Bakersfield Night Sky – November 16, 2013
By Nick Strobel
Last week was filled with exoplanet news from the Kepler science conference held at NASA's Ames Research Center in Moffett Field. Although, the Kepler spacecraft is no longer currently looking for planets due to hardware failure of a couple of its stabilizer reaction wheels, there is a plan to use some clever orbital mechanics to look for exoplanets around other stars in the same plane as our solar system. Using the pressure of sunlight to help stabilize the spacecraft along with the remaining two stabilizer reaction wheels, Kepler would search for planets around the very common cooler stars in a wider variety of environments than its original mission that focused on a small section of the sky in the constellation Cygnus. Details are still being worked out before the Kepler team submits its proposal to NASA.
In the meantime, there are A LOT of data from the original mission phase that the Kepler team is still poring over. The Kepler science conference last week presented results from the first three years of the four-year mission. The number of exoplanet candidates is now over 3500 in over 2600 star systems. The word "candidates" is important, so let me explain how Kepler detected exoplanets. Kepler found exoplanets by looking at over 150,000 stars non-stop for four years to see which ones had planets with orbits that would have the planet cross in front of star ("transit the star"), causing it to become slightly dimmer for a few hours during the transit. A transit would have to be repeated at least three times to be sure the dip in star brightness wasn't some fluke of the instrument or something else passing between us and the star by accident. Bona fide periodic transits are followed up by the largest ground-based telescopes to rule out the possibility of the transit being caused by a dim star in orbit around the brighter star or background contamination from another binary system in nearly the same direction as the star of interest. Once the exoplanet candidate passes all of that intense cross-checking, it becomes a "confirmed exoplanet". There are currently 170 confirmed exoplanets from the Kepler mission and along with the numbers of candidates that were later rejected, we now have a very good idea of how well the Kepler team's transit vetting process works in finding possible planets. Just ten percent of the exoplanet candidates turn out to be something else. That leaves 90% of the exoplanet candidates as actual exoplanets. Ninety percent of 3500 is 3150. That's a lot of real exoplanets to explore further and the Kepler team still has another year's worth of observations to find even more exoplanets! The team gave just a slight smirk at the late October news story about the number of confirmed exoplanets from all the other exoplanet searches reaching the 1000 exoplanet mark---"you ain't seen nothing yet!"
Other results from the latest data release include: over 600 of the candidate exoplanets are Earth-size or smaller; 104 candidates are in their star's habitable zone with 24 of them smaller than 2 Earth diameters in size; 22% of the stars with planets have more than one candidate orbiting them (the other research teams have a harder time finding systems with multiple planets like our solar system); nature makes planets of a variety of sizes up to 3 Earth diameters with equal ease and has more difficulty with larger planets like Saturn or Jupiter; 23% of sun-like stars (that's better than 1 in 5) have planet less than 3 Earth diameters in size; at least 70% of ordinary stars including those hotter than the Sun and the great majority cooler than the Sun have a planet of some size orbiting them; and about 50% or so of the very common cool stars have a planet between 0.5 to 1.4 Earth diameters orbiting within their habitable zone. That last statistic means that the closest Earth-size exoplanet having its orbit aligned just right with our line of sight so the exoplanet transits its star is just 29 light years away---within very easy reach of the proposed TESS mission that will look for transiting exoplanets around stars in all directions but at closer distances than the Kepler mission.
Although the Kepler team has not found the exact Earth analog of an exoplanet of identical size to the Earth orbiting a star with the same temperature as the Sun at the exact same distance as the Earth is from the Sun, it has become quite clear that there are PLENTY of small rocky-world exoplanets orbiting within the habitable zone of their star in just this one small section of the Milky Way we have searched.
Now that we know that such exoplanets exist, there is intense work going on to figure out how many of them would actually be habitable. What combination of atmosphere thickness and composition; carbon, silicon, nitrogen and oxygen abundance, and water in liquid and vapor form would enable a rocky planet to have decent temperatures on the surface so life could form on or near the surface? Surface life is key because that form of life would be able to modify the atmosphere in a way that we could detect from light years away here on the Earth. Remember that even our fastest spacecraft would take many tens of thousands of years to travel to the next star, so we're pretty much "stuck" here in our solar system for a while.
