The Moon is thought to have formed mainly from material within a giant impactor that struck the proto-Earth, so it seems odd that the compositions of the Moon and Earth are so similar, given the differing composition of other Solar System bodies. Alessandra Mastrobuono-Battisti et al. track the feeding zones of growing planets in a suite of computational simulations of planetary accretion and find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are more similar to the planets they impact. A significant fraction of planet–impactor pairs have virtually identical compositions. The authors conclude that the similarity in composition between the Earth and Moon could be a natural consequence of a late giant impact.
A primordial origin for the compositional similarity between the Earth and the Moon
Alessandra Mastrobuono-Battisti, Hagai B. Perets & Sean N. Raymond
Nature 520, 212–215 (09 April 2015) doi:10.1038/nature14333
Received 10 November 2014 Accepted 10 February 2015
Most of the properties of the Earth–Moon system can be explained
by a collision between a planetary embryo (giant impactor) and the
growing Earth late in the accretion process1–3. Simulations show that
most of the material that eventually aggregates to form the Moon
originates from theimpactor1,4,5. However, analysis of the terrestrial
and lunar isotopic compositions show them to be highly similar6–11.
In contrast, the compositions of other Solar System bodies are significantly
different from those of the Earth and Moon 12–14, suggesting
that different Solar System bodies have distinct compositions. This
challenges the giant impact scenario, because the Moon-forming
impactor must then also be thought to have a composition different
from that of the proto-Earth. Here we track the feeding zones of
growing planets in a suite of simulations of planetary accretion 15, to
measure the composition of Moon-forming impactors.We find that
different planets formed in the same simulation have distinct compositions,
but the compositions of giant impactors are statistically
more similar to the planets they impact. A large fraction of planet–
impactor pairs have almost identical compositions. Thus, the similarityin
composition between the Earth and Moon could be a natural
consequence of a late giant impact.
Four versions of the same supernova explosion have been captured because a large galaxy between us and the event is distorting the path on which the light travels to reach us. The event not only makes visible a supernovae more distant than we normally see but provides the opportunity astronomers have been dreaming of to test three of the biggest questions in cosmology. Even more opportunities should arise in future.
One of the key predictions of General Relativity is that mass bends spacetime, and therefore light. Einstein predicted that very massive objects could focus light in a manner analogous with glass lenses, an effect finally observed in 1979.
Depending on the locations of the relevant objects we often see multiple images of the same distant quasar or galaxy. Since this light follows different paths to reach us the distance traveled on each will not be identical, so we are seeing some slightly delayed relative to the others. This makes little difference for an object whose brightness barely varies.
However, in 1964 Sjur Refsdal pointed out that different images of the same supernova would capture different moments in the explosion’s evolution, and might be used to test the rate at which the universe is expanding. Great efforts have been made to find such an example of such a valuable case. Dr Patrick Kelly of the University of California, Berkeley was looking for distant galaxies and came across the sight of four images of a nine billion year old supernova around a galaxy in the MACS J1149.6+2223 cluster.
Astronomers have glimpsed a far off and ancient star exploding, not once, but four times.
The exploding star, or supernova, was directly behind a cluster of huge galaxies, whose mass is so great that they warp space-time. This forms a cosmic magnifying glass that creates multiple images of the supernova, an effect first predicted by Albert Einstein’s General Theory of Relativity 100 years ago.
Dr Brad Tucker from The Australian National University (ANU) says it’s a dream discovery for the team.
“It’s perfectly set up, you couldn’t have designed a better experiment,” said Dr Tucker, from ANU Research School of Astronomy and Astrophysics.
“You can test some of the biggest questions about Einstein’s theory of relativity all at once – it kills three birds with one stone.”
Astronomers have mounted searches for such a cosmic arrangement over the past 20 years. However, this discovery was made during a separate search for distant galaxies by Dr Patrick Kelly from University of California, Berkeley.
“It really threw me for a loop when I spotted the four images surrounding the galaxy – it was a complete surprise,” he said.
The lucky discovery allows not only testing of the Theory of Relativity, but gives information about the strength of gravity, and the amount of dark matter and dark energy in the universe.
Because the gravitational effect of the intervening galaxy cluster magnifies the supernova that would normally be too distant to see, it provides a window into the deep past, Dr Tucker said.
“It’s a relic of a simpler time, when the universe was still slowing down and dark energy was not doing crazy stuff,” he said.
“We can use that to work out how dark matter and dark energy have messed up the universe.”
