Binary asteroids

Planetary scientists Seth Jacobson and Daniel Scheeres, both from University of Colorado, have developed a theory that explains the formation of synchronous binary asteroids, as well as all other dynamical classes of Near Earth Asteroids (NEAs). Their model follows the dynamical evolution of rotationally fissioned asteroids – asteroids whose rotation has increased to break-up speed.

Contrary to what you might think, astronomers believe that most asteroids are not solid monoliths of rock orbiting the Sun, but are better described as “rubble piles” – a collection of smaller boulders bound together by their mutual gravitational force. It is thought that low-speed collisions amongst asteroids can fracture them, but as long as the relative velocity amongst the shattered fragments of an asteroid is less than their mutual escape velocity, the fragments can remain gravitationally bound as a rubble pile.

Evidence that asteroids may indeed be rubble piles include their relatively low rotation rates (which needs to be low so as not to fling the small fragments away from each other), low bulk density (regardless of their surface composition, a rubble pile would have high porosity and hence low bulk density), and the fact that binary asteroids – or asteroids with satellites – are relatively common.  The first asteroid satellite, the Ita-Dactyl system, was discovered in 1993 by the Galileo spacecraft on its way to Jupiter. Since then many more have been discovered, and about 2% of Main Belt asteroids and 15% of NEAs are binaries.

Animation of the orbit of the binary Near-Earth Asteroid (66391) 1999 KW4 (Credit: NASA/JPL)

When sunlight hits the day-side of an asteroid, some of the light will be scattered while some will be absorbed and then re-emitted as heat or thermal radiation. For a rotating, irregularly shaped asteroid, this results in asymmetric re-emission of thermal radiation. This causes a small torque that changes the rotation rate and orientation of the spin axis of the asteroid.  This is known as the YORP (or Yarkovsky–O’Keefe–Radzievskii–Paddack) effect, and while the torque is very small, over long periods of time the asteroid’s spin rate can increase.  The smaller the asteroid, the larger the effect and small asteroids (generally less than 10 km) can be rotationally fissioned by the YORP effect, effectively turning one asteroid into two asteroids.

If the original asteroid was a rubble pile, then Jacobson and Scheeres argue that both asteroids in the binary must be rubble piles. They suggest that the smaller of the pair can undergo a ‘secondary fission’ – not due to the YORP effect this time, but due to a coupling of the spin and orbital states for the binary system.  The smaller secondary asteroid can “steal” rotational energy from the larger primary asteroid, increasing its rotation rate (while decreasing the rotation of the larger asteroid and changing the shape of their mutual orbit). The smaller asteroid can rotationally fission, producing a chaotic ternary (3-body) system. Energy can swap between the spins and orbits of these three bodies, as well as be damped by tidal effects, which can result in a wide range of outcomes. Impacts between members of the ternary or the escape of one member will help stabilise the resulting binary system.

They conclude that rotational fission can explain all NEA systems, including synchronous binaries (where the spin rate of the secondary equals the rotation rate of the binary pair), high eccentricity binaries (which are asynchronous and have highly oval orbits), ternary systems (3-body systems thought to form after a binary asteroid  undergoes a secondary rotational fission), and contact binaries (which results when gravity pulls the two asteroids together until they touch, forming a single irregularly shaped asteroid).

“NEAs are actively evolving systems driven by these processes and the observed asteroid classes are stages in this evolution”, the authors conclude. This work sheds light on the  mechanism that contributes to the formation and potential disintegration of small asteroids.

For more information, see

[Sheridan Lacey & Sarah Maddison]

** Note: the 2014 arXiv paper by Jacobson & Scheeres (arxiv:1404.0801) was in fact published in Icarus in 2011 (Icarus, 2011, 214, 161) !

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Asteroid rings discovered

Last week astronomers announced the surprise discovery of an asteroid with its own ring system. Until now, rings have only been detected around the giant planets in our Solar System – Jupiter, Saturn, Uranus and Neptune – and it was thought that only massive planets could host rings.  The discovery of two narrow rings around the minor planet Chariklo, a Centaur in the outer Solar System, has forces astronomers to revise their ideas of rings.

