Planets, planets everywhere. That is the thinking among astronomers now that the first results of NASA’s Kepler space mission to find planets around other stars have been released.
Kepler is a space observatory that was launched on March 7, 2009, exactly 400 years after its namesake, Johannes Kepler, accurately described how the planets orbit the sun. Housing a specially designed telescope, the Kepler spacecraft is dedicated to monitoring more than 100,000 stars continuously in a region of the sky between the constellations Cygnus the swan and Lyra the harp.
Kepler has a telescope mirror that is about half the size of the one in the Hubble space telescope but, unlike Hubble, is not designed for taking “pretty pictures.” It is a light meter for its target stars.
With its large array of extremely sensitive electronic light detectors, it can carefully measure the brightness of stars with a technique that is ideally suited for space.
Kepler watches for the exceptionally minute dimming of a star caused when an object passes directly between the star and Earth. If the drop in the star’s light is predictable and repeatable, the object is orbiting the star. This approach for finding objects around other stars is called the transit method.
Stellar transits can tell us about the size of an object as compared to its star and, by watching for the next transit, how long it takes the object to loop around its star. Knowing this time, we can find the distance of the object from its star. Since we know what type of star is being orbited, we know its size and temperature. With this information, we can deduce the absolute size of the orbiting object, and with its distance, the temperature of the object. Ultimately, we want to know whether the object is Earth-sized and if liquid water could exist there.
It would seem extremely unlikely that an object around a distant star would be aligned so exactly with our line-of-sight as to dim the star’s light. It is. For Earth-sized planets, the likelihood of alignment amounts to one-half of one percent, for giant, Jupiter-sized and larger planets that are orbiting close to their star, the chances increase to about ten percent.
To overcome this slim possibility for us to detect an Earth-sized planet when it transits its star, Kepler continuously observes more than 100,000 stars to increase the odds of detection.
Further, the reduction in the star’s brightness is so tiny that it amounts to only one or two percent for a Jupiter-sized planet transiting its star but less than one part in ten thousand for an Earth-sized planet — that’s similar to the dimming caused by a flea walking across a car’s headlight as seen from several miles away!
Kepler has found massive planets orbiting close to their parent star. Astronomers call these “hot Jupiters” because the amount of material these planets contain is greater than our planet Jupiter — the largest and most massive planet in our solar system and, like Jupiter, they are composed mainly of gases. Now the goal is to find planets that are approximately Earth-sized, meaning in the range from one-half to twice the size of Earth.
In the ideal scenario, Kepler would find Earth-like planets, that is, ones that orbit around their star at a distance where liquid water could exist. Astronomers call this region the habitable zone, or Goldilocks zone, because it is not too hot or too cold for the presence of liquid water, one of the necessary ingredients for life.
Earlier this year, the preliminary results of Kepler’s first observing run were released by NASA. It was announced that the data contained 1,235 planet candidates. Of these, 68 are Earth-sized and the rest are larger, with 19 of those larger than Jupiter. Fifty-four of the planetary candidates are located within their star’s habitable zone and five of these candidates are Earth-sized.
For stars like our sun, planets in the habitable zone transit their star about once a year. Scientists want at least three transits to confirm their observations, so the nominal Kepler mission has been funded for 31⁄2 years with the possibility of extension.
Unlike the Kepler observatory, we Earth-bound observers can’t view Cygnus and Lyra all year, but these constellations are just two of the many star figures that make our summer night sky.
By the time evening twilight ends — about 10:15 p.m. during the first week of August — one of the brightest stars of the summer sky is almost overhead. This star is Vega, the marker star of Lyra. Vega is thought of as the handle of a small harp (lyre); an adjacent rectangle of much fainter stars is imagined to outline the rest of the harp.
The Kepler spacecraft is aimed at the star field between Vega and a star called Deneb that is to the northeast of Vega. Deneb indicates the tail of Cygnus the swan.
A line of stars from Deneb that’s pointed southwest during mid-evening makes the swan’s long neck and another line of stars outstretched east to west outlines its wings.
Saturn is low in the south-southwest during the evening and can be located to the right of Spica, the brightest star of Virgo. A rather robust lunar crescent will appear below Saturn on Aug. 3.
Bright Jupiter rises over the east-northeastern horizon just before midnight. Watch for the waning gibbous moon above Jupiter the night of August 19-20.
The Perseid meteor shower will be at its best before dawn on August 13, but that’s the night of the full moon, so this year only the brightest “shooting stars” will shine though the moonlit sky.
Richard Monda is an astronomer in the Capital Region.