November 4, 2013
Over the past 18 years, astronomers have discovered 1038 planets orbiting distant stars. Disappointingly, though, the vast majority don’t seem like candidates to support life as we know it—they’re either so close to their home star that all water would likely evaporate, or so far away that all of it would freeze, or they’re made up of gas instead of rock and more closely resemble our solar system’s gas giants than Earth.
Or so we thought. Today, a group of scientists from UC Berkeley and the University of Hawaii published a calculation suggesting that we’ve overlooked evidence of a vast number of Earth-sized exoplanets in the habitable zone of their stars, simply because these planets are harder to detect with current methods. They believe that, on average, 22% of Sun-like stars (that is, stars with a size and temperature similar to the Sun) harbor a planet that’s roughly Earth-sized in their habitable zones.
“With about 100 billion stars in our Milky Way galaxy, that’s about 20 billion such planets,” said Andrew Howard, one of the study’s co-authors, in a press conference on the findings. “That’s a few Earth-sized planets for every human being on the planet Earth.”
The team, led by Erik Petigura, came to these conclusions by taking an unconventional approach to planet-finding. Instead of counting how many exoplanets we’ve found, they sought to determine how many planets we’re unable to see.
Exoplanets are detected as a result of rhythmic dimming in a star’s brightness, which indicates that there’s a planet orbiting it and passing between the star and our vantage point. Because of this method, large planets that orbit closely to their stars have been the easiest to find—they block more light, more often—and thus disproportionately dominate the list of known exoplanets.
To estimate the number of exoplanets this technique misses, the Berkeley team wrote a software program that analyzed data from the Kepler mission, an exoplanet-hunting NASA telescope launched into orbit in 2009. Initially, to confirm the program’s accuracy, they fed it the same data from 42,557 Sun-like stars that had already been scrutinized by other astronomers, and it indeed detected 603 candidate planets, all of which had already been found.
When it parsed the data further to find Earth-like planets—using the length of time between dimmings to indicate how far out the planet orbits the star, and the degree of dimming to indicate us how much of the star is blocked by the planet, and thus the exoplanet’s size—it found 10 potential exoplanets that are between one and two times the size of Earth and orbit in what is likely the star’s habitable zone. This, too, aligned with previous findings, showing the program could accurately detect planets.
But what the researchers really wanted to do was determine the overall prevalence of Earth-like exoplanets. To calculate this number, they first had to determine just how many weren’t detected in the survey. “One way of thinking of it is that we’re doing a census of habitable exoplanets, but not everyone’s answering the door,” Petigura explained.
There are a few reasons that a planet might not be detected. If its orbit doesn’t take it into a location that would block the path of light between its star and our telescopes, we’d have no way of seeing it. Alternately, it could successfully block starlight, but the event could be lost amid natural variation in the brightness of the star as we perceive it on Earth.
Both of these possibilities, it turns out, make it disproportionately hard to find Earth-like exoplanets. “Planets are easier to detect if they’re bigger, and closer to their host stars,” Howard said. “Thus it’s no accident that hot Jupiters were the first planets to be discovered.” Simply by virtue of physics, smaller, Earth-sized planets that may orbit a bit farther out are less likely to pass directly in front of their stars, from our perspective.
To finding out how many Earth-like planets we likely miss as a result, the scientists altered the Kepler data by artificially introducing 40,000 more exoplanets similar to Earth—roughly one per star—then feeding the resulting data back into the planet detection software. This time, it only found about one percent of the Earth-like planets introduced, because the vast majority didn’t cause a detectable dimming of their star.
This means that, with current detection methods, 99 out of 100 Earth-like aren’t coming to the door when to answer our interstellar census. Accounting for this level of imperfection, the researchers calculated that far more Sun-like stars are home to a potentially habitable, Earth-sized exoplanet than we previously thought.
It’s important to note that this is a theoretical calculation: The scientists didn’t actually discover these sorts of planets orbiting 22% of the stars. But if the underlying assumptions are accurate, it does give hope to the possibility that we’ll find more potentially habitable planets in the future. In fact, the researchers calculated that if the prevalence of these sorts of planets is uniform across the galaxy, odds are that one can be found tantalizingly nearby—about 12 light years away from Earth.
It’s still unknown whether these planets might have the other ingredients that we believe are likely necessary for life: a protective atmosphere, the presence of water and a rocky surface. But the researchers say another recent finding makes them hopeful that some of them have potential. Earlier this week, scientists found a rocky, Earth-sized exoplanet roughly 700 light-years away. Although that planet is certainly too hot to harbor life, it has density similar to that of Earth—suggesting that at least some of the Earth-sized planets we’ve failed to detect so far have a geologic composition similar to our own planet’s.
