Super-Earths, also known as mini-Neptunes since they may be gassy or rocky, are among the most common planets in the Milky Way. Hundreds have been spotted in other solar systems over the past two decades, but even so there appeared to be none back home.

But now and , astrophysicists at the California Institute of Technology, say they can make a convincing case for a super-Earth here in our solar system.

Only a giant planet will do

“We didn’t want to invoke a distant giant planet to explain anything about the outer solar system,” says Batygin. “We tried every trick in the book, and some that aren’t in the book. Simply nothing else worked.”

“We usually talk about how the most common type of planet, two to three times the size of Earth, is not found in our solar system, yet now we might have one,” says at the Massachusetts Institute for Technology. “That’s cool.”

But even if there is a super-Earth out on the edges of our solar system, we’re still pretty weird. “Even with Planet Nine bringing us a bit closer to the galactic planetary catalogue, the solar system does still appear to be an overall outlier,” says Batygin, as super-Earths in other star systems are much closer to their stars.

Jupiter is the culprit

Simulations of the young solar system suggest that the early formation of Jupiter probably prevented this from happening here. But those simulations also suggest that a super-Earth couldn’t have formed as far away as Planet Nine would have to be, at least, not then – there simply wasn’t enough material out there to make a planet that big.

So how did Planet Nine, if it exists, get all the way out there?

The answer must lie in the very beginnings of the solar system, when our stellar neighbourhood was a very different – and much more dangerous – place. “We know the process of planet formation is not placid,” says Batygin. “There was a very chaotic sequence of events.” Planets were thrown around by their gravitational pulls on each other, occasionally colliding, as in the process that is thought to have formed Earth’s moon.

Planet Nine was the “core of a giant planet, which was forming with the other planets we know and love, that was thrown out of the solar system,” says Batygin.

Violent births

“We know that Uranus and Neptune probably had a very violent final phase of formation,” says of Bordeaux Observatory in France – they essentially came together through a series of collisions. Last year he and colleagues simulated this process and found that planets around five times the mass of Earth were ejected from the solar system during the pileup.

At the time the team didn’t think too much about their eventual fate, but hints of Planet Nine suggest one could have settled down into a lengthy orbit. “This extra planet could be representative of the building blocks of the ice giants,” says Raymond.

That gives us a new mystery: what prevented the planet’s escape? Brown believes that gas and dust in the very early solar system may have slowed down the planet, but Batygin has his eye on a different culprit: the other stars nearby in the early days of our solar system. He thinks that gravitational tugs from those stars, are the best candidates for slowing down Planet Nine enough to keep it in a huge, elongated orbit around the sun.

Random pattern or real object?

But Planet Nine might not even be there. While Batygin and Brown have pointed out compelling evidence for the existence of Planet Nine by studying the unusual orbits of six distant rocks called Kuiper belt objects (KBOs), that is not the same as a discovery, says Brown. “People are good at seeing patterns in places, and maybe we have just seen a random pattern and convinced ourselves it’s statistically significant. That’s my biggest worry.”

Future discoveries of KBOs will provide the true test. “If you find more of these objects and they really are randomly distributed around the sky, then it’s time for us to hang our heads in shame and retreat from the national media,” says Brown. But if the anomalies in the outer Kuiper belt remain, then Planet Nine will remain a likely explanation for them.

“Further work is needed to explain the origin of Planet Nine,” says Lucie Jílková of Leiden Observatory in The Netherlands, who last year suggested the strange orbits of these KBOs could be explained if they were stolen from a passing star. “Observations of more outer solar system bodies will help to further constrain and distinguish the theories.”

Lightbulb on the moon

If Planet Nine really is out there, with any luck astronomers will be able to spot it soon, despite the scant light it reflects from the sun. “Finding this object is like finding a lightbulb on the moon,” says Batygin. “But our calculations have provided a roadmap for where to look for this very dim object in the sky. Hopefully, that will trigger a hunt.”

