with varying success, details of the surface of the brain that were imprinted on the walls of the braincase while an individual was alive. Such brain impressions are not always clear, which has sparked debate over how to interpret them.

Two grooves identified on an endocast from a partial H. naledi skull frame the language-related section of Broca’s area in humans today, Hurst said. H. naledi’s brain also possessed folds of tissue that largely covered a surface section where the grooves converged. Similar folds of tissue typically cover the surface of Broca’s area in modern human brains.
The general shape of that part of the frontal brain in humans differs greatly from that of living apes and fossil hominids dating to at least 700,000 to 1 million years years ago, Hurst added.

H. naledi also displays a humanlike pattern of surface features at the back of the brain, although to a lesser extent than at the brain’s front, Holloway said. Endocasts for this analysis came from two other partial H. naledi skulls.

Specific protrusions and other features at the back of H. naledi’s brain are more pronounced on the left side, Holloway said. In people today, the same left-sided bias in brain organization is associated with right-handedness.

In the past, Holloway and anthropologist Dean Falk of Florida State University in Tallahassee have sharply disagreed over how to identify neural features on fossil endocasts, including a key groove in tissue at the back of the brain. After hearing Hurst and Holloway’s presentations, Falk expressed doubt that H. naledi’s brain was as humanlike as they concluded.

Shortly after the presentations, Hurst and Falk hashed out their differences head-to-head as they jointly studied a solid cast of the partial H. naledi brain surface displaying proposed signs of Broca’s area. They agreed on much about the fossil species’ neural setup, with one major exception. “I’m skeptical that two frontal [grooves] frame an area that corresponds to Broca’s area,” Falk said. If she’s right, then H. naledi communicated much less like present-day people than proposed by Hurst. Falk plans to study the new endocasts more closely and compare them with endocasts of other fossil hominids.

Seabirds called common murres appear to use preening as a way to negotiate whose turn it is to watch their chick and who must find food. And when one parent is feeling foul, irregularities in this grooming ritual may send the other a signal that all is not well, researchers report in the July issue of The Auk: Ornithological Advances.

“The fascinating part of this study is the inference that communication between mates allows murres to negotiate the level of effort that each member of the pair puts into the breeding effort,” says John Piatt, a wildlife biologist with the U.S. Geological Survey in Anchorage, Alaska. “Reproductive success of this species requires a high degree of cooperation by each mate as they switch duties.”
Common murres (Uria aalge) lay only one egg each breeding season. Parental roles aren’t determined by gender for the birds; mothers and fathers take turns watching over their chick and foraging for fish. When one parent returns with a fish for the chick, the couple preen each other and switch roles. This swapping ceremony typically happens three to four times a day.

But study coauthor Carolyn Walsh noticed that switches don’t always go smoothly. Video of 16 pairs of murres, documenting a total of 198 role swaps, showed that sometimes both birds appeared indecisive. Each parent would hop on and off the chick several times before the birds preened each other and one left to fish. “It’s as if they’re resisting leaving the colony; neither bird actually wants to go,” says Walsh, an animal behavior researcher at Memorial University of Newfoundland in Canada.
For about a fifth of all switching ceremonies, the brooding parent was slow to preen its mate and then refused to switch, forcing the parent that had just returned with a fish to go back out and fish some more.
Irregular behavior also occurred when the parent on fishing duty returned without food, which happened about 10 percent of the time. The empty-beaked bird would quickly start preening its mate, but the mate would be slow to preen back, or might not preen at all. “The brooder is basically communicating, ‘The chick still needs a fish, you better go get one,’” Walsh says.
The ceremony could be a way for the seabirds to communicate their well-being, Walsh says. By withholding preening and delaying the switching ceremony, a murre in poor condition may be trying to negotiate with its partner to have the easier job of brooding. Staying in the nest may allow the bird to rest and recover its strength.

Flying out to sea to fish is energetically costly for murres because they aren’t very aerodynamic. The seabirds are “absolutely ridiculous looking” when they fly, Walsh says. “Their wings are really meant for swimming in the water.”

In physical tests, Walsh and colleagues found a correlation between body condition and ceremony irregularities. Her team captured birds, weighed them and sampled their blood for beta-hydroxybutyrate, a metabolite associated with continual weight loss.