October 10, 2013
The star GD61 is a white dwarf. As such, it’s insanely dense—similar in diameter to Earth, but with a mass roughly that of the Sun, so that a teaspoon of it is estimated to weigh about 5.5 tons. All things considered, it’s not a particularly promising stellar locale to find evidence of life.
But a new analysis of the debris surrounding the star suggests that, long ago, GD61 may have provided a much more hospitable environment. As part of a study published today in Science, scientists found that the crushed rock and dust near the star were once part of a small planet or asteroid made up of 26 precent water by volume. The discovery is the first time we’ve found water in a rocky, Earth-like planetary body (as opposed to a gas giant) in another star system.
“Those two ingredients—a rocky surface and water—are key in the hunt for habitable planets,” Boris Gänsicke of the University of Warwick in the UK, one of the study’s authors, said in a press statement. “So it’s very exciting to find them together for the first time outside our solar system.”
Why was water found in such a seemingly unhospitable place? Because once upon a time, GD61 wasn’t so different from our Sun, scientists speculate. But roughly 200 million years ago, when it exhausted its supply of fuel and could no longer sustain fusion reactions, its outer layers were blown out as part of a nebula, and its inner core collapsed inward, forming a white dwarf. (Incidentally, this fate will befall an estimated 97 percent of the stars in the Milky Way, including the Sun.)
When that happened, the tiny planet or asteroid in question—along with all the other bodies orbiting GD61—were violently knocked out of orbit, sucked inward, and ripped apart by the force of the star’s gravity. The clouds of dust, broken rock and water that the scientists recently discovered near the star are the remnants of these planets.
Even in its heyday, the watery body was probably still very small—perhaps comparable in size to our solar system’s dwarf planet Ceres, which orbits in the asteroid belt and is about .015 percent the mass of Earth. Additionally, like Ceres, the ancient planet or asteroid was extremely water-rich (26 percent water, far more than Earth’s .023 percent), and this water was similarly constituted as ice locked within a rocky crust.
To find all this out, the group of scientists (which also includes Jay Farihi of the University of Cambridge and Detlev Koester of the University of Kiel) used observations from two sources: a spectrograph on board the Hubble Space Telescope, through which they obtained data on ultraviolet light emitted by GD61, and a telescope at the W.M. Keck Observatory on Mauna Kea on Hawaii.
By looking at the light emitted from the star, which glows in certain patterns depending on the chemical signatures of gases present, they were able to determine the proportions of a number of elements (including oxygen, magnesium, aluminum, silicon, calcium and iron) contained within the cloud of dust that surrounds it. Using computer simulations of this stellar atmosphere, they were able to rule out a number of alternate possibilities that could have accounted for the abundance of oxygen, leaving only the explanation that it was brought there in water form.
Based on the amount of water and rocky minerals detected in the star’s atmosphere—and assuming it all came from one body—scientists speculate that the small planet or asteroid ripped up by the white dwarf was at least 56 miles in diameter, but perhaps much larger.
Although the star certainly isn’t home to any life at the moment due to its relatively cold temperature, the finding makes it seem more likely that other exoplanets contain water, which is necessary for life as we know it. Many scientists have speculated that small planets and asteroids like Ceres delivered water to Earth in the first place, so finding evidence of a watery body like this in another star system raises the possibility that the same process may have brought water to an Earth-sized planet elsewhere too.
“The finding of water in a large asteroid means the building blocks of habitable planets existed—and maybe still exist—in the GD 61 system, and likely also around a substantial number of similar parent stars,” Farihi said. “These water-rich building blocks, and the terrestrial planets they build, may in fact be common.”
September 4, 2013
Sometime in the next two or three months, something special will happen: the magnetic field that emanates from the Sun and extends throughout the entire solar system will reverse in polarity.
“It’s really hard to say exactly when it’s going to happen, but we know it’ll be in the next few months, for sure,” says Andrés Muñoz-Jaramillo, a researcher at the Harvard-Smithsonian Center for Astrophysics who studies the Sun’s magnetic cycle. “This happens every solar cycle, and it’s a very special day when it does.”
First, the basics: the Sun, like Earth, naturally generates a magnetic field. The massive solar magnetic field is a result of the flow of plasma currents within the Sun, which drive charged particles to move from one of the Sun’s poles to another.
Every 11 years, the strength of this magnetic field gradually decreases to zero, then emerges in the opposite direction, as part of the solar cycle. It’s as if, here on Earth, compasses pointed towards the Arctic as “North” for 11 years, then briefly wavered, then pointed towards Antarctica as “North” for the next 11 years (in fact, the Earth’s magnetic field does reverse as well, but it occurs with much less regularity, and takes a few hundred thousand years to do so).
