Nearly 1,500 light years away is a strange and mysterious star. It’s behavior seems to defy explanation, leading some to conclude its appearance could be due to an advanced alien civilization. Is the alien conclusion just clickbait hype, or is it a legitimate answer? 

As scientists we really don’t like jumping on the alien bandwagon. Regular radio pulses in space! Aliens? No, just rapidly rotating neutron stars. Strong unexplained radio bursts! Aliens? No, just a microwave oven. The Wow! signal, radioactivity on Mars, it could be aliens, right? Probably not. That’s not to say that aliens could never be the answer to strange astronomical behavior, it’s just that it’s bad form to use aliens as a filler for “I don’t know.” That said, KIC 8462852, commonly called Tabby’s star after its discoverer Tabetha Boyejian, continues to defy clear explanation, so the aliens conclusion keeps lurking in the background.

Whenever we have a mystery like this, the best approach is to be cautious and follow where the evidence leads. Even if the evidence doesn’t support a clear solution, it can at least rule out some of the options. So what does the evidence say?

One point of evidence is what originally drew our attention to Tabby’s star in the first place. KIC 8462852 is an F-type star, about 45% more massive than our Sun. It is one of the stars the Kepler spacecraft has studied, looking for exoplanets by watching the star dim as a planet transits the star. A transiting planet typically only dims the star by a few percent at most, and that’s typically for planets orbiting small red dwarfs. For larger stars like our Sun and Tabby’s star, the dip in brightness should be even less. If a distant civilization were to see Jupiter pass in front of our Sun, the Sun’s brightness would only decrease by about 1%.

It turns out that while Jupiter is not the most massive a planet can be, it is about as large as a planet can be. With more mass comes more gravity, so planets more massive than Jupiter are about the same size, just more dense. Since Tabby’s star is a bit larger than our Sun, we would expect a transiting planet to cause a dimming of no more than about 1%. Kepler observed brightness dips that look like planetary transits, but were often much more than a few percent. An orbiting planet couldn’t create such large dips.

Kepler data for Tabby’s star.

It is possible for such large dips in brightness to be caused by an interstellar body to pass between us and the star. If a rogue planet passed much closer to us than the star, then it could cause a significant dimming while still being planet-sized. But we’ve observed is multiple large dips. One rogue planet transiting the star would be rare enough, but the odds of several in succession is basically zero. So that likely doesn’t explain things either. What’s more perplexing is that there are also dimming events that don’t agree with transits at all. Rather than a sharp dip, the star shows some longer term variations in brightness, as if it is being obscured by some type of dust cloud or protoplanetary disk. The problem with that idea is that KIC 8462852 is well into the main sequence stage, so a protoplanetary disk is unlikely. Infrared observations of the star also find no evidence of a protoplanetary disk.

So not a planet, not an odd interstellar transit, and not a protoplanetary dust cloud. What else could it be? The leading contender has been a large swarm of comets. The data could be explained by clusters of large comets with a total mass roughly equal to that of Ceres. So if an object similar to Ceres was tidally ripped apart by some close planetary encounter, and fragmented into lots of large comets, it might, just might, be able to explain the data. If you think that isn’t a very likely answer you’d be right. It’s really the least unlikely of a range of wild ideas such as a complex ring system, a catastrophic collision of planets, or even that the star is strangely variable.

The brightness of Tabby’s star found in the Harvard data.

Clearly what’s needed is more data. So off scientists went to get some. A SETI radio search of the star turned up nothing, infrared searches didn’t find anything strange, but an examination of historical records did. It turns out that photographs of KIC 8462852 have been captured since 1890. The Harvard College Observatory has photographic plates spanning more than a century, and these have been digitized and put online. Tabby’s star happens to be in this data, and when the brightness of the star is plotted over the years it shows a gradual dimming over time. This is really strange because main sequence stars should not show this kind of dimming effect. It’s also a very controversial result. While the data is clear, it isn’t clear whether it shows an actual dimming of the star, or if it’s an artifact of the way photographs were taken over the years. The brightness of some other stars in the historical data also show some dimming effect, so it could easily be due to poor calibration of the photographic plates. However, an analysis of the average Kepler data also shows a slight overall dimming consistent with the dimming trend.