We might even send a probe there one day. “It’s not as crazy as it sounds,” says Brown. “The way you do it is head straight for the sun, and as you swing around the sun and you’re getting this gravitational slingshot, you fire a whole bank of rocket engines.” Depending on the distance to Planet Nine, this could take anywhere from a few years to 20 years.

Other schemes like this have been proposed in the past, some with nuclear-powered rockets, but they never got past the planning stages. But a target like Planet Nine could make all the difference. “The societal impact of having that destination would be very exciting,” says at the University of California, Santa Cruz. “The stars are just too far away, we’re not going there. But this is a place where we could get to.”

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Such theories are difficult to test in the lab because they probe such extreme scales. But quantum effects have a way of showing up unexpectedly. In a strange quantum phenomenon known as the Casimir effect, two sheets of metal held very close together in a vacuum will attract each other.

The effect occurs because, even in empty space, there is an electromagnetic field that fluctuates slightly all the time. Placing two metal sheets very close to one another limits the fluctuations between them, because the sheets reflect electromagnetic waves. But elsewhere the fluctuations are unrestricted, and this pushes the plates together.

suggests that we might be able to observe the equivalent effect for gravity. That would, in turn, be direct evidence of the quantum nature of gravity: the Casimir effect depends on vacuum fluctuations, which are only predicted by quantum physics.

But in order to detect it, you would need something that reflects gravitational waves – the ripples in space-time predicted by general relativity. Earlier research suggested that superconductors (for example, metals cooled to close to absolute zero such that they lose all electrical resistance) might act as mirrors in this way.

“The quantum properties of superconductors may reflect gravitational waves. If this is correct, then the gravitational Casimir effect for superconductors should be large,” says Quach. “The experiment I propose is feasible with current technology.”

It’s still unclear if superconductors actually reflect gravitational waves, however. “The exciting part of this paper has to do with a speculative idea about gravitational waves and superconductors,” says at Lehman College in New York. “But if it’s right, it’s wonderful.”

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NASA’s champion planet-hunter, which recently suffered a fatal breakdown, is now looking on the sunny side. Despite the loss of its precision steering capability earlier this year, the may be able to hunt for planets once more, using a helpful push from the sun.

Engineers behind Kepler discussed the plan, dubbed K2, at the at the NASA Ames Research Center in Moffett Field, California, this week.

K2 will look at a much larger section of the sky than Kepler’s original mission – potentially capturing a bigger diversity of planets. It won’t be able to stare at the same patch of sky for as long, so K2 will be restricted to hunting for planets that orbit their stars much more closely than Earth does the sun. However, in the cases when such stars are cooler than the sun, some of these could still host life.

“There have been people saying ‘game over’,” says , one of the scientists on the Kepler team. “I think it’s very premature to say that.”

Sunlight spin

Kepler was designed to stare intently at a single patch of the sky for years on end. It hunted for planets by measuring the brightness of the stars in its field of view to exquisite precision.

When planets passed in front of those stars, Kepler noticed them dim slightly. Since its launch in 2009, the space telescope’s vigil has paid off with a haul of more than 3500 possible exoplanets. The existence of some of these are confirmed, others are still classed as candidate planets, but the results point to a Milky Way teeming with potentially habitable worlds.

To do that patient work, Kepler needed to be able to point at the same patch of sky without interruption. In late 2012, one of the four reaction wheels that hold it steady failed. It managed to carry on for a while, because only three are needed, but when a second wheel failed earlier this year, it spelled the end of Kepler’s original mission.

Kepler can still point at any location in the sky, but it can’t control its own rotation. Sunlight falling on the craft gives it a tiny push, and when that sunlight falls on the craft unevenly, it can set the telescope spinning.

Whizzy planets

“If we could turn off the sun, then we could probably go back and point at the Kepler field of view and stay perfectly stable. We could do what we needed to do,” says , the Kepler deputy project manager. “Because Kepler is out in the middle of nowhere, the only disturbance we have is the solar pressure.”

Now, rather than turning off the sun, the Kepler team has devised a way to keep the telescope from spinning. They have found a way to manoeuvre the craft into a position that should keep the amount of sunlight falling on both sides relatively even.