Recent observations indicate that the next solar magnetic reversal is imminent—in August, NASA announced that it was three or four months away. The reversal, explains Muñoz-Jaramillo, won’t be a sudden, jarring event but a gradual, incremental one. “The strength of the polar field gradually gets very close to zero,” he says. “Some days, it’s slightly positive, and other days, it’s slightly negative. Then, eventually, you see that it’s consistently in one direction day after day, and you know the reversal has occurred.” His research group’s measurements of the magnetic field suggest this reversal is a few months away, but it’s impossible to say for sure which day it’ll occur.
Because the region that the solar magnetic field influences includes the entire solar system, the effects of the reversal will be felt widely. “The magnetic field flows out into interplanetary space, and it forms a bubble that encloses the solar system as it travels through the galaxy,” Muñoz-Jaramillo says.
One aspect of this bubble—formally known as the heliosphere—is an invisible electrically-charged surface called the current sheet pervades the solar system and resembles a twisted ballerina’s skirt, because the rotation of the Sun twists its far-flung magnetic field into a spiral. The reversal of the field will cause the sheet to become more rippled, which in turn will lead the Earth to pass through the sheet more frequently as it orbits the Sun.
Passing through more often could cause more turbulent space weather, potentially leading to disruptions in satellite transmissions and telecommunications equipment. On the other hand, the current sheet also blocks high-energy cosmic rays that arrive from other areas of the galaxy, so a more wavy sheet could provide satellites and astronauts in space more robust protection from harmful radiation.
Additionally, the magnetic field reversal coincides with the maximum of other solar activity, which means a greater number of sunspots, more powerful solar flares, brighter aurorae and more frequent coronal mass ejections. Most of these events have little or no effect on Earth, but an especially powerful flare or plasma ejection aimed in the right direction could knock out Earth-based telecommunications systems. At the same time, this solar cycle has been especially weak—NASA solar physicist David Hathaway called it “wimpy” in an interview with Scientific American—so there’s not a ton to worry about with this particular reversal.
For Muñoz-Jaramillo, who spends his days monitoring and analyzing the Sun’s magnetic activity, the reversal will also have personal significance. “Because the cycle is such a long process, in terms of a human’s lifetime, a solar scientist is going to see maybe four reversals in a career,” he says. “That makes every turning point special—and this is the first time I’m seeing one of these since I started studying solar physics.”
For more on the solar reversal, take a look at NASA’s video:
August 29, 2013
A rocky, icy body the size of Rhode Island is playing follow the leader with the seventh planet from the Sun, whizzing along Uranus’ orbit one-sixth of a revolution ahead of the planet. The body, temporarily dubbed 2011 QF99, is the first of its type found to circle with Uranus. Researchers reporting in the journal Science document its detection and show that it is probably not alone
, promising a clearer picture of the ongoing celestial pinball game in the solar system’s outer reaches.
Thousands of similarly positioned bodies are known to exist around Jupiter; they are called Trojans because each is named for a mythological character in the Trojan War. But scientists had believed that gravitational tug around Uranus and Saturn, particularly the pull of Jupiter, made similar companions there unlikely.
What exactly are Trojans? Their story dates back to the 18th century, when a famous mathematician named Joseph-Louis Lagrange wrote an essay on the problem of three bodies, identifying five positions where the gravitational effects of a body orbiting another body (think of the Earth-Moon system as a single body circling the Sun) would allow a third smaller body to stay balanced. When located at any of these five Lagrange Points, the third body would appear stationary relative to the other two. Three of these five positions, called L1, L3 and L3, would be unstable–if the third body drifted just a bit off course from any of these positions, it could never recover from the misstep. L1 and L2 are ideal locations for placing artificial satellites that study the Sun and space, although the spacecrafts’ trajectories have to be constantly tweaked so that they stay at these points.
But at two Lagrange Points, dubbed L4 and L5, the body would be pulled right back regardless of which way it drifted, causing it to swing around the point like a gymnast on a high bar. In fact, multiple bodies–many thousands–could dance around each point within an elongated region of stability that contours to the orbital path of the planet. One of these points sits 60 degrees ahead on that orbital path and another 60 degrees behind.
Other three-body systems have these same balance points, and in 1906 astronomers found an asteroid in the
L4 region of Jupiter’s orbit around the Sun, naming the body Achilles. In the following years, more Trojan asteroids were discovered around Jupiter’s L4 and L5 and, more recently, Trojans have been found along other planets’ orbits, including Mars’, Neptune’s and even Earth’s.
