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Movies and other media are full of mixed messages about the risks and rewards of building machines with minds of their own. For every manipulative automaton like Ex Machina’s Ava (SN: 5/16/15, p. 26), there’s a helpful Star Wars droid. And while some tech titans such as Elon Musk warn of the threats artificial intelligence presents, others, including Mark Zuckerberg, dismiss the doomsayers.

AI researcher Toby Walsh’s Machines That Think is for anyone who has heard the hype and is seeking a critical assessment of what the technology can do — and what it might do in the future. Walsh’s conversational style is welcoming to nonexperts while his endnotes point readers to opportunities for deeper dives into specific aspects of AI.
Walsh begins with a history of AI, from Aristotle’s foundation of formal logic to modern facial-recognition systems. Excerpts from computer-composed poetry and tales of computers trouncing humans at strategy games (SN: 11/11/17, p. 13) are a testament to how far AI has come. But Walsh also highlights weaknesses, such as machine-learning algorithms’ reliance on so much data to master a single task.

This 30,000-foot view of AI research packs a lot of history, as well as philosophical and technical explanation. Walsh personalizes the account with stories of his own programming experiences, anecdotes about AI in daily life — like his daughter’s use of Siri — and his absolute, unapologetic love of puns.

Later in the book, Walsh speculates about technical hurdles that may curb further AI development and legal limits that society may want to impose. He also explores the societal impact that increasingly intelligent computers may have.
For instance, Walsh evaluates how likely various jobs are to be outsourced to AI. Some occupations, like journalist, will almost certainly be automated, he argues. Others, like oral surgeon, are probably safe. For future job security, Walsh recommends pursuing careers that require programming acumen, emotional intelligence or creativity.

AI also has the potential to revolutionize warfare. “Like Moore’s law, we are likely to see exponential growth in the capabilities of autonomous weapons,” Walsh writes. “I have named this ‘Schwarzenegger’s law’ to remind us of where it will end.” Walsh isn’t resigned to a Terminator-like future, though. If governments ban killer robots and arms developers use automation to enhance defensive equipment, he believes military AI could actually save many lives.

In fact, Walsh argues, all aspects of AI’s future impacts are in our hands. “Artificial intelligence can lead us down many different paths, some good and some bad,” he writes. “Society must choose which path to take.”

No anniversary list is ever complete. Just last month, for instance, my Top 10 scientific anniversaries of 2018 omitted the publication two centuries ago of Mary Shelley’s Frankenstein. It should have at least received honorable mention.

Perhaps more egregious, though, was overlooking the 125th anniversary of the physics journal Physical Review. Since 1893, the Physical Review has published hundreds of thousands of papers and has been long regarded as the premier repository for reports of advances in humankind’s knowledge of the physical world. In recent decades it has split itself into subjournals (A through E, plus L — for Letters — and also X) to prevent excessive muscle building by librarians and also better organize papers by physics subfield. (You don’t want to know what sorts of things get published in X.)
To celebrate the Physical Review anniversary, the American Physical Society (which itself is younger, forming in 1899 and taking charge of the journal in 1913), has released a list, selected by the journals’ editors, of noteworthy papers from Physical Review history.

The list comprises more than four dozen papers, oblivious to the concerns of journalists composing Top 10 lists. If you prefer the full list without a selective, arbitrary and idiosyncratic Top 10 filter, you can go straight to the Physical Review journals’ own list. But if you want to know which two papers the journal editors missed, you’ll have to read on.