Artist’s impression of the rings surrounding the minor planet Chariklo, the first non-planetary body in our Solar System discovered to have its own ring system. (Credit: Lucie Maque)

Chariklo is the largest of the Centaurs and orbits between Saturn and Uranus with an average distance of nearly 16 AU (16 times further from the Sun than the Earth).  Centaurs are small, rocky-icy minor planets – or asteroids – on unstable orbits in the outer Solar System. Because their orbits carry them close to the giant planets, their orbits cannot be stable for more than a few million years.  The name Centaur - which is a half-man-half-horse from Greek mythology – comes from the fact that these minor planets share properties with both asteroids and comets, including cometary coma-like activity.

Chariklo was discovered in 1997 and had a very surprising property: both its brightness and composition were seen to change over time.  After its discovery, its brightness dropped dramatically and the water-ice features initially seen in the spectra of Chariklo fading away. Equally mysteriously, this trend has reversed since 2008, with the brightness increasing and the water-ice features returning.

To learn more about this mysterious object, Felipe Braga-Ribas, of the Observatório Nacional in Brazil, devised an experiment to study the size and shape of Chariklo during a stellar occultation.  From a vantage point in South America, Chariklo was predicted to pass in front of a star on 3 June 2013. Using seven different telescope across Brazil, Argentina, Uruguay and Chile, Braga-Ribas and collaborators watched as Chariklo blocked the starlight, but were surprised to see a dip in the star’s light a few second before and after the main occultation.  Using data from the 7 different telescopes revealed that these extra dips in stellar brightness were caused by two thin rings around Chariklo, just 7 and 3 km wide. The rings on Uranus were discovered in the same way during stellar occultation in the 1977.

The light curve of UCAC4 248-108672 during Chariklo’s occultation on 3 June 2013. The light curve shows the stellar light as a function of time, with the central dip caused by Chariklo transiting in front of the background star, and the two dips either side due to Chariklo’s two rings blocking some of the stellar light. (Credit: Braga-Ribas et al. 2014, Nature)

The ring system nicely explains the mystery of the changing brightness and composition of Chariklo. The reflective, icy rings would add to the brightness of the asteroid when viewed face-on, but the brightness and the water-ice signal would drop when the rings are seen edge-on.

What remains a mystery is how the rings formed, and remain stable, around such a small object. Chariklo is just 250 km in diameter. The most likely explanation is that the rings formed from debris left over from a collision, either from a small body catastrophically colliding with Chariklo and ripping off some of its outer icy layer, or perhaps the collision of pre-existing icy satellites.  Given 5% of Centaurs are known to have small companions, the idea of colliding satellites is not unreasonable. For the rings to be stable, any collision that caused them must have occurred at very low speeds to ensure that the ring particles remained within the gravitational attraction of the asteroid’s tiny mass.  And to have two thin rings, the ring material must be confined by tiny km-sized ‘shepherding’ satellites.

While Chariklo is the first satellite found to host rings, it is unlikely to be unique in our Solar System.

For more information, see

[Sarah Maddison]

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Cosmic dust census of the local Universe

Astronomers have recently completed the largest census of dust in the local Universe via the Herschel Reference Survey. An international team, led by Luca Cortese from Swinburne University, used ESA’s Herschel Space Observatory to study the properties of dust grains in more than 300 nearby galaxies.

When we think about the various components of galaxies dust does not immediately come to mind. However, it turns out that these tiny particles (with sizes that vary between 10-3 and 1 micron) play a vital role in the formation of stars & planets and in the overall evolution of galaxies. For example, dust grains are needed to form molecular hydrogen (see our earlier article on dust grain alignment for an explanation of how H2 forms on the surface of grains), and stars form in molecular hydrogen clouds. So without dust it would be more difficult for galaxies to form new stars.

