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Kepler April 10, 2006

Posted by dr. gonzo in Space.

Artists conception of Kepler. (Courtesy NASA).Babbling on and on about Kepler. Kepler will use the transit method to detect extrasolar planets, both terrestrial and gas giants.

Here’s a quick blurb from NASA on the transit method of planet detection:

A transit occurs each time a planet crosses the line-of-sight between the planet's parent star that it is orbiting and the observer. When this happens, the planet blocks some of the light from its star, resulting in a periodic dimming. This periodic signature is used to detect the planet and to determine its size and its orbit.

The Kepler web site, which looks as if it hasn't been updated since the new budget request estimated its launch in June 2008 as opposed to October of that year, has some neat resources.

I found the expected results page of particular interest.

NASA has provided some estimates on the number of extrasolar planets we can expect to discover based on a number of assumptions which I list below. The main thing to remember is that the program is prepared for the results of the Kepler search to turn up few if any terrestrial planets. This, too, would be an important discovery because it would mean that our theories about the origins of Earth could well be fundamentally flawed.

Assumptions used to make estimates:

-One-hundred thousand main-sequence stars are monitored;
-The average white-light variability of most F-, G- and K-main-sequence stars on the time scale of a transit is similar to that of the Sun after excluding the most active 25% of the dwarf stars in the FOV;
-Most main-sequence stars, including binaries, have terrestrial planets in or near the habitable zone;
-On an average two Earth-size or larger planets exist in the region between 0.5 and 1.5 AU, based on our solar system and the accretion model of Wetherill (1996);
-The transit probability for planets in or near the HZ is 1/2% per planet;
-The transit is near-grazing in a 1 year orbit;
-Each star has one giant planet in an outer (jovian-like) orbit;
-On average, 1% of the main-sequence stars have giant planets in orbits

These assumptions, as you can see, include binaries which many star systems are not single stars, many consist of two, three or even more stars. NASA says that if binary systems are found not to have planets, which could be a possiblity, then expected number of planetary systems decreases by 46%.

Another thing to consider is the assumption, above, that each star has one giant planet in an outer (Jovian-like) orbit, may well be a bad one. The frequency of discovery of planets of the type “hot jupiter” may well mean that the Solar System is somewhat rare in its structure.

I am inclined to think that the estimates provided by the Kepler team may be on the liberal end of potential discoveries but nonetheless, even cutting these estimates in half presents the discovery of hundreds of new planets.

Here is the summary of the estimates from the Kepler page, in case you didn’t click the link. If you did, just skip them, you’re almost done.

Based on these assumptions and the capability of the Kepler Mission, we expect to to perform a census of planets with periods from days to a few years and to detect:

Terrestrial inner-orbit planets based on their transits:

About 50 planets if most have R ~ 1.0 Re
About 185 planets if most have R ~ 1.3 Re
About 640 planets if most have R ~ 2.2 Re
(Or possibly some combination of the above)
About 12% of the cases with two or more planets per system

Giant inner planets based on the modulation of their reflected light:

About 870 planets with periods less than one week

Giant planets based on their transits:

About 135 inner-orbit planets along with albedos for 100 of these planets
Densities for 35 of the inner-orbit planets, and
About 30 outer-orbit planets.

Bottom line, lots of new planets are awaiting discovery to add to the 163 known (to date) extrasolar planets. Optionally, the mission could extend for two years beyond its planned four year operational phase. This could push the number of planets discovered to exceed current estimates.

All indications are that planets are not uncommon. Many stars have them. Finding an Earth-like planet in a habitable zone orbit may be reality in just a few short years.



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