Bakersfield College

July 6, 2013

By Nick Strobel

Last week a couple of more habitable exoplanets orbiting a nearby star system were announced. The two new exoplanets (well, new to us anyway) are in a triple star system called "Gliese 667" about 22 light years away from us in the direction of the Scorpius constellation. A total of six, and possibly seven, exoplanets orbit one of the stars in the triple star system, Gliese 667C, and three of them could reside in the star's habitable zone, the region around a star in which an exoplanet's surface would be not too hot nor too cold for liquid water to exist on the surface. Now, notice that I say "could reside" because some allowance must be made for the composition and thickness of the planet's atmosphere that determine how much solar energy the planet traps via the greenhouse effect and how well it distributes that heat energy around from the day side to the night side of the planet. Another one or two of the Gliese 667C exoplanets might also be habitable if they had more unusual atmospheres, bringing the total to four or five in the star's habitable zone. However, there is greater certainty of the three exoplanets in the star's habitable zone––we don't need to invoke really bizarre atmospheres to make them habitable, so let's stick with the number of three habitable zone exoplanets.

To clinch the case, we need detailed spectra of the exoplanet's atmospheres. A spectrum is what you see when you spread out of the light from an object into a rainbow of its individual colors to make a display of the brightness vs. color or wavelength. The spectrum of an object will usually have some distinct features where certain colors/wavelengths of the light have been absorbed so it looks like dark lines. The pattern of lines and how dark they are depends on the composition, temperature, pressure, etc. of the gas. A heck of a lot of information is encoded in that bar code! Well, it is easy to take the spectrum of a star but extremely difficult to do so for an exoplanet because they are so darn faint compared to their host star. It's like looking at a firefly right next to the light from a lighthouse as seen from several miles away (actually, the situation is worse than this but you get the idea). In fact, until recently, most astronomers thought it would be impossible get the spectrum of the exoplanet. Fortunately, a few astronomers were stubborn and very creative and they figured how to take the spectrum of at least some of the exoplanets. With their success we now know that it IS possible to get the spectrum of those super-faint exoplanets, though it is still darn hard to do.

The reason why the Gliese 667C is noteworthy is that the habitable zone is packed with exoplanets and the exoplanets are super-Earths, not big planets like Neptune or Jupiter. In our solar system there is just one habitable planet, Earth. Mars might have been habitable if it was bigger to retain a thicker atmosphere and have plate tectonics working on it for long-term habitability. Another system announced by the Kepler team a few months ago called Kepler 62 has two super-Earth's in its habitable zone but Kepler 62 is much farther away: 1200 light years away, over 50 times farther away than Gliese 667C. In addition, Gliese 667C is a low-mass star, smaller than our Sun and those cooler, lower mass stars are more common in the Galaxy than stars like the Sun. If there are potentially billions of habitable exoplanets orbiting stars like the Sun in our Galaxy, then there are potentially (yes, you guessed it) "billions and billions" of habitable exoplanets orbiting the cooler, lower mass stars in our Galaxy.

Earlier this week the Earth was at its farthest distance from the Sun. This distance is called the "aphelion" and we arrived at the point yesterday, July 5th, at around 8 AM in our timezone. That farthest distance of 94,506,000 miles is no help to us here in Kern County in cooling things off. The high altitude of the mid-day Sun and the long time it is above the horizon are what make our summer days quite warm despite the greater distance. Things could be worse, though. We could be at our closest distance ("perihelion") during our summer season and with most of the land area of the Earth in the northern hemisphere, that would make our seasons slightly more extreme. The southern hemisphere has proportionally more ocean area than land area, so, although it is summer in January in the southern hemisphere when we're at perihelion, all of that water moderates the temperature change.

The Moon will be at new phase on the night of July 8th, so tomorrow morning you may be able to see an extremely thin Waning Crescent Moon rising just before sunrise. This morning it was near the planet Mars on our sky, close enough that you would have been able to see both in the same field of view of your binoculars. By July 10th, you may be able see an extremely thin Waxing Crescent Moon low in the west in the evening sky just after sunset. It will be below Venus, though too far away for both to fit in your binoculars. The next night, a fatter crescent will be up higher below the bright star at the end of the Sickle of Leo, Regulus. On July 15th, the First Quarter Moon will almost cover up the bright star of Virgo, Spica. The chart below shows the Moon covering Spica because of the scale of the chart, but we will see the Moon pass slightly below Spica. Those in parts of Central American and South America will see the Moon cover up Spica. The following night (July 16th), the Moon will be close enough to Saturn so that they may just barely fit together in your binoculars. Saturn has now stopped its retrograde drift toward Spica and is now inching its way eastward. Saturn, Spica, and the bright star Arcturus, nearly straight overhead, will continue to make a prominent triangle in our evening sky all throughout July. As you watch the Moon move from night to night over the next couple of weeks, also keep track of Venus getting closer and closer to Regulus. They will be very close together on July 22nd, two days after the free monthly public star party with the Kern Astronomical Society at Russo's. [Corrected date reference to KAS public star party: the star party is on July 20th, so the close pairing of Venus and Regulus on July 22nd is two days AFTER the star party. My apologies!]

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.

Kern Community College District