Astronomers are using NASA's Hubble Space Telescope to study auroras stunning light shows in a planet's atmosphere on the poles of the largest planet in the solar system, Jupiter. The auroras were photographed during a series of Hubble Space Telescope Imaging Spectrograph far-ultraviolet-light observations taking place as NASA's Juno spacecraft approaches and enters into orbit around Jupiter. The aim of the program is to determine how Jupiter's auroras respond to changing conditions in the solar wind, a stream of charged particles emitted from the sun. Auroras are formed when charged particles in the space surrounding the planet are accelerated to high energies along the planet's magnetic field. When the particles hit the atmosphere near the magnetic poles, they cause it to glow like gases in a fluorescent light fixture. Jupiter's magnetosphere is 20,000 times stronger than Earth's. These observations will reveal how the solar system's largest and most powerful magnetosphere behaves.
Researchers have developed a new method for detecting and measuring one of the most powerful, and most mysterious, events in the Universe – a black hole being kicked out of its host galaxy and into intergalactic space at speeds as high as 5000 kilometres per second.
The method, developed by researchers from the University of Cambridge, could be used to detect and measure so-called black hole superkicks, which occur when two spinning supermassive black holes collide into each other, and the recoil of the collision is so strong that the remnant of the black hole merger is bounced out of its host galaxy entirely. Their results are reported in the journal Physical Review Letters.
Earlier this year, the LIGO Collaboration announced the first detection of gravitational waves – ripples in the fabric of spacetime – coming from the collision of two black holes, confirming a major prediction of Einstein’s general theory of relativity and marking the beginning of a new era in astronomy. As the sensitivity of the LIGO detectors is improved, even more gravitational waves are expected to be detected – the second successful detection was announced in June.
As two black holes circle each other, they emit gravitational waves in a highly asymmetric way, which leads to a net emission of momentum in some preferential direction. When the black holes finally do collide, conservation of momentum imparts a recoil, or kick, much like when a gun is fired. When the two black holes are not spinning, the speed of the recoil is around 170 kilometres per second. But when the black holes are rapidly spinning in certain orientations, the speed of the recoil can be as high as 5000 kilometres per second, easily exceeding the escape velocity of even the most massive galaxies, sending the black hole remnant resulting from the merger into intergalactic space.
The Cambridge researchers have developed a new method for detecting these kicks based on the gravitational wave signal alone, by using the Doppler Effect. The Doppler Effect is the reason that the sound of a passing car seems to lower in pitch as it gets further away. It is also widely used in astronomy: electromagnetic radiation coming from objects which are moving away from the Earth is shifted towards the red end of the spectrum, while radiation coming from objects moving closer to the Earth is shifted towards the blue end of the spectrum. Similarly, when a black hole kick has sufficient momentum, the gravitational waves it emits will be red-shifted if it is directed away from the Earth, while they will be blue-shifted if it’s directed towards the Earth.
“If we can detect a Doppler shift in a gravitational wave from the merger of two black holes, what we’re detecting is a black hole kick,” said study co-author Davide Gerosa, a PhD student from Cambridge’s Department of Applied Mathematics and Theoretical Physics. “And detecting a black hole kick would mean a direct observation that gravitational waves are carrying not just energy, but linear momentum as well.”
Detecting this elusive effect requires gravitational-wave experiments capable of observing black hole mergers with very high precision. A black hole kick cannot be directly detected using current land-based gravitational wave detectors, such as those at LIGO. However, according to the researchers, the new space-based gravitational wave detector known as eLISA, funded by the European Space Agency (ESA) and due for launch in 2034, will be powerful enough to detect several of these runaway black holes. In 2015, ESA launched the LISA Pathfinder, which is successfully testing several technologies which could be used to measure gravitational waves from space.
The researchers found that the eLISA detector will be particularly well-suited to detecting black hole kicks: it will be capable of measuring kicks as small as 500 kilometres per second, as well as the much faster superkicks. Kick measurements will tell us more about the properties of black hole spins, and also provide a direct way of measuring the momentum carried by gravitational waves, which may lead to new opportunities for testing general relativity.
“When the detection of gravitational waves was announced, a new era in astronomy began, since we can now actually observe two merging black holes,” said study co-author Christopher Moore, a Cambridge PhD student who was also a member of the team which announced the detection of gravitational waves earlier this year. “We now have two ways of detecting black holes, instead of just one – it’s amazing that just a few months ago, we couldn’t say that. And with the future launch of new space-based gravitational wave detectors, we’ll be able to look at gravitational waves on a galactic, rather than a stellar, scale.”
