Exocomets

An image of Halley’s comet. Astronomers have detected around other stars exocomets with masses comparable to Halley’s comet.W. Liller, the International Halley Watch Large Scale Phenomena Network. Hi-res image

There are currently over 3500 confirmed exoplanets known thanks to the remarkable sensitivity of the Kepler spacecraft and to technological advances in space and ground-based methods made over the past dozen years. Relatively little is known, however, about the minor bodies that might orbit within these systems, asteroids and comets for example. Planet-formation theories predict that such minor bodies should be common, but their low masses and small radii present extreme detection challenges. Methods that rely on solid body transits or velocity variations are generally orders-of-magnitude too weak to spot such small objects. The smallest solid body that has been detected so far via the transit method is an object about one-quarter the size of the Earth, while pulsar timing measurements have spotted a lunar-mass object orbiting a pulsar.

In a tour de force analysis of the Kepler data sets spanning 201250 target stars, CfA astronomers Andrew Vanderburg, Dave Latham, and Allyson Bieryla joined eight of their colleagues in discovering and modeling a likely set of six transiting comets around one star, with another comet possible around a second star. The physical characteristic that made these detections possible was unexpected: the comets have large, extended dust tails that can block enough starlight to make themselves recognizable via unique, asymmetrically shaped absorption dips in their transit lightcurves. (The paper reports, in press, finding a prediction of just such an effect published in 1999). The astronomers systematically consider other explanations for the dips, including starspots, as well as possible inconsistencies in their cometary model, like orbital behavior, but reject them all.
The scientists can estimate the mass of the comets from the observed transit properties and simple assumptions, and they conclude that the bodies are probably similar in mass to Halley’s Comet. The scientists also conclude that exocomets are probably not rare given that these seven were spotted without using sophisticated computer tools, although deeper searches will need to be undertaken to find them. Since the two stars hosting exocomets in their study are quite similar in type, they conclude by wondering whether comet transits happen preferentially around certain kinds of stars, although why this might be is not known.

Reference(s): 
 

“Likely Transiting Exocomets Detected by Kepler,” S. Rappaport, A. Vanderburg, T. Jacobs, D. LaCourse, J. Jenkins, A. Kraus, A. Rizzuto, D. W. Latham, A. Bieryla, M. Lazarevic, and A. Schmitt, MNRAS 474, 1453, 2018.

carnegiemuseumnaturalhistory: Anzu wyliei Perhaps better known…

carnegiemuseumnaturalhistory:

Anzu wyliei

Perhaps better known by its colorful nickname, the “Chicken
from Hell,” Anzu wyliei is a
bird-like oviraptorosaurian dinosaur. More specifically, it is a member of the
Caenagnathidae, a poorly understood group of oviraptorosaurs that lived mainly
in North America during the Cretaceous Period. Anzu has distinctive characteristics that are not found in any other
dinosaur, plus other typically oviraptorosaurian features such as a crested
skull and a toothless beak. It grew to a length of at least nine feet and had a
relatively short tail and long, spindly legs with three-toed feet. Its long
arms featured sharp, hooked claws that may been used to catch prey or for
protection.

With a name that translates to “Wylie’s feathered demon,”
this dinosaur presents numerous riddles to scientists. Due to the shape of its
toothless jaws, it is unknown if Anzu
was a carnivore, like most other theropod dinosaurs, or if it was a plant eater.
Anzu may even have been an omnivore,
eating both plants and small animals.

Carnegie Museum of Natural History’s two skeletons of Anzu are the most complete oviraptorosaur
specimens yet found in the Western Hemisphere. Museum scientists and their
collaborators are continuing to study the dinosaur’s bones to gain a better
understanding of the species. Because the real fossils are extremely fragile,
more so than those of most other dinosaurs on display, the skeleton on exhibit
is a cast. It is a combination of replicas of the museum’s two real specimens,
which were discovered in the late 1990s in ~66 million-year-old rocks belonging
to the Hell Creek Formation in Harding County, South Dakota. Anzu wyliei was named in 2014 by
Carnegie Museum paleontologist Matt Lamanna and three of his colleagues.