But none had turned up for Uranus or Saturn–until now. As part of a Canada-France-Hawaii Telescope survey designed to search for small bodies orbiting beyond the most distant planet, Neptune, a team of astronomers spotted 2011 QF99 in three images taken an hour apart on the same patch of sky. The object’s brightness suggested it was 60 kilometers across and its orbit pinned it as distant as Uranus, but further observations in 2011 and 2012 distinguished it from a Centaur, an unstable icy body that orbits the Sun and occasionally crosses, but doesn’t follow or lead, planetary orbits. The team’s study showed 2011 QF99 running out ahead of Uranus like a dog on a leash: It was an L4 Trojan.
“A Uranian Trojan was not the focus of our survey,” says Mike Alexandersen, an astronomer at the University of British Columbia. “When we realized what it was, we were like ‘Whoa, wow.’”
Unlike most other known Trojans, which adopted their current positions early during the solar system’s formation, 2011 QF99 was probably first a Centaur and was captured at L4 later on, caught as it leaked inward from more distant reaches. Numerical analyses of the details of the orbit of 2011 QF99 suggest it will remain as a Trojan for 70,000 years before, after a million years or so, it moves beyond the L4 region of stability and rejoins the Centaurs.
2011 QF99, then, is a temporary Trojan. And simulations by Alexandersen and his team, reported for the first time in the new paper, find that 2011 QF99 is not alone. About 3 percent of the small bodies in the outer solar system share an orbit with Neptune or Uranus at any given time. “There are a lot of asteroids and comets flying around the solar system, and a lot of them cross the orbits of planets and only a tiny fraction get captured,” he says. Capture is “
a low probability event. Intuitively, we thought it had an even lower probability.”
While the more permanent Trojans have quite a lot to to say about primordial jostling, the temporary Trojans–including others discovered orbiting with Neptune and Earth–could reveal information on the amount of Centaurs populating the nether reaches, how exactly they got there and what paths they follow.
“Those unstable objects, the Centaurs, often go on to become Jupiter-family comets, many of which approach the Earth and could, eventually, pose an impact threat,” says Jonti Horner, an astronomer at the University of New South Wales who wasn’t involved in the study. “Being able to study those objects when they’re far from the Sun, and therefore not hidden by a cometary coma, can tell us a lot about comets and other objects that can threaten Earth.”
“It’s a really exciting discovery for me, and for other people who look at the solar system’s small bodies,” he added.
Alexandersen, who notes that the risk of impact is extremely low, says the results speak to how much is still left to know about our solar system. He predicts that more will be revealed as astronomers continue to detect smaller and smaller objects. “If there is one 60-kilometer Trojan, then there are probably dozens of one-kilometer Trojans,” he says. “We just can’t see them yet.”
August 28, 2013
If the phenomena of Star Trek, Area 51, Ancient Aliens, or War of the Worlds can be taken as anthropological clues, humanity is consumed with curiosity about the possibility of life beyond Earth. Do any of the 4,437 newly discovered extrasolar planets contain traces of life? What would these life forms look like? How would they function? If they came to Earth, would we share ET-esque embraces or would the visit be more a Battle Los Angeles style throw down?
Life outside of Earth has spawned endless interest, but less public interest seems to be given to how life on Earth began 3 to 4 billion years ago. But the two topics, it turns out, might be more connected than one would believe–in fact, it’s possible that life on Earth really began outside of Earth, on Mars.
At this year’s Goldschmidt conference in Florence, Steve Benner, a molecular biophysicist and biochemist at the Foundation for Applied Molecular Evolution will present this idea to an audience of geologists. He’s well aware that half the room will be adamantly against his idea. “People will probably throw things,” he laughs, hinting at a consciousness of how out-of-this-world his ideas sound. But there’s scientific basis for his assertion (PDF), a logical reason for why life maybe truly did begin on Mars.
Science holds a number of paradoxes: If there are an infinite number of stars in the sky, why is the night sky dark? How can light act as both a particle and a wave? If the French eat so much cheese and butter, why is the incidence of coronary disease in their country so low? The origins of life are no different; they, too, are dictated by two paradoxes: the tar paradox, and the water paradox. Both, according to Benner, make it difficult to explain the creation of life on Earth. But both, he also notes, can be solved by placing the creation of life on Mars.