Davide Gerosa and Christopher J. Moore. ‘Black-hole kicks as new gravitational-wave observables.’ Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.117.011101
Black holes are the most powerful gravitational force in the Universe. So what could cause them to be kicked out of their host galaxies? Cambridge researchers have developed a method for detecting elusive ‘black hole kicks.’We now have two ways of detecting black holes, instead of just one – it’s amazing that just a few months ago, we couldn’t say that.Christopher MooreSXS LensingComputer simulations motivated by GW150914
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A Neptune-sized transiting planet closely orbiting a 5–10-million-year-old star
Nature 534, 7609 (2016). doi:10.1038/nature18293
Authors: Trevor J. David, Lynne A. Hillenbrand, Erik A. Petigura, John M. Carpenter, Ian J. M. Crossfield, Sasha Hinkley, David R. Ciardi, Andrew W. Howard, Howard T. Isaacson, Ann Marie Cody, Joshua E. Schlieder, Charles A. Beichman & Scott A. Barenfeld
Theories of the formation and early evolution of planetary systems postulate that planets are born in circumstellar disks, and undergo radial migration during and after dissipation of the dust and gas disk from which they formed. The precise ages of meteorites indicate that planetesimals—the building blocks of planets—are produced within the first million years of a star’s life. Fully formed planets are frequently detected on short orbital periods around mature stars. Some theories suggest that the in situ formation of planets close to their host stars is unlikely and that the existence of such planets is therefore evidence of large-scale migration. Other theories posit that planet assembly at small orbital separations may be common. Here we report a newly born, transiting planet orbiting its star with a period of 5.4 days. The planet is 50 per cent larger than Neptune, and its mass is less than 3.6 times that of Jupiter (at 99.7 per cent confidence), with a true mass likely to be similar to that of Neptune. The star is 5–10 million years old and has a tenuous dust disk extending outward from about twice the Earth–Sun separation, in addition to the fully formed planet located at less than one-twentieth of the Earth–Sun separation.
A hot Jupiter orbiting a 2-million-year-old solar-mass T Tauri star
Nature 534, 7609 (2016). doi:10.1038/nature18305
Authors: J. F. Donati, C. Moutou, L. Malo, C. Baruteau, L. Yu, E. Hébrard, G. Hussain, S. Alencar, F. Ménard, J. Bouvier, P. Petit, M. Takami, R. Doyon & A. Collier Cameron
Hot Jupiters are giant Jupiter-like exoplanets that orbit their host stars 100 times more closely than Jupiter orbits the Sun. These planets presumably form in the outer part of the primordial disk from which both the central star and surrounding planets are born, then migrate inwards and yet avoid falling into their host star. It is, however, unclear whether this occurs early in the lives of hot Jupiters, when they are still embedded within protoplanetary disks, or later, once multiple planets are formed and interact. Although numerous hot Jupiters have been detected around mature Sun-like stars, their existence has not yet been firmly demonstrated for young stars, whose magnetic activity is so intense that it overshadows the radial velocity signal that close-in giant planets can induce. Here we report that the radial velocities of the young star V830 Tau exhibit a sine wave of period 4.93 days and semi-amplitude 75 metres per second, detected with a false-alarm probability of less than 0.03 per cent, after filtering out the magnetic activity plaguing the spectra. We find that this signal is unrelated to the 2.741-day rotation period of V830 Tau and we attribute it to the presence of a planet of mass 0.77 times that of Jupiter, orbiting at a distance of 0.057 astronomical units from the host star. Our result demonstrates that hot Jupiters can migrate inwards in less than two million years, probably as a result of planet–disk interactions.
Why ultra-powerful radio bursts are the most perplexing mystery in astronomy
Nature 534, 7609 (2016). http://www.nature.com/doifinder/10.1038/534610a
Author: Elizabeth Gibney
Strange signals are bombarding Earth. But where are they coming from?
NASA’s Juno spacecraft prepares to probe Jupiter’s mysteries
Nature 534, 7609 (2016). http://www.nature.com/doifinder/10.1038/534599a
Author: Alexandra Witze
The mission will peek through the gas giant’s swirling clouds in search of a planetary core.