10. Millikan measures the electron’s charge, 1913.
    When J.J. Thomson discovered the electron in 1897, it was by proving the rays in cathode ray tubes were made up of a stream of particles. They carried a unit of electrical charge (hence their name). Thomson did not publish in the Physical Review. But Robert Millikan did in 1913 when he measured the strength of the electric charge on a single electron. He used oil drops, measuring how fast they fell through an electric field. Interacting with ions in the air gave each drop more or fewer electric charges, affecting how fast the drops fell. It was easy to calculate the smallest amount of charge consistent with the various changes in speed. (OK, it was not easy at all — it was a tough experiment and the calculations required corrections for all sorts of things.) Millikan’s answer was very close to today’s accepted value, and he won the Nobel Prize in 1923.
11. Wave nature of electron, Davisson and Germer, 1927.
    J.J. Thomson’s son George also experimented with electrons, and showed that despite his father’s proof that they were particles, they also sometimes behaved like waves. George did not publish in the Physical Review. But Clinton Davisson and Lester Germer did; their paper established what came to be called the wave-particle duality. Their experiment confirmed the suspicions of Louis de Broglie, who had suggested the wave nature of electrons in 1924.
12. Particle nature of X-rays, Compton, 1923.
    Actually, wave-particle duality was already on the physics agenda before de Broglie’s paper or Davisson and Germer’s experiment, thanks to Arthur Holly Compton. His experiments on X-rays showed that when they collided with electrons, momentum was transferred just as in collisions of particles. Nevertheless X-rays were definitely a form of electromagnetic radiation that moved as a wave, like light. Compton’s result was good news for Einstein, who had long argued that light had particle-like properties and could travel in the form of packets (later called photons).
13. Discovery of antimatter, Carl Anderson, 1933.
    In the late 1920s, in the wake of the arrival of quantum mechanics, English physicist Paul Dirac was also interested in electrons. He applied his mathematical powers to devise an equation to explain them, and he succeeded. But he got out more than he put in. His equation yielded correct answers for an electron’s energy but also contained a negative root. That perplexed him; a negative energy for an electron seemed to make no physical sense. Still, the math was the math, and Dirac couldn’t ignore his own equation’s solutions. After some false steps, he decided that the negative energy implied the existence of a new kind of particle, identical to an electron except with an opposite electric charge (equal in magnitude to the charge that Millikan had measured). Dirac did not publish in the Physical Review. But Carl Anderson, who actually found Dirac’s antimatter electron in 1933, did. In cloud chamber observations of cosmic rays, Anderson spotted tracks of a lightweight positively charged particle, apparently Dirac’s antielectron. He titled his paper “The Positive Electron” and referred to the new particles as positrons. They were the first example of antimatter.
14. How stars shine, Hans Bethe, 1939.
    Since the dawn of science, astronomers had wondered how the sun shines. Some experiments in the 19th century suggested gravity. But a sun powered by gravitational contraction would have burned itself out long ago. A new option for powering the sun appeared in the 1930s when physicists began to understand the energy released in nuclear reactions. In the simplest such reaction, two protons fused. That made sense as a solar power source, because a proton is the nucleus of a hydrogen atom and stars are made mostly of hydrogen. But at a conference in April 1938, experts including Hans Bethe of Cornell University concluded that proton fusion could not create the temperatures observed in the brightest stars. On the train back to Cornell, though, Bethe figured out the correct, more complicated nuclear reactions and soon sent a paper to the Physical Review. He asked the journal to delay publishing it so he could enter it in a contest (open to unpublished papers only). Bethe won the contest and then OK’d publication of his paper, which appeared in March 1939. For winning the contest, he received $500. For the published paper, his prize was delayed — until 1967. In that year he got the Nobel Prize: $61,700.
15. Is quantum mechanics complete? Einstein, Podolsky and Rosen, 1935.
    Einstein was famous for a lot of things, including a stubborn resistance to the implications of quantum mechanics. His main objection was articulated in the Physical Review in May 1935 in a paper coauthored with physicists Nathan Rosen and Boris Podolsky. It presented a complicated argument that is frequently misrepresented or misunderstood (as I’ve discussed here previously), but the gist is he thought quantum mechanics was incomplete. Its math could not describe properties that were simultaneously “real” for two separated particles that had previously interacted. Decades later multiple experiments showed that quantum mechanics was in fact complete; reality is not as simple a concept as Einstein and colleagues would have liked. The “EPR paper” stimulated an enormous amount of interest in the foundations of quantum mechanics, though. And some people continue to believe E, P and R had a point.
16. Is quantum mechanics complete? (Yes.) Bohr, 1935.
    Here’s one of the missing papers. Physical Review’s editors somehow forgot to include Niels Bohr’s reply to the EPR paper. In October 1935, Bohr published a detailed response in the Physical Review, outlining the misunderstandings that EPR had perpetrated. Later EPR experiments turned out exactly as Bohr would have expected. (An early example from 1982 is among the Physical Review anniversary papers, but not this Top 10 list.) Yet some present-day critics still believe that somehow Bohr was wrong and Einstein was right. He wasn’t.
17. Gravitational waves detected by LIGO, 2016.
    Einstein was right about gravitational waves. After devising his general theory of relativity to explain gravity, he realized that it implied ripples in the very fabric of spacetime itself. Later he backed off, doubting his original conclusion. But he was right the first time: A mass abruptly changing its speed or direction of movement should emit waves in space. Violent explosions or collisions would create ripples sufficiently strong to be detectable, if you spent a billion dollars or so to build some giant detectors. In a hopeful sign for humankind, the U.S. National Science Foundation put up the money and two black holes provided the collision in 2015, as reported in February 2016 in Physical Review Letters and widely celebrated by bloggers.
18. Explaining nuclear fission, Bohr and Wheeler, 1939.
    On September 1, 1939, the opening day of World War II, the Physical Review published a landmark paper describing the theory of nuclear fission. It was a quick turnaround, as fission had been discovered only in December 1938, in Germany. While Einstein was writing a letter to warn President Roosevelt of fission’s potential danger in the hands of Nazis, Bohr and John Archibald Wheeler figured out how fission happened. Their paper provided essential theoretical knowledge for the Manhattan Project, which led to the development of the atomic bomb, and later to the use of nuclear energy as a power source.
19. Oppenheimer and Snyder describe black holes, 1939.
    The process of black hole formation was first described by J. Robert Oppenheimer and Hartland Snyder in the same issue of the Physical Review as Bohr and Wheeler’s fission paper. Of course, the name black hole didn’t exist yet, but Oppenheimer and Snyder thoroughly explained how a massive star contracting under the inward pull of its own gravity would eventually disappear from view. “The star thus tends to close itself off from any communication with a distant observer; only its gravitational field persists,” they wrote. Nobody paid any attention to black holes then, though, because Oppenheimer soon became director of the Manhattan Project (requiring him to read Bohr and Wheeler’s paper). It wasn’t until the late 1960s when black holes became a household name thanks to Wheeler (who eventually got around to reading Oppenheimer and Snyder’s paper). Yet for some reason the Physical Review editors omitted the Oppenheimer-Snyder paper from their list, verifying that no such list is ever complete, even if you have dozens of items instead of only 10.

