Bakersfield Night Sky – January 21, 2012
By Nick Strobel
In planet-hunting news, the Kepler spacecraft, has discovered two more planetary systems where the planet orbits two stars. Most stars in our galaxy have at least one stellar companion—two or more stars orbiting each other. Lone stars like our Sun are the minority. Up to a few months ago we thought it would be difficult for planets in stable orbits to exist in multiple star systems. Then in September the Kepler team announced with great fanfare the discovery of the "Tatooine" planet, after the science fiction character Luke Skywalker's home planet. In that system with a very creative name of "Kepler 16", the Saturn-sized planet Kepler-16b orbits two stars that are each smaller and cooler than our Sun. Kepler-16b orbits well outside the habitable zone of the combined stars.
Was this system a fluke? Just over a week ago (by the time this column goes to print), the Kepler team answered this question with a definite "NO". Two more "circumbinary" planetary systems were announced: Kepler 34 and Kepler 35. The new planets with the original names, yes, you guessed it, "Kepler-34b" and "Kepler-35b", are both gaseous Saturn-size planets. The planet orbits are too elliptical for them or any moon orbiting them to be habitable. Complex life (multi-cellular) will need to have a stable temperature regime to form so the planet orbit cannot be too elliptical. Simple life like bacteria might be able to withstand large temperature changes on a planet with a significantly elliptical orbit but complex life is the much more interesting case. Suitable binary stars would be those systems where either the binary stars orbit very close to each other with the planet(s) orbiting both of them at a large distance (the "circumbinary" case) or the binary stars orbit very far from each other so the planet(s) could reside in stable orbits near each of the stars—the one star's gravity acting on a planet would be much stronger than that of the other star. The two new circumbinary planets have elliptical orbits that should create very wild climate swings, cycling through the four seasons many times a year. Furthermore, their "year" is shorter than our 365-day year. Kepler-34b orbits its two Sun-like star stars every 289 days and Kepler-35b orbits its two smaller stars (80% and 89% the Sun's mass) every 131 days.
The planets themselves are too large to be habitable since their Jupiter-Saturn-like atmospheres would have large up and down drafts so lifeforms would cycle between too-extreme temperature and pressure swings. Smaller worlds are the key and Kepler's goal is to find an Earth-size planet orbiting in a star's habitable zone, where the surface temperatures on a planet would allow liquid water to exist. Kepler has discovered some smaller worlds. At the same time as the Kepler 34 & 35 announcement, the Kepler team announced the discovery of KOI-961, a mini-planetary system with three planets all smaller than Earth. The smallest is about half the size of the Earth or slightly larger than Mars. The three small planets orbit a faint red-dwarf star at about the same distance as Jupiter's large moons orbit Jupiter. This is well inside the star's habitable zone, so the planets would be too hot.
A month ago, the Kepler team announced the discovery of two planets orbiting Kepler 20 that are the size of Earth. Unfortunately, both of them and the three other Neptune-size planets all orbit their Sun-size star within a distance equal to Mercury's orbit around the Sun. Therefore, Kepler-20e, the smallest of them with a size slightly smaller than Venus, has a surface temperature of around 1400º F and Kepler-20f, a planet very slightly larger than Earth has a surface temperature of about 800º F. At the beginning of December the Kepler team announced the discovery of a planet, Kepler 22b, 2.4 times the size of Earth (a "super-Earth") orbiting within its star's habitable zone. So these confirmed planet discoveries show that Kepler has the capability of finding an Earth-size planet orbiting within a star's habitable zone. Kepler finds planets by the "transit method"—looking for periodic drops in a star's light as a planet crosses in front of the star and blocks some of its light. There are over 2300 other planet candidates that have not been confirmed yet. Of these, 207 are about the size of Earth and there are 48 planet candidates orbiting within their star's habitable zone. Candidate planets are those that have not been verified yet through follow-up observations to make sure the periodic star's dimming is not due to another star or a dead star called a white dwarf passing front of the star. However, this technique assumes that the stars are calm and steady like our Sun. The Kepler team has found that a number of the stars are a bit more active, more variable, than our Sun, so they will need more observations to tease out the dimmings due to transiting planets from those due to the intrinsic variability of the stars themselves. Will NASA have the funds to continue the mission beyond the 3.5-year original mission? The Kepler website is at http://kepler.arc.nasa.gov.
In solar system news the first near-Earth asteroid discovered, Eros, will make a rare fly-by of the Earth over the next few weeks. Its last close approach was in January 1975 and this month's fly-by will be the last close pass until January 2056. Its closest distance will be on January 31st when it will get to within about 70 times the Moon's distance from us. You'll need a good pair of binoculars to see it. Eros made history in 1898 by enabling the best determination of the "astronomical unit" that is the average distance between the Earth and the Sun and is the distance on which all other solar system distances are based. Another close fly-by in 1931 was used to fine-tune the astronomical unit even more. You can participate in a re-enactment of the 1931 campaign with equipment as simple as a DSLR and an ordinary camera lens.
In this evening's sky at around 6 PM the two bright stars you'll see are the planets Jupiter in the south and Venus in the southwest (see the first star chart below). While Jupiter continues poking along just left of Pisces, speedier Venus is now in the middle of Aquarius. Also in Aquarius a bit to the left of Venus is the second-most massive asteroid, Vesta. The Dawn spacecraft is in its lowest altitude orbit around Vesta mapping its gravity field and gathering data on the kinds of atoms on Vesta's surface. Dawn will be in the very close orbit until near the end of February. The very thin Waxing Crescent Moon may be visible near the western horizon just after sunset on the 24th but it will probably be washed out by the twilight glow. It should be bright and high enough to see by the following evening when it will be 8.5 degrees (almost a fist-width at arm's length) to the right of Venus and then 8 degrees above Venus on the 26th. By the 29th the fat waxing crescent Moon will be just right Jupiter and by the 30th the First Quarter phase will have moved beyond Jupiter.
Looking east at 6 PM you will see that all of the stars of the Winter Hexagon will be above the horizon but after 8 PM will offer a better view. The Winter Hexagon's stars are starting from the top at 8 PM and going clockwise: Capella almost directly overhead in Auriga then downward to the right is Aldebaran at the eye of Taurus the bull then further downward to Rigel at one knee of Orion then the bottom of the hexagon is Sirius at the head of Canis Major (one of Orion's hunting dogs) then upward to the left to Procyon in the hindquarters of Canis Minor (little dog) then further up and left to Pollux or Castor at the heads of the twins of Gemini and finally back to Capella. See the second star chart below this view. By 9:45 PM you should be able to see orange-red Mars below the tail of Leo low in the east and Saturn will be visible in the east by about 12:45 AM with the slightly dimmer Spica in Virgo about half a fist-width to the right. By an hour before sunrise, the sky will have rotated so that Saturn will be just past due south and Mars and Leo will be in the southwest—see the third star chart below. At that time Pollux and Castor in Gemini will be setting in the west and almost directly overhead facing south will be the bright star, Arcturus at the foot of Bootes.
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