So why not admit it could be aliens?

In all honesty it could be aliens, but even that seems to be contradicted by evidence. If the dimming were caused by some kind of alien superstructure, then the blocked light being captured would emit some of that energy as heat. Based on the data, the alien superstructure would be absorbing about 20% of the star’s light, and that would generate a great deal of waste heat. So there should be an excess of infrared light coming from the star, but observations have found no significant infrared excess.

That doesn’t mean aliens are ruled out, but they aren’t any more compelling than some strange natural phenomena. In short, it’s a mystery, and that’s definitely worth studying further.

Paper:  T. S. Boyajian, et al. Planet Hunters IX. KIC 8462852 – where’s the flux? MNRAS 457 (4): 3988-4004. (2016) doi: 10.1093/mnras/stw218

Paper: Eva H. L. Bodman, Alice Quillen. KIC 8462852: Transit of a Large Comet Family. The Astrophysical Journal Letters, Volume 819, Number 2 (2016) arXiv:1511.08821 [astro-ph.EP]

Paper: Bradley E. Schaefer. KIC 8462852 Faded at an Average Rate of 0.165+-0.013 Magnitudes Per Century From 1890 To 1989. arXiv:1601.03256 [astro-ph.SR] (2016)

The post An Alien Star appeared first on One Universe at a Time.

Just when you think you’ve got a mystery solved, new data revives the mystery again. It’s a common story in science, and this time its about fast radio bursts. 

Fast radio bursts (FRBs), as you might recall are short, intense, bursts of radio waves. They have indications of being distant in origin, but similar bursts known as perytons were found to be due to local radio noise. Because of their short duration they are difficult to study, or even to verify their origin in space. Recently there was observation of a particularly strong FRB that seemed to be from a distant galaxy. The evidence to support this idea came from two radio telescopes. The first observed the FRB, while the second observed a radio afterglow in the same general region. From theses two observations the source could be triangulated as a distant galaxy.

But new evidence indicates that the radio afterglow isn’t from the FRB, but rather from the material surrounding a supermassive black hole in the distant galaxy, completely independent from the fast radio burst. So the origin of these radio bursts is still a mystery after all.

If that wasn’t bad enough, there’s also evidence that FRBs can repeat. So far these bursts seemed to be one-time events, which would imply they are caused by catastrophic events such as colliding neutron stars or a neutron star collapsing into a black hole. If they repeat that would indicate a transitory origin, such as flares from highly magnetized neutron stars. It’s also possible that FRBs can have multiple causes, with some repeating and some not.

At this point what’s clear is that we don’t have a good understanding of FRBs after all.

Paper: P. K. G. Williams and E. Berger. Cosmological Origin for FRB 150418? Not So Fast. arXiv:1602.08434 [astro-ph.CO] (2016)
Paper: L. G. Spitler, et al. A repeating fast radio burst. Nature doi:10.1038/nature17168 (2016)

The post A Repeating Mystery appeared first on One Universe at a Time.

Last year the Juno spacecraft made a close approach to Earth. Such near-Earth flybys are used by spacecraft to give them a boost of speed needed to reach the outer planets. The initial launch of Juno only gave it enough speed to reach a bit farther than the orbit of Mars, so its orbit was aimed to come close to Earth. As it approached Earth from behind, Earth’s gravity gave it a boost of speed (at the cost of slowing Earth down very slightly). Juno thus gains enough speed to reach Jupiter.

The main purpose of the flyby is simply to get the spacecraft to Jupiter. But a close flyby of a spacecraft is a rare thing, so it is worth taking advantage of. Amateur astronomers have imaged the spacecraft during close approach, Juno itself took some images of the Earth and Moon during the flyby. But perhaps the most interesting data is about Juno’s speed and trajectory after the flyby.