Domesticated bunnies may need a new origin story.

Researchers thought they knew when rabbits were tamed. An often-cited tale holds that monks in Southern France domesticated rabbits after Pope Gregory issued a proclamation in A.D. 600 that fetal rabbits, called laurices, are fish and therefore can be eaten during Lent.

There’s just one problem: The story isn’t true. Not only does the legend offer little logic for rabbits being fish, but the proclamation itself is bogus, according to a new study of rabbit domestication.
“Pope Gregory never said anything about rabbits or laurices, and there is no evidence they were ever considered ‘fish,’” says Evan Irving-Pease, an archaeologist at the University of Oxford.

He and his colleagues discovered that scientists had mixed up Pope Gregory with St. Gregory of Tours. St. Gregory made a passing reference to a man named Roccolenus who in “the days of holy Lent … often ate young rabbits.” The misattribution somehow led to the story of rabbits’ domestication.

What’s more, DNA evidence can’t narrow rabbit domestication to that time period, Irving-Pease and colleagues report February 14 in Trends in Ecology and Evolution. Rabbit domestication wasn’t a single event, but a process with no distinct beginning, the researchers say. For similar reasons, scientists have found it difficult to pinpoint when and where other animals were first domesticated, too (SN: 7/8/17, p. 20).
Geneticist Leif Andersson of Uppsala University in Sweden agrees that genetic data can’t prove rabbit domestication happened around 600. But he says “it is also impossible to exclude that domestication of rabbits happened around that time period.”

Domestication practices were well known by then, Andersson says, and it’s possible that French monks or farmers in Southern France with a taste for rabbit meat made an effort to round up bunnies that eventually became the founding population for the domestic rabbit.

Ancient DNA from old rabbit bones may one day help settle the debate.

An amateur astronomer caught a supernova on camera during the explosion’s earliest moments, giving physicists a glimpse of a long-sought phase of stellar death.