Switching ceremonies lasted about two minutes longer for the lightest birds, around 900 grams, compared with the heaviest birds weighing in at about 1,000 grams. Birds with lower mass and higher metabolite levels also were more likely to preen irregularly, Walsh says.

The longer ceremonies may also be a sign that there’s unrest in the nest. Murres usually mate for life, but pairs can “divorce.” A previous study by Walsh found that mates heading for a split take more time to switch roles.

The benefits of statins for people older than 75 remain unclear, a new analysis finds. Statins did not reduce heart attacks or coronary heart disease deaths, nor did they reduce deaths from any cause, compared with people not taking statins, researchers report online May 22 in JAMA Internal Medicine.

Recently published guidelines cited insufficient data to recommend statins for people older than age 75 who don’t have a history of cardiovascular disease. The new analysis considered a subset of older adults enrolled in a study of heart attack prevention and mortality conducted from 1994 to 2002. The sample included 2,867 adults ages 65 and older who had hypertension, 1,467 of whom took a statin.

There was no meaningful difference in the frequency of heart attacks or coronary heart disease deaths between those who took statins and those who did not. There was also no significant difference in deaths from any cause, both overall and among participants ages 65 to 74 or those 75 and older.

Statin use may be associated with muscle damage and fatigue, which could especially impact older adults and put them at higher risk for physical decline, the authors say.

For a third time, scientists have detected the infinitesimal reverberations of spacetime: gravitational waves.

Two black holes stirred up the spacetime wiggles, orbiting one another and spiraling inward until they fused into one jumbo black hole with a mass about 49 times that of the sun. Ripples from that union, which took place about 3 billion light-years from Earth, zoomed across the cosmos at the speed of light, eventually reaching the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, which detected them on January 4.
“These are the most powerful astronomical events witnessed by human beings,” Michael Landry, head of LIGO’s Hanford, Wash., observatory, said during a news conference May 31 announcing the discovery. As the black holes merged, they converted about two suns’ worth of mass into energy, radiated as gravitational waves.
LIGO’s two detectors, located in Hanford and Livingston, La., each consist of a pair of 4-kilometer-long arms. They act as outrageously oversized rulers to measure the stretching of spacetime caused by gravitational waves. According to Einstein’s theory of gravity, the general theory of relativity, massive objects bend the fabric of space and create ripples when they accelerate — for example, when two objects orbit one another. Gravitational ripples are tiny: LIGO is tuned to detect waves that stretch and squeeze the arms by a thousandth of the diameter of a proton. Black hole collisions are one of the few events in the universe that are catastrophic enough to produce spacetime gyrations big enough to detect.
The two black holes that spawned the latest waves were particularly hefty, with masses about 31 and 19 times that of the sun, scientists report June 1 in Physical Review Letters. LIGO’s first detection, announced in February 2016, came from an even bigger duo: 36 and 29 times the mass of the sun (SN: 3/5/16, p. 6). Astrophysicists don’t fully understand how such big black holes could have formed. But now, “it seems that these are not so uncommon, so clearly there’s a way to produce these massive black holes,” says physicist Clifford Will of the University of Florida in Gainesville. LIGO’s second detection featured two smaller black holes, 14 and eight times the mass of the sun (SN: 7/9/16, p. 8).
Weighty black holes are difficult to explain, because the stars that collapsed to form them must have been even more massive. Typically, stellar winds steadily blow away mass as a star ages, leading to a smaller black hole. But under certain conditions, those winds might be weak — for example, if the stars contain few elements heavier than helium or have intense magnetic fields (SN Online: 12/12/16). The large masses of LIGO’s black holes suggest that they formed in such environments.

Scientists also disagree about how black holes partner up. One theory is that two neighboring stars each explode and produce two black holes, which then spiral inward. Another is that black holes find one another within a dense cluster of stars, as massive black holes sink to the center of the clump (SN Online: 6/19/16).

