Kepler Mission Produces Results – Ramps Up Search for Extra-solar Planets

by Chris Gebhardt

After one and one half years in space, NASA’s Kepler observatory, a telescope designed to search for Earth-like, terrestrial planets in the Cygnus and Lyra constellations, has produced the first-ever simultaneous discovery of two extra-solar planets (via the transit method of detection) orbiting the same parent star.

Mission Recap:

Just over one-third of the way into its initial three and half year mission, Kepler has proven itself a pivotal component of NASA’s desire to understand extra-solar system dynamics and planet frequency in star systems in our “local” part of the Milky Way Galaxy.

Launched March 6, 2009 on a Delta II rocket from the Cape Canaveral Air Force Station, FL, Kepler was inserted into an Earth-trailing heliocentric orbit – a solar orbit that trails the Earth by an increase of one week per year. In other words, Kepler completes one revolution of the sun every 372.5 days (~53 weeks) – one week longer than that of Earth’s heliocentric orbit.

By placing Kepler in this unusual orbit, program managers ensured that Earth would neither occlude the stars in Kepler’s field of vision nor generate light interference. The solar-trailing heliocentric orbit, likewise, allowed Kepler’s scientists to eliminate the constant gravitational tugs and wobbles prevalent to satellites/telescopes in Earth orbit, in turn creating a more stable viewing platform for Kepler’s photometer.

Dedicated to the detection and study of extra-solar terrestrial planets that orbit their parent stars inside the habitable zone, the Kepler mission is the first in a proposed series of missions that will study the characteristics of Earth-like, extra-solar planets.

The habitable zone – or area where water can exist in its three forms (most importantly in a liquid state) – is the main area of interest for Kepler and its science teams, with Kepler’s main objective to determine the commonality or frequency of terrestrial planets.

To accomplish this task, Kepler’s primary mission was broken down into six distinct questions: How many terrestrial planets (large and small) are there in or near the habitable zones of stars of varying spectral types; what is the assortment of sizes and shapes of the orbits of such planets; how many planets are contained within multiple-star star systems; what are the various characteristics (i.e. orbit size, luminosity, mass) of short-period gas giants; are there additional planetary members in already discovered extra-solar planetary systems; what are the properties of stars that harbor planetary systems?

Unlike previous observations conducted by orbital and ground-base telescopes, Kepler looks at a swath of the sky roughly equivalent to the size of one’s hand if held at arm’s length.

This area of observation yields approximately 156,000 stars for Kepler’s science team to monitor for any trace of the tell-tale “dimming” effect caused when a planetary body passes in front of its parent star – a detection method known as the transit method of discovery.

However, this form of direct observation requires the elliptic planes of the extra-solar planetary systems to be inclined in a direct, or near direct, line-of-sight with Kepler. If the supposed planetary systems’ elliptic planes are inclined at too great an angle, any planets contained within the systems would not pass between the observed star and Kepler – therefore negating the firm detection of any planets around those stars via the transit method of detection.

In fact, the odds of Kepler detecting a terrestrial planet in or near the habitable zone of an observed star are 1 in 210. This means that if every star Kepler observed contained an Earth-like, terrestrial planet that orbited its parent star at roughly the same distance as Earth orbits the sun (and if that terrestrial planet was similar in size to Earth) Kepler would detect a total of 480 terrestrial planets in its three and a half year mission.

While Kepler’s mission is slated to last for three and half years, the science team has taken the necessary steps to preserve for the possible extension of Kepler’s mission for an additional three years – a plan that could prove vital in our search for Earth-like planets as Kepler’s science team needs to observe three transits of a “detected planet” before they can confirm that they have, in fact, discovered a new exoplanet.

Following initial focal calibrations, Kepler’s science team conducted one final refinement to Kepler’s focus – a move which significantly increased the scientific return of the telescope. This refinement was accomplished by moving the primary mirror 40 microns (or 1.6 thousandths of an inch) toward the focal plane array – the area where light is focused – and tilting it 0.0072 degrees.

Kepler successfully completed its commissioning phase on May 12, 2009 at 20:01 EDT, officially beginning its search for extra-solar planets.