Víctor Buso spotted the supernova from his rooftop observatory in Rosario, Argentina, on September 20, 2016, when he aimed his telescope straight overhead at spiral galaxy NGC 613 to test a new camera. To avoid letting in too much light from the city sky — Rosario is a city of about 1.2 million people — he took a series of about 100 images that were each exposed for 20 seconds, spanning about an hour and a half.
Over the last half-hour of Buso’s observations, the supernova appeared and then doubled in brightness. In 2013, astronomers spotted a supernova within hours of its explosion (SN Online: 2/13/17), but this is one of the first to be spotted before it exploded.

Because there is no way to predict when and where a supernova will go off, this sort of observation is extremely rare, says astrophysicist Melina Bersten of the National University of La Plata in Argentina, who reports details of the supernova in the Feb. 22 Nature.
“This is completely unusual, and was something that many people were searching for around the world without success,” Bersten says. “It was incredible.”

Bersten and her colleagues analyzed the light from the supernova and found that it matches models of the first phase of a supernova called the shock breakout phase, in which a shock wave from a massive star’s collapse ricochets back from the star’s core and pushes stellar material outward.

Termites are the new cockroach.

Literally. The Entomological Society of America is updating its master list of insect names to reflect decades of genetic and other evidence that termites belong in the cockroach order, called Blattodea.

As of February 15, “it’s official … that termites no longer have their own order,” says Mike Merchant of Texas A&M University in College Station, chair of the organization’s common names committee. Now all termites on the list are being recategorized.
The demotion brings to mind Pluto getting kicked off the roster of planets, says termite biologist Paul Eggleton of the Natural History Museum in London. He does not, however, expect a galactic outpouring of heartbreak and protest over the termite downgrade. Among specialists, discussions of termites as a form of roaches go back at least to 1934, when researchers reported that several groups of microbes that digest wood in termite guts live in some wood-eating cockroaches too.

Once biologists figured out how to use DNA to work out genealogical relationships, evidence began to grow that termites had evolved as a branch on the many-limbed family tree of cockroaches. In 2007, Eggleton and two museum colleagues used genetic evidence from an unusually broad sampling of species to publish a new tree of these insects (SN: 5/19/07, p. 318). Titled “Death of an order,” the study placed termites on the tree near a Cryptocercus cockroach.

Cryptocercus roaches live in almost termitelike style in the Appalachian Mountains, not too far from chemical ecologist and cockroach fan Coby Schal at North Carolina State University in Raleigh. Monogamous pairs of Cryptocercus roaches eat tunnels in wood and raise young there. The offspring feed on anal secretions from their parents, which provide both nutrition and starter doses of the wood-digesting gut microbes that will eventually let the youngsters eat their way into homes of their own.
Termites are “nothing but social cockroaches,” Schal says. Various roaches have some form of social life, but termites go to extremes. They’re eusocial, with just a few individuals in colonies doing all of the reproducing. In extreme examples, Macrotermes colonies can grow to 3 million individuals with only one queen and one king.

After several years of debate, the common names committee of the American entomologists’ organization voted it was time to switch to the new view of termites. At a February meeting of the society board, there was no objection. The common names of individual termite species, of course, will remain as something-something “termite.”

Considering whether to demote a whole order of insects is an uncommon problem, says Whitney Cranshaw of Colorado State University in Fort Collins, a longtime member of the society’s naming committee. “Probably some of us, including myself, didn’t want to make the change because we liked it the way it was,” he says. Termites and cockroaches as separate orders were easy to memorize for the undergraduates he teaches. Yet, he voted yes. “It’s what’s right.”

Two human mummies housed at the British Museum in London for more than a century boast the world’s oldest known — and longest hidden — tattoos of figures and designs, a new investigation finds. These people lived in Egypt at or shortly before the rise of the first pharaoh around 5,100 years ago.