Despite its importance, until now astronomers have had very little information on how the amount of dust and dust properties vary across the different types of galaxy in the Universe. This was mainly due to the lack of telescopes able to perform deep surveys in the far-infrared and sub-millimetre parts of the electromagnetic spectrum. Cosmic dust is heated by starlight to temperatures of only a few tens of degrees Kelvin, causing it to emit radiation in the wavelength range mainly between 100 microns and 1 millimetre – covered by the far-infrared and sub-millimetre bands.

Previous space missions such as IRAS and Spitzer only covered a small part of this frequency range, while ground-based sub-millimetre telescopes like the James Clerk Maxwell Telescope (JCMT) and the SubMillimeter Array (SMA) are not sensitive enough to observe a large number of galaxies in a reasonable amount of time, since the emission from cosmic dust is generally faint. Only with Herschel have astronomers have been able to provide the first census of cosmic dust in the local Universe.   Herschel had the largest mirror (3.5 metres) ever sent to space and three incredibly sensitive instruments.

The sample observed in the Herschel Reference Survey includes 323 galaxies spanning the entire range of morphologies (from ellipticals to spirals to irregulars) and a wide range of star formation activities.

Galaxies in the Herschel Reference Survey. These false-colour images highlight different dust temperatures, blue representing colder and red warmer dust respectively (Credit: ESA/Herschel/HRS-SAG2, HeViCS Key Programmers, L. Cortese, Swinburne University)

The two cameras on board the Herschel satellite, SPIRE and PACS, allowed astronomers to cover the entire wavelength range from 100-500 micron, which provide important information about the physical properties of cosmic dust grains. Although the SPIRE data were obtained three years ago, the team had to wait for the completion of the PACS survey last year.

Such a long wait has been worthwhile, as the combination of the PACS and SPIRE data shows that the properties of grains vary significantly as a function of galaxy type, star formation activity and metal enrichment of the interstellar medium. It now seems clear that dust is much more abundant in irregular and spiral galaxies where active star formation is still present, while it is almost completely absent in lenticular and elliptical galaxies, where star formation has stopped and the hydrogen gas has long been consumed. For example, the mass of dust is between 1% and 0.1% of the stellar mass in spirals and irregulars, decreasing to less than 0.01% in ellipticals and lenticulars.

This can be clearly seen in the two mosaics released by the team, which compare the optical and far-infrared images for galaxies in the Herschel Reference Survey. Galaxies are arranged from dust-rich in the top left to dust-poor in the bottom right. It is immediately clear that elliptical and lenticular galaxies, which are extremely rich in stars and thus shine extremely bright in the optical, are almost completely invisible when observed at sub-millimeter wavelengths.

Collage of galaxies in the Herschel Reference Survey at FIR/sub-mm wavelengths by Herschel (left) and at visible wavelengths from the Sloan Digital Sky Survey (right). The Herschel image is coloured with blue representing cold dust and red representing warm dust; the SDSS image shows young stars in blue and old stars in red. Together, the observations plot young, dust-rich spiral/irregular galaxies in the top left, with giant dust-poor elliptical galaxies in the bottom right. (Credit: ESA/Herschel/HRS-SAG2 and HeViCS Key Programmes/Sloan Digital Sky Survey/ L. Cortese, Swinburne University)

Why dust grains are no longer present in elliptical and lenticular galaxies is still unclear, but the Herschel observations have identified one of the possible physical processes able to remove dust from galaxies. These show that, when galaxies fall into a galaxy cluster, the strong pressure (known as ram-pressure) felt by the galaxy due to the hot gas (~108 K) trapped in the cluster’s potential well may be able to remove the grains from the star-forming disk of spiral galaxies, and scatter them in the intra-cluster medium.

The data obtained for the Herschel Reference Survey have been made publicly available to allow further studies of dust properties in nearby galaxies. Although the Herschel Space Telescope completed its mission in April 2013, these data will represent for quite a long time a local benchmark for studies of dust in the early Universe. In particular, the newly commissioned Atacama Large Millimeter/sub-millimeter Array (ALMA) in Chile will soon allow astronomers to carry out similar studies at very high redshift, and these Herschel data will be the local reference necessary to determine if and how the properties of grains in galaxies have changed with the age of the Universe.