The first, the tar paradox, is simple enough to understand. “If you put energy into organic material it turns to asphalt, not to life,” Benner explains. Without access to Darwinian evolution–that is, without organic molecules having the opportunity to reproduce and create offspring who themselves, mutations and all, are reproducible–organic matter that is bathed in energy (from sunlight or from geothermal heat) will turn into tar. Early Earth was full of organic materials–chains of carbon, hydrogen and nitrogen that are believed to be the building blocks of life. Given the tar paradox, these organic materials should have devolved into asphalt. “The question is, how is it possible that the organic materials on early Earth managed to leap from their asphaltic fate to something that had access to Darwinian evolution? Because once that happens–presumably–you’re off to the races, and then you can manage whatever environment you want,” Benner explains.
The second paradox is the so-called water paradox. The water paradox states that even though life needs water, if organic material could escape its asphaltic fate and move toward Darwinian evolution, you can’t assemble the necessary building blocks in a flood of water. The building blocks of life start with genetic polymers–the well-known player DNA and its less-famous but still very smart friend RNA. Experts agree that RNA was likely the first genetic polymer, partly because in the modern world, RNA plays such an important role in the manufacturing other organic compounds. “RNA is the key to the ribosome, which is what makes proteins. There’s almost no question that RNA, which is a molecule involved in catalysis, arose before proteins arose,” Benner explains. The difficulty is that for RNA to assemble into long strands–which is needed for genetics–you can’t have the assembly taking place in water. “Most people think that water is essential for life. Very few people understand how corrosive water is,” Benner says. For RNA, water is extremely corrosive–bonds cannot be made within water, preventing long-strands from forming.
However, Benner says that these paradoxes can be resolved with the help of two very important groups of minerals
. The first are borate minerals. Borate minerals–which contain the element boron–prevent life’s building blocks from devolving into tar if incorporated into organic compounds. Boron, as an element, is seeking electrons to make itself stable. It finds these in oxygen, and together the oxygen and boron form the mineral borate. But if the oxygen boron finds is already bonded to carbohydrates, the carbohydrates linked with boron form a complex organic molecule dotted with borate that’s less resistant to decomposition.
The second group of minerals that come into play
involve those that contain molybdate, a compound that consists of molybdenum and oxygen. Molybdenum, more famous for its conspiratorial relation to the Douglas Adams classic A Hitchhiker’s Guide to the Galaxy than for its other properties, is crucial, because it takes the carbohydrates that borate stabilized, bonds to them and catalyzes a reaction which rearranges them into ribose: the R in RNA.
Which brings us–however circuitously–back to Mars. Both borate and molybdate are scarce and would have been especially scarce on early Earth. The molybdenum in molybdate is
highly oxidized, meaning that it needs electrons from oxygen or other readily available negatively charged ions to achieve stability. But early Earth was too oxygen-scarce to have readily created molybdate. Plus, returning to the water paradox, early Earth was quite literally a water world–with land making up only two to three percent of its surface. Borates are soluble in water–if early Earth was a flooded planet, as scientists believe, it would have been difficult for an already scarce element now diluted in a huge ocean to find ephemeral organic molecules to bond with. Moreover, Earth’s status as a water-logged planet makes it difficult for RNA to form, because that process can’t easily happen in water on its own.
These concepts become less of an issue on Mars, however. Though water was certainly present on Mars 3 to 4 billion years ago, it was never as abundant as it was on Earth, creating the possibility that Martian deserts–locations where borate and molybdate could concentrate–could have fostered the formation of long strands of RNA. Moreover, 4 billion years ago, Mars’ atmosphere contained much more oxygen than Earth’s. Further, recent analysis of a Martian meteorite confirms that boron was once present on Mars.
And, Benner believes, molybdate was there too. “It’s only when molybdenum becomes highly oxidized that it is able to influence how early life formed,”Benner explains. “Molybdate couldn’t have been available on Earth at the time life first began, because three billion years ago the surface of the Earth had very little oxygen, but Mars did.”
Benner believes that these factors imply that life originated on Mars, our closest neighbor in space equipped with all the right ingredients. But life wasn’t sustained there. “Of course Mars dried out. The process of drying was very important for life originating, but not sustaining,” Benner explains. Instead, a meteor would have to have hit Mars, projecting materials into space–and eventually those materials, including some building blocks of life, might have made it to Earth.
Would the sudden change in environment have been too harsh for the fledgling building blocks to survive
? Benner doesn’t think so. “Let’s say life starts on Mars, and becomes very happy in the Martian environment,” Benner explains. “A meteor comes to hit Mars, and the impact ejects rocks on which your predecessor is sitting. Then you land on Earth, and you discover that there is lots of water that you were treating as a scarce element. Will it find the environment adequate? It certainly appreciated the existence of enough water that it didn’t have to worry.”
So, sorry Lil Wayne, looks like it might be time to relinquish your claim to the fourth rock from the Sun. As Brenner notes, “The evidence seems to be building that we are actually all Martians.”