Radiocarbon analyses of hairs from the mummies date the bodies to between 3351 B.C. and 3017 B.C., says a team led by Egyptologist Renée Friedman of the University of Oxford and bioarchaeologist Daniel Antoine of the British Museum in London. Infrared photography revealed that smudges on a male mummy’s upper right arm depict a wild bull and a Barbary sheep, while a female mummy bears four S-shaped patterns on her right shoulder and a line with bent ends on her right arm. These animals and figures appear in Egyptian art from the same period, the researchers report online March 1 in the Journal of Archaeological Science. Both sets of tattoos — which consist of a carbon-based pigment, possibly soot — may have symbolized power, social status or knowledge of cult activities, but their precise meanings are unclear.
The two were the only mummies found with tattoos, out of seven mummies originally buried at a southern Egyptian site and now held at the British Museum. All of the bodies had been preserved by the desert’s dry heat.

The tattooed Egyptian mummies are approximately as old as Ötzi the Iceman. The mummified man found in the Italian Alps has 61 dark lines tattooed on his limbs and torso, but no pictures or designs (SN: 1/23/16, p. 5). Some of the Iceman’s tattoos covered areas of joint disease and may have been intended as treatments. CT scans of the two Egyptian mummies found no signs of bone disease near or below tattoos.

A new analysis is shedding light on drought in Mongolia, both past and future.

By studying the rings of semifossilized trees, researchers constructed a climate history for the semiarid Asian nation spanning the last 2,060 years — going 1,000 years further back than previous studies.

It was suspected that a harsh drought from about 2000 to 2010 that killed tens of thousands of livestock was unprecedented in the region’s history and primarily the result of human-caused climate change. But the tree ring data show that the dry spell, while rare in its severity, was not outside the realm of natural climate variability, researchers report online March 14 in Science Advances.
“This is a part of the world where we don’t know about the past climate,” says Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y., who was not involved with the study. “Having this record is a great resource for trying to understand past droughts in the region.”

In recent years, many studies have sought to unsnarl the role of anthropogenic climate change from natural variability on extreme weather events (SN: 1/20/18, p. 6). Such work is necessary for more accurately predicting future climate trends and helping governments prepare for the most severe scenarios, says study coauthor Amy Hessl, a physical geographer at West Virginia University in Morgantown. This is especially true in countries like Mongolia that lack certain infrastructure, such as enough water reservoirs, to ease the impact of events like prolonged drought.

Hessl and her colleagues studied tree rings in hundreds of samples of Siberian pines, well-preserved by Mongolia’s naturally dry climate. A ring’s width indicates how much the tree grew in a year. In wet years, the rings are wider; in dry years, skinnier.
The recent dry spell was the severest in recorded history. But the rings showed that an even more severe drought took place around the year 800, long before anthropogenic climate change began.

Still, computer simulations suggest that about a third of the recent drought’s severity could have been caused by elevated temperatures linked to climate change, the researchers found. The finding is consistent with studies on how climate change has affected other recent droughts in South Africa and California.

Using computer simulations, Hessl and her colleagues conclude that droughts in coming decades may not be any worse than those seen in Mongolia’s past. The team predicts that as global temperatures rise over the next century, Mongolia will first become drier, then wetter. Increased heat initially will dry out the plains. But at a certain point, hot air holds more moisture, leading to increased precipitation.

Those climate patterns will likely guide how Mongolia develops, Hessl says, because they have in the past. In 2014, she and colleagues published a paper detailing how a 15-year period of unprecedented temperate and rainy conditions in 13th century Mongolia may have led to the rise of Genghis Khan (SN Online: 3/10/14).

After some initial waffling, President Trump signed a budget bill March 23 that lays out spending details for the rest of fiscal year 2018, which goes through September. The $1.3-trillion spending deal boosts funding for nearly all science agencies, avoiding cuts the White House had proposed.

These increases stem largely from February’s budget deal, which raised caps on discretionary spending. The National Institutes of Health, which got a $3 billion bump over the 2017 level, comes out especially well, as does the Department of Energy’s Office of Science. It receives an $868 million boost, a 16 percent increase. Climate science research and programs across several agencies also avoided big cuts that the administration had proposed.
The one outlier: the Environmental Protection Agency. At about $8.1 billion, its budget remains flat at the 2017 level.

Here are a few details that stood out to us.

