Meet Jupiter’s new friend plus the mystery of Vesta’s dark rays

Jupiter, Aries and planet-bearing Hamal are all high up in the southeastern sky around 7 o'clock local time in late November. Aries is only one fist held at arm's length above Jupiter. Created with Stellarium

Not far from Jupiter in the fall sky is a newly discovered planet with a mass nearly twice as large. Granted it’s 66 light years away, but you can look up anytime it’s clear and imagine another hitherto unknown world up there. Called Alf Ari b, it revolves around Hamal (HAM-al), the brightest star in the little constellation Aries, and was discovered just this year using the radial velocity method of detection.

A team of astronomers employed a spectrograph on the 71-inch Bohyunsan Optical Astronomy Observatory reflecting telescope in Korea to precisely measure the minute tugs on the star from the gravity of an orbiting planet. Based on changes in Hamal’s speed toward and away from Earth (radial velocity) they determined how much matter was doing the tugging.

The new planet tips the scales at 1.8 Jupiter-masses and orbits 111 million miles from its host star with a period of 381 days. That’s only a little more than Earth’s distance from the sun. Most extrasolar planets are of the “hot Jupiter” variety – large planets orbiting very close to their host stars. This one is a little further away and perhaps might be called a “warm” Jupiter. Assuming it has a solid surface, which is by no means certain, if you weigh 100 lbs. on Earth, the additional gravity of this massive planet would pump that number up to around 425 lbs. – most uncomfortable. For a bit of fun, click HERE to see what you’d weigh on other planets.

Hamal is 15 times larger than the sun or about 13 million miles in diameter. If put in the sun's place, it would reach more than 1/3 the way to Mercury. Illustration: Bob King

Hamal is an orange giant star 1.5 times the sun’s mass and 15 times larger. A trained eye might detect a hint of warmth in its hue compared to stars of similar brightness. At magnitude 2.0, which is as bright as the stars in the Big Dipper, Hamal is the second brightest star in the night sky to sport an extrasolar planet. While no one can yet see Alf Ari b, it revealed itself through gravity and now shares a place in our mind’s eye.

A fresh 4.3 mile diameter crater on Vesta displays both bright and dark rays as well as blocks of debris (in its center) from the impact. Click to enlarge. Credit: NASA/ JPL-Caltech

I was browsing the Dawn Mission website the other day to see what’s new with the asteroid Vesta and came across an interesting photo of a fresh, small crater with a splatter of both light and dark rays around it. Rays are formed from secondary impacts of rock blasted from the crust that land in a radial pattern around the crater. What’s unusual is that the impact excavated both light and dark layers of material.

A 1-mile-diameter Vestan crater with dark rays. Credit: NASA/JPL-Caltech

Pale rays systems are much more common across the solar system because falling impact debris digs up fresh materials from beneath the crust that have yet to be exposed to the darkening effects of sunlight.  Dark rays are uncommon except it seems on Vesta, which has a surprisingly large share of them. Clearly the asteroid has layers of different kinds of rocks, but exactly what they’re made of is not yet known. Dawn will use its gamma ray and neutron detector to map the elemental composition of the surface in the weeks and months to come, hopefully shedding light on this dark matter.

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