Bakersfield Night Sky – October 20, 2012
By Nick Strobel
The exoplanet system 55 Cancri was in the news again. This time it was because it is possible that it's innermost planet may have a carbon-rich interior that gets compressed enough to form A LOT of diamond—more than an entire Earth's mass amount. Well, if one talks about that much diamond, you can be darn certain that it will be picked up by the news media. Sure enough, that's what happened with this story. It even made national TV news. Now while it looks like I've jumped on the bandwagon too, re-read my second sentence above and notice the cautionary words "possible" and "may". The original research paper describing the possibility has the same cautionary tone but the press release from the lead researcher's university (Yale University), changes the language to say that the planet is "likely covered in graphite and diamond". Still cautionary but less tentative words. On the TV news the words are now that the planet IS a diamond planet—now that's throwing caution to the wind. Let's take another look at this planet and how it was concluded that it could have a lot of carbon in it, including the form known as diamond.
The stick-figure drawing of Cancer on our sky looks an upside-down letter "Y" (see the first chart below). Slightly to the left of the base of the Y (i.e., opposite to the two prongs of the Y), is the faint star 55 Cancri that is just barely visible without binoculars if you have a dark sky and really good eyes (well, that definitely means I need to use binoculars). 55 Cancri is a binary star system about 41 light years away that consists of a star much like our Sun and a small, cooler red-dwarf star that orbits the Sun-like star (55 Cancri A) at a distance over 25 times further than Pluto orbits the Sun. Much closer to the main star 55 Cancri A orbit at least five planets. The fifth one discovered made news a few years ago because it made 55 Cancri be the first planetary system discovered with at least five planets in it and because that fifth planet orbited inside the habitable zone of 55 Cancri A. The habitable zone is the region around a star in which the surface temperature on a planet could be just right for liquid water to exist. The 55 Cancri exoplanet making the news last week is the closest one to the main star, 55 Cancri e. It has a diameter two times the Earth and a mass eight times the Earth so it is called a "super-Earth". 55 Cancri e orbits so close to the star that it takes just 18 HOURS to orbit the star and the surface temperature is about 3900º F. So diamond-mining companies willing and able to wait the 2.06 million-year round trip in our fastest rocket-powered spacecraft would need equipment made of non-metals with very high melting points and cooling systems for the electronics far beyond anything we have even planned. (So, scratch off "mining 55 Cancri" from the "to-do" list.)
When 55 Cancri e was first discovered, astronomers figured that it could have a layer of liquid water in a supercritical state if the abundances of its elements (types of atoms), particularly oxygen, were like the Earth's. If you assume different abundances, then you will deduce that different types of minerals exist in 55 Cancri e. That's what happened here. The research team led by Nikku Madhusudhan at Yale Univ. looked at what combinations of iron, carbon, silicon-carbide, and other silicates could match the size and diameter of the planet using what we know of how materials behave under different conditions. The main star 55 Cancri A is relatively carbon-rich and iron-rich compared to the Sun. This means that the gas cloud from which 55 Cancri A and its planets formed would have been enriched in iron and carbon compared to the cloud that formed our Sun and planets, including the Earth. Assuming that the 55 Cancri protoplanetary disk that formed the planets had the same composition as the main star (that's a good, reasonable assumption to make), the research team worked through the interior structure computer models and planet-formation computer models to find that 55 Cancri e could have a surface of mostly graphite surrounding a thick layer of diamond that is on top of an outer core of silicon-based minerals and an inner molten core of iron at the center. So assuming a carbon-rich formation scenario, it is possible that 55 Cancri could have up to three times the mass of the Earth in diamond. Future observations of the star's composition in higher resolution and the planet's atmosphere will constrain the possibilities.
The constellation Cancer is the location for another object in astronomy news that could get hyped to an extreme—Comet ISON (aka Comet 2012 S1). The comet is now beyond the orbit of Jupiter and too faint to be seen without a large telescope but it will pass very close to the Sun in late-November 2013 and it has the possibility of becoming bright enough to see in daylight at around Thanksgiving time. That is, if it doesn't get destroyed by its very close passage by the Sun as plenty of other Sun-grazing comets have been. There is some hype on the web about Comet ISON becoming as bright as the Full Moon, despite the fact that no astronomer has said that and no comet in recorded history has ever been that bright. Even if it did get that bright, Comet ISON would be just two fingers width (at arm's length) from the million-times brighter Sun at that point. I haven't heard of any end-of-the-world reports connected to Comet ISON yet, but I'm sure that when we approach the one-year anniversary of the 2012-mayan baktun calendar re-set, the doom-sayers will be sure to put out the word. Anybody recall Comet Elenin? Comet brightnesses are notoriously hard to predict because a comet's composition and structure is extremely difficult to measure from afar. The composition and how the materials are glued together determine the size and brightness of the comet's tail. Although comet orbits are easy to predict, when considering how a comet itself will behave, it is wise to keep in mind famous comet-hunter David Levy's counsel that "comets are like cats; they have tails, and they do precisely what they want."
The first star chart below shows the locations of the featured players of this column in the constellation Cancer. The chart is set for 6 AM of tonight's sky (i.e., tomorrow's pre-dawn morning) looking toward the southeast. Jupiter will be almost directly overhead between the horns of Taurus and super-bright Venus will be below the star Denebola at the tail of Leo, about a quarter of the way up in the sky. The Moon is in Waxing Crescent phase tonight, so it will set by 11 PM and not wash out the early morning sky. Tomorrow night the Moon will be at First Quarter phase. This evening, Mars will be just above the bright red heart of Scorpius, the star Antares but they both will already be low in the sky by 7:30 PM, so you will probably need binoculars to see them through the city-light glow (see the second chart below). The Moon will be above the Teapot part of Sagittarius.
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com