Switching gears, Comet ISON should be getting bright enough to see on a dark sky without binoculars now. ISON will swing around the Sun on November 28th so the week leading up to Thanksgiving and the week or so after Thanksgiving will be good times to look for the comet. The first chart below shows where to look in the eastern pre-dawn sky the week before and after Thanksgiving. The first chart is sized to include the bright stars Arcturus and Spica along with Mercury and Saturn as reference points to help you locate the comet. While you're up early for comet observing, check out Mercury and Saturn's close conjunction on the mornings of November 25 and 26th. On December 1st, a very thin Waning Crescent Moon will be nicely placed midway between Mercury and Saturn. The second and third charts below show the entire eastern pre-dawn sky before and after Thanksgiving to provide more context.
The star charts do assume that ISON doesn't break up when it gets really close to the Sun, of course. ISON is going to approach within just 683,000 miles of the Sun's surface, or 26 times closer than Mercury ever gets to the Sun, so its outer layer of ices are going to be boiling off madly. The ices provide the "glue" holding the chunks of dust, dirt, and rock together in the mile-wide icy nucleus. As I write this, the comet nucleus is still all in one piece. Check the Comet ISON Observing Campaign website at www.isoncampaign.org for the very latest on the status of Comet ISON.
Even if Comet ISON doesn't turn out to be as bright as originally hoped, it has been the object of intense scrutiny by amateur and professional research astronomers alike because it was discovered over a year before its closest flyby of the Sun (called "perihelion"). That has given everyone time to develop their observing plans and in the case of the professional astronomers, enough time to put in their requests for observing with huge telescopes like the ones on the summit of Mauna Kea months in advance of when they want to observe. NASA is also tasking various spacecraft to observe the comet as it passes within reach of the various spacecraft instruments. The twin STEREO spacecraft studying the Sun are also now keeping on eye on ISON as well as is the MESSENGER spacecraft orbiting Mercury.
Another reason why ISON is so interesting is that this is its first passage into the inner solar system since it formed over 4.5 billion years ago. It formed near the current locations of the large jovian planets and then passed near one of the jovian planets which gravitationally flung it very far out from the Sun into the Oort Cloud. This was happening a lot in the early forming solar system. The Oort Cloud is the result of all those billions of flung out icy chunks that didn't crash into the planets or the Sun. This comet cloud is 1250 to 2500 times farther out from the Sun than Pluto. Every now and then a slight nudge from a passing nearby star sends an Oort Cloud comet inward toward the Sun. Some of them will pass very close to the Sun and will be called "sungrazers". A sungrazer coming from the Oort Cloud for the first time like ISON happens about once every 200 years on average, so yes, we do want to check it out while we can.
ISON will pass closest to Earth on December 26th at a very safe distance of 39,900,000 miles, so there is ZERO chance of it hitting the Earth. That hasn't stopped the doomsayers and those eagerly awaiting the end of the world to spread fear about ISON. As I was perusing through some videos about ISON on YouTube I stumbled across one apocalyptic site falsely claiming that the "huge Comet ISON" stirred up many large asteroids before it passed by Mars at the beginning of October and those asteroids would hit the Earth at the end of December or early January. Geesh and big eye roll on this! Comet ISON is way too small in mass and its orbit is way too mis-aligned with the asteroid belt to have done this. In January the shenanigans of Congress are going to be giving us enough stress that we don't need false doomsday accounts of comet or asteroid impacts to add to the noise. YouTube and Yahoo News can be sources of authentic news BUT you have to wade through A LOT of junk stuff to find the good stuff. With all of the intentionally fear-mongering, doomsday, conspiracy-laden junk along with the innocently mis-informed stuff out on the internet, our high school and college teachers have their jobs cut out for them as they teach students how to critically analyze what they see in the media to separate the wheat from the huge pile of chaff. If you had a teacher do that for you, send them a thank you note for Thanksgiving.
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