Features RSS

Asteroid Itokawa

Recent observations of asteroid (335433) 2005 UW163 have added a new member to the mysterious category of “super-fast rotators” — asteroids that rotate faster than should be possible, given current theories of asteroid composition.

Asteroids come in sizes of a few meters to a few hundred kilometers, and can spin at rates from 0.1 to nearly 1000 revolutions per day. Current theories suggest that asteroids smaller than 150m are mostly monolithic (made up of a single rock), whereas asteroids larger than 150m are usually what’s known as a “rubble pile” — a collection of rock fragments from past collisions, bound together into a clump by gravity. “Rubble pile” asteroids have an important structural limitation: they can’t spin faster than once every 2.2 hours without flying apart as the centripetal force overcomes the force of gravity.

Asteroid 2005 UW163 violates this rule: its diameter is 690m, but it rotates once every 1.29 hours. This discovery was made by a team of scientists using telescopes at the Palomar Observatory in California to conduct a large survey of the rotation rates of nearby asteroids. The group, led by Chan-Kao Chang of Taiwan’s National Central University, discovered 11 super-fast rotator candidates — of which asteroid 2005 UW163 is the first to have its rotation rate confirmed by additional observations.

The category of super-fast rotators poses an interesting problem: how are they able to spin so quickly without flying apart? Either the density of these asteroids is unexpectedly high (roughly four times the density of typical “rubble pile” asteroids), or else there must be additional forces besides gravity at work to help hold the asteroid together, such as bonds between the rocks. Future observations of super-fast rotators will help us better understand the peculiar structure of these rocky neighbors.

Citation:

Chan-Kao Chang et al. 2014 ApJ 791 L35 doi:10.1088/2041-8205/791/2/L35

Protoplanetary disk

A team of scientists led by Catherine Walsh (Leiden Observatory) has found evidence of two planets orbiting a young, hot star located relatively nearby, at just 335 light years from Earth. The star, HD 100546, is surrounded by a disk of gas and dust — which is a prime environment for forming planets. Previous observations have hinted at two planets potentially hiding in this disk, but directly imaging planets in disks is very difficult. Walsh’s team took a different approach to finding these subtle planets: rather than looking for them directly, the group instead looked for gaps in the dust of the disk.

The team examined the signature of dust particles in the disk around HD 100546 using observations from the Atacama Large Millimeter/Submillimeter Array (ALMA), a telescope located in Chile. They found that rather than forming a solid disk, the dust particles have settled into two nested rings with a gap between them. That gap is telltale evidence of a planet: planets embedded in disks tend to clear out a path as they orbit, accreting the gas and dust onto themselves.

Walsh’s team followed up the discovery by using different models of planetary formation to try to reproduce ALMA’s observations. They found that the best model required there to be two planets in HD 100546’s system: one located very close to the star, and a second, newly-forming planet located within the disk gap.

This model is consistent with previous estimates of where the two suspected planets might be located, which is promising news for exoplanet enthusiasts. HD 100546 could be our first opportunity to study a planet caught in the act of forming — which is an important step toward understanding how planets are created out of disks around young stars.

Citation:

Catherine Walsh et al. 2014 ApJ 791 L6 doi:10.1088/2041-8205/791/1/L6

1 … 113 114 115