Multiple images of a highly magnified supernova formed by an early-type cluster galaxy lens
BY PATRICK L. KELLY, STEVEN A. RODNEY, TOMMASO TREU, RYAN J. FOLEY, GABRIEL BRAMMER, KASPER B. SCHMIDT, ADI ZITRIN, ALESSANDRO SONNENFELD, LOUIS-GREGORY STROLGER, OR GRAUR, ALEXEI V. FILIPPENKO, SAURABH W. JHA, ADAM G. RIESS, MARUSA BRADAC, BENJAMIN J. WEINER, DANIEL SCOLNIC, MATTHEW A. MALKAN, ANJA VON DER LINDEN, MICHELE TRENTI, JENS HJORTH, RAPHAEL GAVAZZI, ADRIANO FONTANA, JULIAN C. MERTEN, CURTIS MCCULLY, TUCKER JONES, MARC POSTMAN, ALAN DRESSLER, BRANDON PATEL, S. BRADLEY CENKO, MELISSA L. GRAHAM, BRADLEY E. TUCKER
SCIENCE06 MAR 2015 : 1123-1126
Light from a distant supernova at z = 1.491 is detected in four images after being deflected en route by gravitational forces.
In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a strong gravitational lens could be used to measure the rate of cosmic expansion. We report the discovery of such a system. In Hubble Space Telescope imaging, we have found four images of a single supernova forming an Einstein cross configuration around a redshift z = 0.54 elliptical galaxy in the MACS J1149.6+2223 cluster. The cluster’s gravitational potential also creates multiple images of the z = 1.49 spiral supernova host galaxy, and a future appearance of the supernova elsewhere in the cluster field is expected. The magnifications and staggered arrivals of the supernova images probe the cosmic expansion rate, as well as the distribution of matter in the galaxy and cluster lenses.
Passing stars can perturb the Oort Cloud, triggering comet showers and potentially extinction events on Earth. We combine velocity measurements for the recently discovered, nearby, low-mass binary system WISE J072003.20-084651.2 (“Scholz’s star”) to calculate its past trajectory. Integrating the Galactic orbits of this ~0.15 M⊙ binary system and the Sun, we find that the binary passed within only 52+23−14 kAU (0.25+0.11−0.07 pc) of the Sun 70+15−10 kya (1σuncertainties), i.e., within the outer Oort Cloud. This is the closest known encounter of a star to our solar system with a well-constrained distance and velocity. Previous work suggests that flybys within 0.25 pc occur infrequently (~0.1 Myr−1). We show that given the low mass and high velocity of the binary system, the encounter was dynamically weak. Using the best available astrometry, our simulations suggest that the probability that the star penetrated the outer Oort Cloud is ~98%, but the probability of penetrating the dynamically active inner Oort Cloud (<20 kAU) is ~10−4. While the flyby of this system likely caused negligible impact on the flux of long-period comets, the recent discovery of this binary highlights that dynamically important Oort Cloud perturbers may be lurking among nearby stars.
Perturbations by passing stars on Oort cloud comets have previously been proposed as the source of long-period comets visiting the planetary region of the solar system (Oort 1950; Biermann et al. 1983; Weissman 1996; Rickman 2014), and possibly for generating Earth-crossing comets that produce biological extinction events (Davis et al. 1984). Approximately 30%, of craters with diameters <10 km on the Earth and Moon are likely due to long-period comets from the Oort Cloud (Weissman 1996). Periodic increases in the flux of Oort cloud comets due to a hypothetical substellar companion have been proposed (Whitmire & Jackson 1984); however, recent time series analysis of terrestrial impact craters are inconsistent with periodic variations (Bailer-Jones 2011), and sensitive infrared sky surveys have yielded no evidence for any wide-separation substellar companion (Luhman 2014). A survey of nearby field stars with Hipparcosastrometric data (Perryman et al. 1997) by García-Sánchez et al. (1999) identified only a single candidate with a pass of within 0.9 pc of the Sun (Gl 710; 1.4 Myr in the future at ~0.34 pc); however, it is predicted that ~12 stars pass within 1 pc of the Sun every Myr (García-Sánchez et al. 2001). A recent analysis by Bailer-Jones (2014) of the orbits of ~50,000 stars using the revisedHipparcos astrometry from van Leeuwen (2007), identified four Hipparcos stars whose future flybys may bring them within 0.5 pc of the Sun (however, the closest candidate HIP 85605 has large astrometric uncertainties; see discussion in Section 3).
A low-mass star in the solar vicinity in Monoceros, WISE J072003.20-084651.2 (hereafter W0720 or “Scholz’s star”) was recently discovered with a photometric distance of ~7 pc and initial spectral classification of M9 ± 1 (Scholz 2014). This nearby star likely remained undiscovered for so long due to its combination of proximity to the Galactic plane (b = +23), optical dimness (V = 18.3 mag), and low proper motion (~01 yr−1). The combination of proximity and low tangential velocity for W0720 (Vtan 3 km s−1) initially drew our attention to this system. If most of the star’s motion was radial, it was possible that the star may have a past or future close pass to the Sun. Indeed, Burgasser et al. (2014) and Ivanov et al. (2014) have recently reported a high positive radial velocity. Burgasser et al. (2014) resolved W0720 as a M9.5+T5 binary and provided a trigonometric parllax distance of 6.0+1.2−0.9 pc. Here we investigate the trajectory of the W0720 system with respect to the solar system, and demonstrate that the star recently (~70,000 years ago) passed through the Oort Cloud.