By Nick Strobel
It looks like the Kepler spacecraft that has been looking for Earth-sized exoplanets in the habitable zones is not going to finish its extended mission. The spacecraft lost the use of a second component in its super-precise pointing system called a reaction-control wheel. Three reaction-control wheels are needed for the spacecraft's super-precise pointing at stars and it is now down to just two reaction-control wheels. The super-precise pointing is necessary in its search for tell-tale minute dips in a star's brightness as an exoplanet passes in front of the star. There are over 2700 exoplanet candidates still to be analyzed. Even if the spacecraft cannot find any new exoplanet candidates, the current list of candidates will take over two years to verify which ones are bona fide exoplanets. The spacecraft did achieve its original mission length and has found over 130 confirmed exoplanets, including several in the habitable zones and some planets as small as the Moon. Although no exoplanet as small as the Earth has been confirmed in the habitable zone of a star yet, there are still plenty of possibilities in that list of over 2700 exoplanet candidates. The extended mission was needed to gather more data on the already discovered exoplanet candidates to tighten up the statistics as well as find new exoplanet candidates.
The first step in creating the exoplanet candidate list was compiling a list of raw data transit events of which there are over 13,000 to sift through. The Kepler team has developed computer algorithms to look for possible exoplanet candidates but computers can be made only so smart. The Kepler team released their over 13,000 raw data transit events to the public for others including the many people in the general public interested in the exoplanet search. You can look for planets too using the Kepler data with Zooniverse's Planet Hunters project at planethunters.org. The interface is simple and it doesn't take long to learn how to find those dips in the brightness. This project takes advantage of the excellent pattern-finding abilities of the human brain that is much better than any computer's. The first discovery from Planet Hunters was an exoplanet in a quadruple star system---yes, FOUR stars! The exoplanet named PH1 (short for "Planet Hunters 1") is slightly larger than Neptune and has less than half a Jupiter mass. It orbits the inner two stars (the primary pair) every 138.5 days at a distance that is inside the primary star pair's habitable zone. The other two stars are in a binary system that orbits the primary pair at about 25 times the distance that Pluto is from our Sun. There are probably more exoplanets in the system waiting to be discovered. Almost 19 million observations in the Planet Hunters project have been analyzed with 34 candidates identified. That 19 million observations includes many duplicate observations of any given transit event by multiple, independent people in order to verify a possible candidate. There are over 68,000 people in the general public working with Planet Hunters and they need more help, so give it a try!
The Kepler mission also opened up a new field of astronomy in the study of stars themselves, called asteroseismology. Asteroseismology is the study of the internal structure of stars from their pulsations. We can determine the temperature, pressure, density, and composition of the different layers inside the stars from very careful measurements of minute brightness changes (microvariability) of the stars. From the ground, these pulsations can be seen for only a few brighter stars because of the distorting effects of the Earth's atmosphere. In space much more precise observations are possible so asteroseismology can be extended to smaller, very stable stars, including solar-type stars. You can learn more about asteroseismology in my Astronomy Notes site in the Sun and Stellar Structure chapter at www.astronomynotes.com/starsun/s5.htm .
Now that the Kepler mission has proved quite convincingly that exoplanets can be found using the transit technique, NASA recently approved the continued development of the Transiting Exoplanet Survey Satellite ("TESS" for short) with proposed launch in 2017. TESS has been in development since 2006 and its search field will be different than Kepler's. While Kepler went narrow and deep, TESS will go broad and shallow. TESS will look at a field of sky over 400 times wider than Kepler but focus on stars much closer to us than the ones Kepler has been studying. TESS will also be particularly sensitive to the stars that are smaller and cooler than the Sun. These are the types of stars that are most common in the Galaxy. Many of these nearby stars will be brighter than the ones Kepler looked at and any exoplanets found will therefore be easier to follow-up with the James Webb Space Telescope and ground-based telescopes.
The multiple exoplanet systems discovered by the Kepler mission are incredibly flat, meaning that all of the orbits in a system are very closely aligned with each other. The planet orbits in our solar system are not so exquisitely aligned with Mercury being the odd ball. Its orbit alignment is 7 degree off of the Earth's. Next comes Venus' mis-alignment of 3.4 degrees. The other planet orbits are within 2.5 degrees of ours. Because Mercury and Venus have such large orbit tilts, their positions on our sky can vary by quite a lot between one evening or morning star apparition and the next. Mercury has been putting on its best evening star performance for 2013. It looks like a bright star low in the west a little over a fist width at arm's length above the western horizon at 30 minutes after sunset. Mercury continues to climb up away from the Sun until it reaches its greatest "elongation" distance on June 12th. However, tonight Mercury will be at its brightest, not on the 12th because of how its orbit is aligned with ours. Below Mercury less than fist width above the western horizon will be even brighter Venus. The separation between Mercury and Venus will grow until June 6th and 7th and then Mercury will move back down and to the left of Venus over the next several days. Although Jupiter is also still above the horizon just after sunset, it is too close to the Sun on our sky now for us to spot it. A very thin Waxing Crescent Moon will pass by Mercury and Venus on the nights of June 10th and 11th.
The first chart below shows the positions of Mercury, Venus, and the Moon with respect to the horizon. For the next couple of weeks, Mercury's and Venus' horizon positions stay pretty much the same even as their positions with respect to the stars are shifting. Tonight the positions of Gemini, Auriga, Leo and the rest will be about a third of a constellation width higher up than what is shown on the first chart below.
Looking to the southeast, you will see that Saturn is already a third of the way up in sky shortly after sunset. It will be due south at 10:30 PM. By that time the upper part of Scorpius will be visible with the bright red supergiant Antares at its heart. The second chart below shows this view. The tip of the spout of the Teapot part of Sagittarius is beginning to poke above the horizon by then as well. Just off the spout is the dark rift of interstellar dust that hides the center of the Galaxy at which a supermassive black hole bides its time just waiting for a star or gas cloud to get too close and get gobbled up. We might be able to see such a feeding later this year. One team of astronomers seems to have found a dusty gas cloud heading right toward the supermassive black hole. However, another team of astronomers believes the object is a star embedded within a dusty envelope. If the object is a dusty gas cloud, it will be tidally stretched apart and the parts closest to the black hole will heat up enough to produce X-rays just before they cross the event horizon. If the object is a star, it will be compact enough to not be torn apart and we won't have any "fireworks show". We'll have to wait and see!
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.