Results to Date:

Within the first 43-days of observation, scientific data from Kepler, combined with follow up observations from ground-based telescopes, confirmed the discovery of five new exoplanets – including Kepler-7b, the least dense planet discovered to date.

Kepler has also been credited with the baffling discovery of two “super hot” orbiting companions – companions that appear to be hotter than their respective parent stars.

The discovery, first announced at the 215th American Astronomical Society meeting in Washington D.C. on January 4, 2010 revealed that the Kepler data, combined with ground-base observations, had yet to confirm just what these objects are.

In contrast to other observations of exoplanets, these two objects are significantly hotter than their parent stars. One object – called KOI (Kepler Object of Interest) 74b – is a whopping 70,000 degrees Fahrenheit! Its parent star is only 17,000 degrees Fahrenheit. At roughly the size of Jupiter, this object orbits its parent star every 23-days.

In contrast to KOI 74b’s 70,000 degree F temperature, the hottest confirmed exoplanet to date registers at a mere 3,700 degrees Fahrenheit.

While an official explanation and identification of these objects has not yet been reached, a leading theory has suggested that these objects might in fact be white dwarfs which “wondered” too close to their now-parent stars and were subsequently stripped of their mass – thereby allowing them to swell to their current sizes.

Following this formal announcement, the next significant Kepler data release occurred on June 15, 2010 when it was revealed that Kepler’s data had identified 706 stars “from its first data set [that] have exoplanet candidates with sizes from as small as that of the Earth to larger than that of Jupiter,” notes the “Characteristics of Kepler Planetary Candidates Based on the First Data Set: The Majority are Found to be Neptune-Size and Smaller” report.

The June 15 report only released information on 306 of these exoplanet system candidates. Data on the remaining 400 stars is due to be released in February 2011.

Then, on August 26, Kepler scientists announced another unprecedented discovery – the confirmation of two new exoplanets orbiting the same star as discovered via the transit method of detection.

As NASA reported, the tell-tale transit signatures of two planets were seen in the data beamed back from Kepler.

The two planets, orbiting a star dubbed Kepler-9 located approximately 2,300 light years from Earth, have been designated Kepler-9b and 9c and were discovered over a seven month observational period.

The transit method of detection, which measures the “dimming” of a star’s apparent light as an exoplanet passes between the star and Earth (Kepler in this case), is used in addition to planetary discovery to determine the size of any confirmed exoplanets by measuring the amount of “dimming” during transit.

Likewise, the approximate distance of the exoplanet from its parent star can be determined by measuring the time between successive “dimmings” as the planet orbits its star. The mass of the exoplanets can then be calculated by variations in the regularity of the “dimmings.”

Using information from Kepler and the W.M. Keck Observatory in Hawaii, scientists have estimated the masses of the two confirmed Kepler-9 exoplanets. Based on current calculations, Kepler-9B is confirmed to be the larger of the two planets; however, both exoplanets have masses slightly less than that of Saturn – the second most-massive planet in our solar system.

Kepler-9b also lies closer to the parent star than sister planet Kepler-9c. Whereas 9b orbits the parent star once every 19-days, 9c orbits every 38-days.

In this situation, because of the short orbital period and the extended duration of observation, scientists were able to analyze data from numerous transits.

According to Matthew Holman, a Kepler mission scientist, “This discovery is the first clear detection of significant changes in the intervals from one planetary transit to the next, what we call transit timing variations. This is evidence of the gravitational interaction between the two planets.”

Nonetheless, while this discovery is certainly unprecedented and a major leap forward in our evolving understanding of planetary system dynamics, a greater discovery could be on the verge of breaking in the Kepler-9 system – the confirmation of a super-Earth-sized planet about 1.5 times the radius of Earth.

Orbiting the parent start at a break-neck 1.6-day interval, Kepler scientists believe they have identified the transit signature of this smaller planet – although additional observations will be required to confirm the transit signature and verify that the signature is in fact being caused by a super-Earth-sized planet and not an astronomical phenomenon that simply mimics the appearance of a transit signature.

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