“It’s like balancing a pencil on your finger. As long as you can keep that pencil balanced with your finger below, it’s not going to tip over,” says Sobeck.

However, the team says they could only observe the same patch of sky with this method for two or three months at a time before they would have to shift position again to keep the sulight from entering Kepler’s field of view and blinding it.

Because Kepler needs to see a planet pass in front of its star at least three times in order to make a discovery, this frequent shifting will limit Kepler to spotting planets that take just 20 to 30 days to whizz around their host stars.

Cooler than Mercury

Such speedy planets would be huddled closer to their parent stars than Mercury, which takes 88 days to go around the sun. But they need not roast like Mercury does. Around dim red dwarf stars, planets this close can lie within the habitable zone – the region around the star where temperatures allow liquid water to exist.

The constant shifting will also let Kepler view more of the sky. “We’re opening the doors to some very different kinds of science, a much bigger section of the sky, a more diverse set of target stars to look at,” says Sobeck. “There are things that we can do in K2 that we never had an opportunity to look at before in the Kepler field of view.”

Since its breakdown, there have been other suggestions for how to give Kepler a new lease of life but this is the one that the team has decided to ask NASA to fund.

The K2 proposal still needs to work its way through the NASA funding evaluation process before the new mission can start in full. But preliminary tests of the new steering method are already under way, and are likely to continue into early 2014. If those tests go well, the Kepler telescope may have years of planet hunting to come.

Dark matter is believed to make up about 80 per cent of the matter in the universe, based on its gravitational effects on visible matter. The leading candidates for the elusive stuff are weakly interacting massive particles (WIMPs), which would only interact with normal matter via gravity and the weak force.

Most models suggest that WIMPs don’t interact with themselves to build larger structures, the way that normal matter does. But earlier this year, scientists proposed a model in which 15 per cent of dark matter can build dark atoms and larger structures, including shadow versions of galaxies.

If so, we should be able to see evidence for dark matter interactions in the large-scale structure of the universe, says at the Jet Propulsion Laboratory in Pasadena, California. In the heat of the very early universe, matter existed as dense, soupy plasma. As things cooled, denser regions started to collapse, which set the gas ringing.

Fainter darkness

When the plasma cooled enough to form atoms, the sound waves became frozen in the cosmic microwave background radiation (CMB) – the first light emitted about 380,000 years after the big bang – and in the current pattern of galaxy clusters.

If some dark matter can form atoms, we should see evidence of their formation in the early universe as dark sound waves, says Cyr-Racine. We wouldn’t be able to see these ripples directly, but their gravitational pull would affect visible matter.

His team examined the latest data from the European Space Agency’s Planck spacecraft and the BOSS galaxy survey looking for ripples of dark sound in the CMB and in galaxy clusters. So far they have come up empty-handed. Self-interacting dark matter might still exist, but the current finding puts limits on how “loud” dark sound can be: the team suggest that no more than 5 per cent of dark matter should be able to build atoms. More sensitive surveys could pick this up, they say.

“We don’t rule out self-interacting dark matter,” says Cyr-Racine. “We do give the model some escape routes.”

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Satellites stay in their orbits thanks in part to the Earth’s squashed shape – something we have only just discovered.

Our planet is ringed with more than 1000 working satellites, plus thousands of tonnes of space junk, and for the most part they stay up there quite happily. But surprisingly, it is only now that we properly understand why.

Ideally, a tiny satellite orbiting a perfectly spherical planet will remain there forever, assuming nothing nearby disturbs it. But Earth is not a perfect sphere, and there are plenty of other objects that can disturb artificial satellites in low-Earth orbit – first and foremost, the moon. According to the laws of motion, the moon’s influence alone should cause satellites to crash into the Earth’s atmosphere, where they would burn up.

Saving grace

It turns out that Earth’s imperfections are a satellite’s saving grace. Because of its rotation, Earth is slightly flattened at the poles and bulges around the equator.