For more information, see

[Luca Cortese & Sarah Maddison]

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Detection of gravitational waves

Scientists have found evidence for the existence of gravitational waves and support for cosmic inflation. Gravitational waves are a prediction of Einstein’s theory of general relativity, which states that mass and energy (which are equivalent, as we know from E=mc2) can distort space-time. Gravitational waves can be thought of as waves in the fabric of space-time, which simultaneously stretch and compress different regions of space-time.  Massive objects, such as black holes, bend space-time around them, and when objects move they create ripples in space-time. Imagine a duck swimming through a lake: as she moves forward she pushes the water away from her and creates a wake behind herself. This is what massive objects do when they move through space. Stars in a binary that orbit one another will create ripples in space-time that propagate outwards as gravitational waves.

Ripples in space-time are created by moving masses, in this case a black hole binary (Credit: T. Carnahan, NASA/GSFC)

Energy – being equivalent to mass – can also induce gravitational waves. So what created the gravitational waves that scientists claimed to have detected? The answer is inflation.  Right after the Big Bang, when our Universe was very young and very hot, matter could not exist and all that existed was energy. During this time, the four fundamental forces – gravity, electromagnetism, the strong and weak forces – were one unified force. However, the Universe quickly began to cool, which allowed the fundamental forces to separate into individual components. This kick-started a chain reaction that lead to our Universe inflating from 6 x 10-28 meters to almost 1 meter in under a second!  During cosmic inflation, massive amounts of energy were released into space-time which would have created gravitational waves. So the detection of these primordial gravitational waves lends support to the idea that the Universe was created via the Big Bang and then expanded through cosmic inflation.

How do astronomers detect primordial gravitational waves created at the dawn of time, some 13.8 billion years ago? Because the Universe continues to expand, albeit very slowly compared to the period of cosmic inflation, the signature of gravitational waves would be too weak to detect in the nearby Universe. However, the discovery of the cosmic microwave background (CMB) has given us a time stamp of the distant Universe. Some 372,000 years after the Big Bang, the Universe was cool enough that matter became decoupled from radiation.  Before this time, the Universe was opaque to radiation and light could not escape. The cosmic microwave background can be thought of as the most distant part of the Universe that we can observe, when the Universe first because transparent and light could travel freely through space.

If gravitational waves were present in the early Universe, they would have left a distinct pattern on the cosmic microwave background  because they literally alter the space-time in which the photons moved. As gravitational waves propagate through space-time, they will condense space-time in one direction, making it look a little hotter, and stretch space-time in another direction, making it look a little cooler.  These temperature variations are very, very small, but detectable. As photons move through  rippled space-time, they will scatter in a preferred direction, resulting in polarization. The type of polarization that would have been created by primordial gravitational waves is called B-mode polarization, producing a curly, vortex-like pattern.

B-mode polarization pattern seen in the cosmic microwave background as measured by the BICEP2 instrument. The colours show the spin intensity and orientation (red clockwise, blue anti-clockwise) and the lines show the polarization strength and direction. (Credit: BICEP2 collaboration)

To detect the very faint signal of B-mode polarization in the cosmic microwave background requires a precision of one ten-millionth of a kelvin to measure the tiny temperature fluctuations.  Astrophysicists from Harvard-Smithsonian Center for Astrophysics, led by John Kovac, used the BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) instrument at the South Pole to study the “southern hole”, a patch of sky free of other forms of emission, to measure temperature variations in the cosmic microwave background to extremely high precision. And what they found was amazing – a signature in the cosmic microwave background that is consistent with the pattern left by primordial gravitational waves.

Of course additional measurements have been made to confirm this ground-breaking discovery. The Keck Array, also located at the South Pole has provided data with the same implications as BICEP2, and will continue to run for another two  years. The Planck telescope, which has provided the most precise measurements of the Cosmic Microwave Background to date, will be used to provide a more extensive all-sky map of the B-mode polarization.