The new detection provides some support for the star cluster theory: The pattern of gravitational waves LIGO observed hints that one of the black holes might be spinning in the opposite direction from its orbit. Like a cosmic do-si-do, each black hole in a pair twirls on its own axis as it spirals inward. Black holes that pair up as stars are likely to have their spins aligned with their orbits. But if the black holes instead find one another in the chaos of a star cluster, they could spin any which way. The potentially misaligned black hole LIGO observed somewhat favors the star cluster scenario. The measurement is “suggestive, but it’s not definite,” says astrophysicist Avi Loeb of Harvard University.

Scientists will need more data to sort out how the black hole duos form, says physicist Emanuele Berti of the University of Mississippi in Oxford. “Probably the truth is somewhere in between.” Various processes could contribute to the formation of black hole pairs, Berti says.

As with previous detections of gravitational waves, the scientists used their measurements to test general relativity. For example, while general relativity predicts that gravitational waves travel at the speed of light, some alternative theories of gravity predict that gravitational waves of different energies travel at different speeds. LIGO scientists found no evidence of such an effect, vindicating Einstein once again.

Now, with three black hole mergers under their belts, scientists are looking forward to a future in which gravitational wave detections become routine. The more gravitational waves scientists detect, the better they can test their theories. “There are already surprises that make people stop and revisit some old ideas,” Will says. “To me that’s very exciting.”

Paper wasps have a knack for recognizing faces, and a new study adds to our understanding of what that means in a wasp’s brain.

Most wasps of a given species look the same, but some species of paper wasp (Polistes sp.) display varied colors and markings. Recognizing these patterns is at the core of the wasps’ social interactions.

One species, Polistes fuscatus, is especially good at detecting differences in faces — even better than they are at detecting other patterns. To zero on the roots of this ability, biologist Ali Berens of Georgia Tech and her colleagues set up recognition exercises of faces and basic patterns for P. fuscatus wasps and P. metricus wasps — a species that doesn’t naturally recognize faces but can be trained to do so in the lab. After the training, scientists extracted DNA from the wasps’ brains and looked at which bits of DNA or genes were active.

The researchers found 237 genes that were at play only in P. fuscatus during facial recognition tests. A few of the genes have been linked to honeybee visual learning, and some correspond to brain signaling with the neurotransmitters serotonin and tachykinin. In the brain, picking up on faces goes beyond basic pattern learning, the researchers conclude June 14 in the Journal of Experimental Biology.

It’s possible that some of the same genes also play a broader role in how organisms such as humans and sheep tell one face from another.

Ancient rock artists were drawn to echo chambers. Members of early farming communities in Europe painted images in rock-shelters where sounds bounced off walls and into the surrounding countryside, researchers say.

Rock-shelters lacking such sound effects were passed up, at least in the central Mediterranean, report archaeologist Margarita Díaz-Andreu of the University of Barcelona and colleagues in the July Journal of Archaeological Science. In landscapes with many potential rock art sites, “the few shelters chosen to be painted were those that have special acoustic properties,” Díaz-Andreu says.
Some hunter-gatherer and farming groups studied over the past couple centuries believed in spirits that dwell in rock and reveal their presence via echoes. But acoustic evidence of special echoing properties at rock art sites is rare.

Díaz-Andreu’s team studied two rock art sites in 2015 and 2016. Baume Brune is a kilometer-long cliff in southeastern France. Of 43 naturally formed cavities in the cliff, only eight contain paintings, which include treelike figures and horned animals. Rock art in the Valle d’Ividoro, on Italy’s east coast, appears in an 800-meter-long section of a gorge. Only three of its 11 natural shelters contain painted images. Researchers generally date these French and Italian paintings to between roughly 6,500 and 5,000 years ago, several thousand years after the Stone Age had ended, Díaz-Andreu says.

To investigate the acoustics of the decorated and unadorned shelters, the researchers developed a new technique for determining the direction, intensity and timing of sound waves arriving at a particular point from every direction. A special microphone connected to a digital recorder measured the acoustic properties of any echoes set off by balloons popped just outside each rock-shelter. This setup was moved to various spots outside the caves to record the acoustic reach of reflected popping sounds. Echo measurements in France were taken at distances ranging from 22 to 36 meters from cliff shelters. Due to rougher terrain in Italy, measurements there were taken at distances ranging from 77 to 300 meters.
Then, the acoustic data were transformed into 3-D, slow-motion depictions of echoes, represented by moving circles, indicating where sound reflections originated. At both sites, shelters with rock paintings displayed better echoing properties than undecorated shelters, Díaz-Andreu says. And in each location, the shelter that best reflected echoes had the highest number of paintings.
“This novel technique shows a clear correlation between audible echoes and decorated shelters,” says music archaeologist Riitta Rainio of the University of Helsinki in Finland, who did not participate in the new study.