According to computer simulations and analysis by at the Institute for Advanced Study in Princeton, New Jersey, and Tomer Yavetz of Princeton University, the gravitational pull of that bulge shifts satellites’ orbits over time, preventing tugs from the moon and other sources from pulling them too far in one direction or another. If the Earth were closer to being a perfect sphere, many satellites would crash into the atmosphere and burn up in a matter of months or years.

“It’s interesting that there are lots of things that could destabilise low-Earth orbits, but that things happen to combine in such a way that we have a good environment for satellites,” says , a physicist at the University of California, Santa Cruz, who was not involved with this research. “It makes you pause to think a little bit – when you look in detail at how things work, you can find surprises.”

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You might have wondered, looking up at the night sky, how many other beings are out there looking back at us. Help is at hand. Using data from NASA’s Kepler Space Telescope, ��è����Ƶ has made an .

The grid of squares to the right represents the patch of sky that Kepler stared at for nearly four years. So far, the space telescope – nicknamed the Planet Hunter – has confirmed the existence of 151 exoplanets and identified more than 3500 strong candidates.

Now, using what we know from Kepler, and simulations from its data by and of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, ��è����Ƶ has estimated and mapped the density of habitable worlds across the whole sky. Given that the Milky Way is thought to contain between 100 and 200 billion stars, our best estimate of the total number of such planets in our galaxy is 15 to 30 billion.

“This illustrates the wow factor emerging from the Kepler mission,” says of the SETI Institute in Mountain View, California, who wrote the software that analyses the Kepler data. “The galaxy is just full of potentially habitable planets.”

How many of these worlds harbour life? We don’t know, but if we are alone in our galaxy, it’s not for a lack of accommodation.

The result is consistent with the standard model of particle physics – something of a disappointment for those hoping nature’s weirdest force would reveal something more exotic. But it’s also a preliminary finding, so the force could yet point the way to new physics beyond the standard model.

Nature has four known forces – the electromagnetic force, which gives rise to positive and negative charge, gravity and the strong and weak forces. But the weak force, which acts only on the subatomic level and is responsible for radioactive decay, has an odd quirk.

If you’re on a water slide, gravity pulls you along at the same rate, whether the slide turns clockwise or counter-clockwise. But in the subatomic equivalent of these situations, the weak force behaves differently. “If you do an experiment and compare it with a mirror-image of that experiment, you get different results,” says , a physicist at the University of Manitoba, Canada, and part of the , which made the latest measurement.

Physicists speculate that further examination of the weak force might hold clues to other exotic behaviour. They had previously measured its pull on other particles, including the electron, but had never managed to do the proton.

“We’re looking for a sensitivity of this weak charge to something that might be there that isn’t in the standard model,” says Page. “If there’s something out there that the proton could be feeling, that would show up as a difference between the proton’s expected value and what we find.”

Switching spins

Q-weak researchers measured the weak force’s pull on the proton – a first – by taking advantage of this unique feature. Using equipment at the in Newport News, Virginia, they shot a beam of electrons at a target of protons while switching the “spin” – a quantum mechanical property – of the electrons between clockwise and counterclockwise, at a rate of 1000 times per second.

The behaviour of the electron beam was dominated by the electromagnetic attraction between the protons and electrons, with only a tiny influence from the weak force. But electromagnetism treats clockwise and counter-clockwise electrons the same way, unlike the weak force. So the Q-weak team subtracted the counter-clockwise behaviour from the clockwise behaviour, leaving behind nothing but the small effect of the weak force on the electrons’ interaction with the protons.

They found that the “weak charge” of the proton, the weak force’s analogue to the electric charge, was consistent with the prediction of the standard model, offering no immediate clues to new physics.

But Q-weak is still holding out hope of a surprise as the measurement is preliminary, involving only 4 per cent of their data. “This is a demonstration that you can measure the proton’s weak charge, but to do the science Q-weak really wants to do, they’re going to have to analyse the whole data set,” says , a physicist at the California Institute of Technology who was not involved in the experiment.

Page agrees: “We weren’t expecting to get a rigorous test until later,” she says. “If the final measurement agrees with the standard model, fine. If it doesn’t, that’ll be very interesting.”