NIH
NIH gets an extra $414 million for Alzheimer’s disease research, along with $400 million for the BRAIN Initiative, a research project announced by President Obama in 2013 that aims to improve our understanding of the human brain.
DOE Office of Science
Trump had proposed eliminating the Advanced Research Projects Agency-Energy, which funds research into long-shot but potentially high-reward projects. But the bill allocates an additional $47 million to the agency, a resounding rejection of the administration’s request.

NASA
The Wide-Field Infrared Survey Telescope, or WFIRST, is a proposed mission to study, in part, planets orbiting stars outside the solar system. It was recommended by the most recent decadal survey for astrophysics and remains one of NASA’s top astrophysics priorities after the James Webb Space Telescope. Trump has proposed canceling WFIRST, but the spending package includes $150 million for the telescope. Four earth science programs that Trump had targeted for elimination were also funded.

CDC
The agreement includes wording clarifying that the Centers for Disease Control and Prevention isn’t barred from studying gun violence under a 1996 amendment. The CDC has shied away from such research since that time. On Twitter, some celebrated the news, but others hailed it as a partial victory. Megan Ranney, an emergency physician and researcher at Brown University in Providence, R.I., who studies firearm injuries, tweeted the “announcement changes nothing. The issue has been, and continues to be, lack of appropriations for research.”

EPA
The agency’s overall budget remains flat as do projects with EPA’s Office of Science & Technology. Although not research related, the agency’s regulatory programs are cut by $23.5 million. The bill includes wording that prohibits the regulation of lead content in ammunition and fishing tackle and exempting livestock producers from EPA greenhouse gas regulations.

USGS
The bill includes $23 million for ShakeAlert, an earthquake early warning system on the West Coast that could be rolled out as early as October, another project that had been facing elimination. All eight of the U.S. Geological Survey’s climate science centers are funded in the agreement. The administration had proposed cutting their numbers in half, to four.

It’s tough to be a frog once a killer skin fungus moves in. But, in Panama, the amphibians might be fighting back, researchers propose.

More than a decade ago, an amphibian-killing chytrid fungus nicknamed Bd swept through the country. Now some frog species that had nearly vanished from three regions are growing easier to spot again. But tests of the pathogen find no signs that it is weakening, says disease ecologist Jamie Voyles at the University of Nevada, Reno. With the fungus as dangerous as ever, frogs becoming resistant to the pathogen might be enabling the recovery, Voyles and her colleagues report in the March 30 Science.
Despite any glimmer of hope, it’s too early to celebrate frog recovery, protests ecologist Karen Lips at the University of Maryland in College Park. She doesn’t doubt that researchers have found frogs in the devastated regions, but wants more rigorous monitoring before talking population trends.

The three areas in the study have special resonance. They’re where Lips and other scientists set up a disaster watch as they realized that Bd, short for Batrachochytrium dendrobatidis, was sweeping through Central America, killing many amphibians in its path. As the deadly wave approached, the researchers collected data and living animals in the hope they would help in before-and-after studies. The fungus attacked Lips’ site, El Copé, in 2004 and the other two sites in 2006 and 2007.

Voyles was working at El Copé as a graduate student when Bd arrived. She remembers the abundance of sick and dying animals, and the task of collecting the dead. “That was shocking, really — there’s just no other word for it.”

Using archived, frozen samples of the fungus from those sites, Voyles and colleagues present the first comparison of Bd as it was in 2004 versus in 2012 to 2013, when amphibian communities showed signs of recovering. At first, Voyles suspected that the pathogen was growing less dangerous. Bd attacks a wide range of hosts but evolutionarily speaking, pathogens don’t persist if they drive too many of their hosts extinct.
A waning fungal menace could have explained how nine frog species that had almost disappeared in the area were now easier to spot. But the pathogen appears as lethal as ever, she and her colleagues report. Compared with the older versions, the more recent fungus samples grew and reproduced at about the same speeds, and inhibited immune cells at about the same rates. All fungi killed the test frogs of two species.