Echoes that bounce off steep rock cliffs bordering three lakes in northern Finland also attracted ancient artists, Rainio says. She and her colleagues took acoustic measurements at Finland’s painted cliffs from 2013 to 2016. Microphones placed on boats positioned at different spots on nearby lakes measured sound waves generated, in most trials, by a starter’s pistol. These Finnish paintings date to between around 7,200 and 3,000 years ago, Rainio says.

In some cases, echoes reflect directly from cliff paintings. “That, and possible drumming figures painted on the cliffs, suggest that sound played some role in rituals at these sites,” Rainio says. Her team will report its findings in an upcoming Journal of Archaeological Method and Theory.

Creators of older Stone Age cave art also appear to have focused on sites where echoes abounded, says archaeologist Paul Pettitt of Durham University in England. For instance, many roughly 14,000- to 12,000-year-old animal drawings and engravings at France’s Niaux Cave cluster in a cathedral-like chamber where sounds echo loudly.

“The new study provides convincing evidence that echoes, which were scientifically inexplicable to prehistoric people, played a determining role in how art was created,” Pettitt says.

Big, rogue planets — ones without parent stars — are rare.

A new census of free-floating Jupiter-mass planets determined that these worlds are a tenth as common as previous estimates suggested. The results appear online July 24 in Nature.

Planets can go rogue in two ways: They can get kicked out of their parent planetary systems or form when a ball of gas and dust collapses (SN: 4/4/15, p. 22).

In the new study, Przemek Mróz of the Astronomical Observatory of the University of Warsaw and colleagues estimated the number of large, rogue planets in our galaxy using a technique called microlensing. When an object with a mass of a planet passes in front of a distant, background star, the gravity of the planet acts as a gravitational magnifying glass. It distorts and focuses the light, giving up the planet’s existence.
Mróz and colleagues looked at 2,617 microlensing events recorded between 2010 and 2015 and determined which were caused by a rogue planet. For every typical star, called main sequence stars, there are 0.25 free-floating Jupiter-mass planets, the analysis suggests.

The new result sharply contrasts an estimate published in 2011, which suggested that rogue Jupiters are almost twice as common as main sequence stars. About 90 percent of stars in the universe are main sequence stars, so if that estimate were accurate, there should be a lot of free-floating Jupiters.

“That result changed our conceptual framework of the universe just a little bit,” says astronomer Michael Liu of the University of Hawaii in Honolulu. It challenged long-held ideas about how planets go rogue because the known methods wouldn’t generate enough planets to account for all the wanderers.

The 2011 result was based on a relatively small sample of microlensing events, only 474. Since then, infrared telescope images haven’t detected as many free-floating planets as expected. “Over the years, serious doubts were cast over the claims of a large population of Jupiter-mass free-floaters,” Mróz says.

David Bennett, coauthor of the 2011 study, agrees that the new census failed to find evidence for a large population of Jupiter-mass rogue planets. He notes, however, that the new data do reveal four times as many Jupiter-mass failed stars called brown dwarfs than predicted in the original census. So some of the rogues that were originally classified as planets may, in fact, be failed stars. Bennett, of NASA’s Goddard Space Flight Center in Greenbelt, Md., and colleagues are working on a new analysis of potential rogues with nearly 3,000 microlensing events and plan to compare their results with those from the new census.
Liu says the latest census is much more in line with theories of how planets form. Most rogues should be Earth-mass or a little heavier. Those lighter planets get tossed out of their planetary systems much easier than behemoths like Jupiter. Still, the smaller planets are harder to detect.

The new microlensing analysis did identify several events in which stars brightened and dimmed in less than half a day. Such short events hint at the existence of Earth-mass free-floaters because smaller planets with less gravity should brighten a distant star more briefly than more massive stars. Determining whether those small planets are really rogue and counting how many there are will take better telescopes, the team notes.