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Editorial: “‘Boring’ Higgs has powers we never dreamed of“

THE Higgs boson may have the right mass to wreck the universe – hurray! Death by Higgs is the simplest way to do away with a paradoxical menagerie of disembodied intelligent beings that shouldn’t exist, yet remain in the best cosmological models.

What’s more, the end is a comfy 20 or 30 billion years off. “That’s quite a few billion; it’s not like we should rush out and buy life insurance,” says at the California Institute of Technology in Pasadena, who put forward the idea along with , also at Caltech.

“That’s quite a few billion years away – it’s not like we should rush out and buy life insurance”

The paradox arose a decade or so ago, when physicists realised their models led to a future filled with Boltzmann brains: fully formed conscious entities that pop out of the vacuum. It sounds bizarre, but there’s nothing to stop matter sometimes randomly arranging itself in just the right way for this to occur. The problem arises when you add in the universe’s accelerating expansion.

This provides limitless time, space and energy for Boltzmann brains to form, even after life as we know it has winked out, causing them to eventually outnumber ordinary consciousnesses. But that would make the brains’ experience of the universe more typical than ours, which is a problem as our understanding of the cosmos assumes that we are typical observers.

Theories have been proposed in which the universe ends before the brains take over, solving the paradox, but these are mostly based on untested physics like string theory. By contrast, the Higgs boson can do it using well-accepted physics.

For this to work, the Higgs boson’s accompanying field must be metastable, meaning it can spontaneously settle down into a lower energy state. A bubble of space would then sprout up, with its own physical laws, and expand at the speed of light to destroy everything in its path, including the universe as we know it.

The idea isn’t new, but now that the Higgs has been found, we have a measurement of its mass: about 125 gigaelectronvolts. Carroll and Boddy combined that value with the , and calculated that physical laws favour a metastable Higgs over a stable one ().

That sounds promising, but the metastable Higgs looks only a tad more likely. New measurements of the top quark’s mass are expected in 2015, when the Large Hadron Collider switches on after a two-year rest, and they could make a stable Higgs field more likely.

The new data should also help to pin down another unknown. If the Higgs field is metastable and flips into a lower energy state, will the affected regions grow faster than the universe is expanding? If yes, the Higgs field will destroy the universe and the Boltzmann brain paradox is resolved.

Even if not, the field may yet be able to destroy the universe. But whether it will in this scenario can only be resolved via a thorny problem related to the nature of probability in multiple universes. Anyone wanting a swift answer to Boltzmann brains will be hoping we don’t have to go there.

This article appeared in print under the headline “Death by Higgs rids us of cosmic brains”

Birds cannot read road signs, but they know that some roads have higher speed limits than others. They will take off further away from an approaching car on a faster road than on a slower road – regardless of the speed of the car.

When of the University of Quebec at Rimouski and of McGill University in Montreal, both in Canada, were working together in France in 2006, they began studying the birds they encountered on the drive home from the lab.

They found that where there was a 50-kilometre-per-hour speed limit, birds on the road typically took off when the car was about 15 metres away, whereas on a 110-km-per-hour road, they took off when a car was nearer 75 metres away. They did this even when faced with a car travelling faster on the slow road or slower on the fast road.

Know your limits

“What was really cool is that birds did not respond to the speed of the car but rather to the speed limit of the road section,” says Legagneux. “It was like they were able to read road signs – although they obviously do not.”

The researchers think the birds treat cars as predators, and realise that in some parts of their environment the predators are more dangerous than in others.

The two biologists also discovered that the distance at which the birds took off varied according to season. They let cars get closer in the spring, and behaved more cautiously in autumn. Legagneux and Ducatez think that this is either because birds are more active in the spring feeding their children, or that juvenile birds are first learning about roads then and have less experience with cars.

“Birds are able to associate environments, like forests or roads, with risk,” says , an ornithologist at the University of Hawaii, Manoa. He thinks the work could prompt follow-up studies comparing birds in urban and rural areas, and perhaps encourage more innovative methods. “I just think it’s really cool,” he says. “We don’t do enough of this kind of work.”

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