So what are the consequences of detecting primordial gravitational waves and confirming cosmic inflation? Inflation solves two major issues that arise from the Big Bang theory. First, we know that in any direction you observe, space-time is flat. Secondly, if you measure the temperature of space where there is no matter, it is isotropic. But the Big Bang theory has no explanation for why the Universe would evolve to be this way. Alan Guth, from Massachusetts Institute of Technology, developed the theory of cosmic inflation to understand why particles that should have been created during the Big Bang are not present today, but his theory also solves these other two problems. Essentially, if the Universe began very small but then grow gigantic very quickly, several things could happen. First, if the Universe were small enough the temperature would balance out and be in equilibrium, and so once it expanded it would be the same temperature everywhere. And if the Universe expands enough during the preriod of cosmic inflation, it would wash out any  curvature in space-time.

The confirmation of primordial gravitational waves would also be the first experimental evidence that quantum mechanics and gravity are in fact linked. This is because part of what started cosmic inflation was the existence of quantum fluctuations. These fluctuations are very small waves that propagate through empty space and ignited cosmic inflation. If quantum mechanics and gravity can be linked, than a theory of quantum gravity cannot be dismissed and could be used to explain other extreme phenomena in our Universe, such as the physics that govern the centers of black holes.

For more information, see

[Rebecca Allen & Sarah Maddison]

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Sudden death of very early galaxies

The first galaxies that formed in the universe might have matured sooner than expected. New results from an international team of astronomers, including researchers from Swinburne University, indicate that some of the most distance galaxies are much more ‘mature’ than expected.

When we look into the distant universe, we are also looking back in time. This is because  light takes a finite time to travel through the vast distances of the universe.  The light from galaxies that are, say, 10 billion light years away have taken – you guessed it! – ten billion years to reach the Earth. This means that the light left that distance galaxy when the universe was 10 billions years younger than it is today. From observations we know that young galaxies contain a lot of gas and hence are efficient at forming stars.  These star-forming galaxies are blue in colour, while galaxies that have exhausted their fuel supply and quenched star formation will look red in colour. So when we look back in time, we expect to find young and energetic blue galaxies that are actively forming stars.

The FourStar Galaxy Evolution (ZFOURGE) Survey team has been looking deep into the universe, searching for the universe’s earliest red quiesent galaxies at near-infrared wavelengths. The team have  discovered a new group of 15 massive, passively-evolving galaxies at a distance of 12 billion light years, when the universe was only 1.6 billion years old. This means that these galaxies have matured within the first 12% of the age of the universe. The results, which will continue to puzzle the astronomers who study how galaxies form and evolve, have been published in the Astrophysical Journal Letters.

Ultra Deep Survey (UDS) field image from the Hubble Space Telescope with 4 out of the 15 very old galaxies discovered by the ZFOURGE team. The ZFOURGE galaxies exhibit the typical red colors of mature galaxies. (Credit: Caroline Straatman)

According to the lead author, Leiden astronomer Caroline Straatman, the universe we see  around us today is full of similar massive, “red and dead” galaxies in which very few new stars are formed.  The early universe was different, with most galaxies actively forming stars and growing in mass by accreting gas.  Even the ZFOURGE team were surprised to find such massive, mature galaxies in the early universe. The galaxies must have formed fast, explosively forming stars and then rapidly quenching star formation.

This discovery was made with combination of data gathered over 40 nights from the 6.5-m Magellan Baade Telescope in Chile using the purpose-built FourStar camera, and data from legacy surveys including Chandra COSMOS and GOODS (Great Observatories Origins Deep Survey).

According to Karl Glazebrook, who leads the Swinburne team, cosmological model favoured 15 years did not even predict such mature galaxies in the early universe. In 2004 Glazebrook and co-workers discovered that such galaxies did exist 3 billion years after the Big Bang. With the advent of deep imaging near-infrared surveys in the late 1990′s and more recent improved technologies, astronomers have now been able to push back the discovery of such galaxies to just 1.6 billion years after the Big Bang, which is both surprising and truly exciting.