Some spacecraft that make a close flyby of Earth seem to gain slightly more energy than they should. This extra energy is very small, giving it a speed millimeters per second faster than predicted. Given that the typical speed of a spacecraft at flyby is on the order of 10 kilometers per second, this difference is tiny. But it has been observed, and it called the flyby anomaly.

The anomaly was first observed in the flyby of Galileo in 1990, when careful measurements of its Doppler shift revealed an extra speed of 4 mm/s. On Galileo’s second flyby in 1992, no anomaly was observed. In 1998 NEAR gained 13 mm/s. Cassini had no anomaly in 1999, Rosetta had an anomaly in its 2005 flyby, but not in subsequent ones, and Messenger had no anomaly.

This on-again off-again nature of the anomaly is a bit strange. There seems to be a slight correlation to the orientation of a flyby relative to the Earth’s equator and the appearance of the anomaly. This may be an indication that the anomaly is somehow related to the rotation of the Earth, though studies of relativistic frame dragging on the probes show that effect isn’t enough to account for the anomaly.

Another explanation for the anomalies is an effect known as the transverse Doppler effect. This effect occurs when an object is only moving side to side (transverse) relative to you, not toward you or away from you. Normally the transverse Doppler effect is too small to worry about, but for a close flyby it becomes important. While the transverse Doppler effect can account for the Doppler anomaly, it can’t account for the time-of-flight ranging data that is also observed in some cases.

Since standard physics can’t seem to account for the anomaly, wilder ideas have been proposed, such as modified gravity theories, a dark matter halo surrounding Earth, even a quantum gravity process known as the Hubble scale Casimir effect. Of course explaining the anomaly after the fact isn’t enough to give these wild ideas credence. What they need to be able to do is predict a flyby anomaly, which no one has been able to do yet.

A similar anomaly was seen with the Pioneer probes, with Pioneer 10 and 11 slowing down a bit faster than expected (equivalent to an acceleration of ten billionths of a gee too small). When no clear solution could be found in traditional physics all sorts of exotic physics ideas were proposed to account for the Pioneer anomaly. In the end it turned out to be a very subtle effect of thermal recoil from their power sources.

The most likely explanation for the flyby anomaly is some subtle but mundane effect we just haven’t pinned down. But there is always a chance that it is due to something more exotic. The effect is definitely there, so it is worth exploring each time there is a close flyby.

The post Flyby Night appeared first on One Universe at a Time.

Pioneer 10 was launched in 1972. The next year Pioneer 11 was launched. Their mission was to fly past Jupiter and then Saturn, making the first detailed studies of the planets. Afterwards, they continued their journey to the outer reaches of the solar system. At this point they entered a second phase of their mission, to study the diffuse gases in the solar system on their way to interstellar space. Their observations indicated that something rather strange was going on.

The Pioneer spacecraft gradually slow down as they speed away from the Sun. Most of this is due to the Sun’s gravity, but part is due to the drag of the gases that exist in the solar system. When all the factors were taken into account, it was found that the Pioneer probes were slowing down a bit faster than expected. This extra deceleration was tiny, amounting to about a ten billionth of a gee (Earth’s surface gravity). This tiny discrepancy became known as the Pioneer anomaly.

The Pioneer probes were the first missions to travel beyond the planets, so in many ways it was the first direct experimental test of our understanding of gravity at stellar distances. It was possible, then, that the Pioneer anomaly was due to some deviation from the gravitational models of Newton and Einstein. Perhaps something such as modified Newtonian dynamics (MOND) was coming into play. Perhaps they were experiencing the effects of dark matter. Interestingly, the Pioneer acceleration was roughly equal in magnitude to the speed of light times the Hubble constant, which suggested some type of cosmic connection.

But there were several problems with these ideas. Most significantly, the orbital motions of the planets agreed with Newton and Einstein perfectly. If the effect was due to dark matter, or a similar modified gravity model, then the planets should be affected as well. That they weren’t was a serious problem. So modified inertia models were proposed, as were more mundane ideas such as gas leaks from the probes themselves, or even simply systematic errors in the measurement of their motion.