“That’s an important result,” says evolutionary ecologist James Collins of Arizona State University in Tempe. Biologists who study the interplay of pathogen and host often ask how the two change in relation to one another over time. But having real data on the relationship is much rarer. Also important, he says, is recognizing that “many species at the Panama sites are still missing, so wholesale recovery isn’t occurring.”
To explain the encouraging cases, the researchers propose that in some species, frog skin secretions may be getting better at repelling the fungus. For wild animals of six amphibian species, secretions from pre-disease days were worse at inhibiting fungal growth in a lab test than secretions from the same species at sites that the fungus had hit. In another test, secretions from wild variable harlequin frogs (Atelopus varius) were about eight times as effective at inhibiting the fungus as were those from zoo animals descended from frogs collected before Bd reached their site.

Of the many amphibian populations that Bd savaged around the world (SN: 3/5/16, p. 14), a few have bounced back in numbers. Sierra Nevada yellow-legged frogs now abound again in the California mountains, ecologist Vance Vredenburg of San Francisco State University points out. Yet these animals are descendants of a worryingly tiny percentage of the original, diverse population. Despite rebounds, attacks by deadly fungi are still “a really big deal,” he says.

NASA is stepping up its search for planets outside our solar system. Its next exoplanet hunting telescope, the Transiting Exoplanet Survey Satellite (TESS), is due to launch from Cape Canaveral on the evening of April 16.

Following the Kepler space telescope’s discovery of more than 5,000 possible exoplanets since 2009, TESS will continue the galactic census — flagging more planetary candidates for further study.

Astronomers expect TESS to find about 20,000 planets in its first two years in operation, focusing on nearby, bright stars that will be easy for other telescopes to investigate later. About 500 of those expected exoplanets would be less than twice the size of Earth — and therefore may be good places to look for life.
The TESS mission is “a whole new opening for exoplanet studies,” MIT astronomer Sara Seager, TESS’ deputy science director, said during a news conference describing the upcoming launch.
TESS will be the first NASA science mission launched on the SpaceX Falcon 9 rocket. Once in orbit, the spacecraft will trace an unusual, elliptical path between Earth and the moon that will enable it to observe at least 85 percent of the sky — 350 times as much sky as Kepler saw.
Most of the planets found by Kepler orbit stars 1,000 light-years away or farther. TESS will focus on 200,000 stars that are a few hundred light-years away at most, and shine between 30 to 100 times brighter on average than Kepler’s.

The brighter the star, the easier it is to determine its planet’s characteristics, such as its mass and whether it has an atmosphere, Seager says. “Photons are our currency — the more, the better,” she says.

That follow-up will help TESS avoid some of Kepler’s pitfalls. Because Kepler’s stars were so far and so dim, some of its planet candidates were confirmed as actual planets only by statistics rather than by other telescopes. And not all those confirmations may stick. A recent paper posted at arXiv.org showed that Kepler 452b, an Earth-sized planet that orbits a sunlike star at the same distance Earth orbits the sun, may be a mirage (SN: 8/22/15, p. 16). Many of TESS’ planets won’t face the same uncertainty.
But the way TESS will search for exoplanets is the same as Kepler: The satellite will watch stars for signs of dimming, which can indicate that a planet is transiting, or crossing in front of, the star. Measuring how much starlight is blocked can tell astronomers the size of the planet.

Once TESS finds a planet, astronomers will need more information to understand its qualities, such as whether it’s rocky or gassy (SN Online: 6/19/17). For that, other telescopes will follow up. Ground-based telescopes will measure the gravitational tug of a planet on its host star to learn the planet’s density, which is a clue to its composition. Astronomers plan to measure masses for at least 50 TESS planets that are smaller than Neptune in the hopes that many of them will have rocky, and therefore potentially habitable, surfaces.
NASA’s James Webb Space Telescope, now scheduled to launch in 2020, will then check some of those planets for signs of life (SN: 4/30/16, p. 32).

“This is one of the major questions that TESS is intended to answer: Where will we be pointing Webb?” said the mission’s principal investigator, MIT astronomer George Ricker, at the press conference. Webb will peer at the starlight filtering through planetary atmospheres to try to detect molecules that could be produced by something living on the surface.

It will take a few months for TESS to swing into its regular orbit before it begins collecting data. At that point, it will be able to use the moon’s gravity to stabilize itself for decades in orbit without using extra fuel. The mission is set to last two years, but could continue taking data almost indefinitely.

“TESS is not going to be limited by any expendable or other aspects,” Ricker said. “It will be basically limited by how long NASA has the patience to fund the mission.”