While it remains a mystery why different galaxies behave in different ways, ZFOURGE team leader Dr. Ivo Labbe from Leiden said that these kinds of results pose new questions, such as how these systems formed so quickly and had a massive boost of size within a short period of time. Much work is yet to be done in understanding the early universe!

For more information, see:

[Themiya Nanayakkara & Sarah Maddison]

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Planetary bonanza

Last month delivered a bumper crop to planet hunters, with  NASA’s Kepler mission announcing the discovery of 715 new planets on 26th February.  All of the planets are in multiple systems orbiting a total of 305 stars; 94% of the planets discovered are smaller than Neptune; and four of the planets are less than 2.5 times the size of Earth and orbit their star’s habitable zone. The habitable zone is a region around a star where liquid water could exist on the surface of solid planets.  This new dataset also shows that small planets in multiple systems appear to have flat and circular orbits that are similar to the planets in our inner Solar System.

Kepler’s multiple-transiting planet systems. For edge-on planetary system, the planets eclipse or ‘transit’ their host star as seen from the vantage point of the observer. (Credit: NASA)

The Kepler spacecraft was launched in 2009, and the mission’s goal has been to search for terrestrial planets in their host star’s habitable zone using the transit method.  The telescope has 1.4-m primary mirror and one instrument: a 0.95-m photometer which comprises an array of 42 CCDs that measure stellar brightness variations over a wide, 15-degree field of view. The primary mission, which started collecting data in May 2009 and ended in November 2012, targeted almost 150,00 stars in a single star field in Cygnus-Lyra region. Each star was continuously monitored over the 3.5 year period.  With the failure of a second of its four gyroscope-like reaction wheel in May 2013, Kepler ceased its terrestrial exoplanet search phase.

However, the Kepler dataset continues to provide fantastic discoveries. The latest batch of 715 new planet discoveries are based on two years of data form May 2009 to March 2011. Of the ~150,000 stars that Kepler monitored for brightness variations during those two years, there are over 2,500 exoplanet candidates, and of these about 450 are potentially multiple planetary systems (meaning more than one planet per star).  Most exoplanet candidates are confirmed planet-by-planet, and three observed transits are generally required for a confirmation of an exoplanet.  This is generally a very time-consuming process.

Jack Lissauer from NASA’s Ames Research Center and collaborators used a statistical technique termed ‘verification by multiplicity’, a method that only works for multiple planet systems, to discover the 715 new planets.  The verification by multiplicity method uses the logic of probability.  The multiplicity method relies on the principal that planets are often clustered in multi-planet configurations, while ‘false positives’ occur randomly.  False positives are mistaken identifications for a potential planet, such as eclipsing binary stars and chance alignments in the Kepler sample. If false positives are random, then they are less likely to occur near the same star. Given this probability, if it appears that a there are several planet ‘signals’ near one star, it is unlikely that they are all false positives. Using this method, the team confirmed in a second paper, led by Jason Rowe from the SETI Institute, that 851 planet candidates in multiple planetary systems are valid planets.

Histogram of the number of planets by size for all known exoplanets. The blue bars represent the exoplanets population prior to the 26 Feb 2014 announcement, and the gold bars add Kepler’s newly-verified planets. (Credit: NASA Ames/W Stenzel)

The Kepler mission now accounts for 57% of all confirmed planets, bringing the total of confirmed exoplanets to 1,690. It is expected that hundreds more planets will be discovered as another two years of Kepler data is processes. The discovery of so many multi-planet systems, and lower mass planet, is extremely valuable for demographic studies and will significantly add to our understanding of the formation of solar systems that have Earth-like planets potentially capable of supporting life.

For more information, see

P.S. Happy birthday Kepler!  5 years strong

[Sheridan Lacey & Sarah Maddison]

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S1-2014 has begun!

Get ready for another semester of awesome SAO astro news! :-)

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