A proven solution wasn’t found until 2012, and the answer happened to be rather mundane. The power source of each probe was a radioisotope thermoelectric generator (RTG), which uses heat produced by radioactive decay to generate electricity. Since RTGs generate heat, and are in the cold vacuum of space, they radiate heat as infrared light. When light is emitted it has a very small amount of momentum, which means the object emitting the light would recoil slightly in the opposite direction. This effect is known as thermal recoil.

Most objects radiate heat uniformly in all directions, so the thermal recoil is negligible. But the design of the Pioneer probes meant that some of the radiated light reflected off the side of the probe facing away from the Sun, which gave the probe a slight deceleration. This produced a thermal recoil that explained the anomaly perfectly.

Now you might think this solution should have been thought of years ago. After all, it seems rather obvious that thermal recoil would play a role for a warm object in the cold of space. In fact the idea was proposed early on, but proving it could account for all of the anomaly was very difficult. We needed to have very precise measurements of the anomaly itself, as well as accurate data on just how the RTGs were cooling over time. It took detailed computer modeling to confirm that thermal recoil could in fact account for the entire anomaly.

There are a few researchers who still explore alternative mechanisms for the Pioneer anomaly, but the thermal recoil explanation has now been confirmed by multiple studies, so the anomaly is generally considered a solved mystery.

Which just goes to show that sometimes new and interesting data doesn’t lead to new and interesting physics.

The post Ten Billionths of a Gee appeared first on One Universe at a Time.

A few days ago I wrote about an interesting type radio signal known as a fast radio burst. These are short, intense pulses of radio energy that have all the hallmarks of being astronomical in origin.  One possible source of FRBs could be a neutron star collapsing to a black hole. But there is still some discussion that such bursts could be terrestrial in origin because of another type of radio burst known as a peryton.

The first FRB is known as the Lorimer burst, named after Duncan Lorimer, whose pulsar research group discovered the burst. The Lorimer burst had a couple of distinguishing features that point to it being astronomical.  The first is that its spectrum is dispersed. That is, instead of being a simple burst with all different frequencies happening at once, the frequencies were spread out, with higher frequencies first and lower ones later.  This whistler effect is characteristic of a pulse that has traveled through the interstellar medium.  It occurs because when an electromagnetic pulse interacts with charged ions, different frequencies are slowed by different amounts, with the lower frequencies slowed down more.  So you get a dispersion effect. Stray bursts or chirps from terrestrial sources generally don’t have the same dispersion because they don’t travel through plasma and they don’t travel far.

Another feature of the Lorimer burst is that it was localized within the detectors.  The Lorimer burst was observed at the Parkes radio telescope (seen in the figure), which has a 13 beam receiver. Each beam is the radio equivalent of a pixel in a digital camera.  The burst maxed out (saturated) one of the beams, but the others were largely unaffected.  This indicates that it was from a single astronomical direction, and not a local electromagnetic burst.

Since the Lorimer burst, there have been detections of at least four similar bursts, which is why there is an effort to determine what their astronomical origin might be.  But this is where it gets interesting, because in addition to the Lorimer-type bursts (FRBs) there is another type known as perytons.  Perytons have a similar intensity, and are also dispersed, but they max out all 13 beam receivers.  This means they are not from a directed astronomical source, and are therefore terrestrial.  These perytons only appear at Parkes, which is also the only place to have observed the FRBs.

Since both FRBs and perytons have only been detected at one radio telescope, you might thing that points pretty strongly to both of them being some kind of local interference or glitch unique to Parkes, but that isn’t the case.  For one, the Parkes radio telescope is particularly suited to detecting this type of signal, so it isn’t too surprising that only Parkes has detected FRBs. For another, perytons are distinctly different from FRBs. While both are dispersed, they are dispersed in different ways, and the peryton dispersion doesn’t match that of an astronomical source.

For now, it seems like Lorimer-type FRBs are astronomical in origin and perytons are terrestrial in origin. Beyond that they are a mystery.

The post Peryton Place appeared first on One Universe at a Time.