Kepler mission and the HZ planet search

[Martin]

Kepler spacecraft was successfully launched on 6 March.

 

It's job is to watch a particular patch of 100,000 stars for several years to detect transiting HZ (habitable zone) planets which are roughly about earth size.

 

If anybody knows more about the Kepler mission please contribute comment.

 

http://kepler.nasa.gov/

This website has an FAQ.

 

My understanding is that HZ planets are ones whose semimajor axis (average distance to star) is 0.95 - 1.37 AU adjusted for luminosity.

 

That might be wrong but it is something like that. The idea is that for a star as luminous as the sun they look for planets whose distance is around the same as the earth's, namely from 95% to 137% of earth's distance.

 

And for a hotter star they adjust the distance out so that the energy balance and equilibrium temperature is the same.

And for a cooler star they adjust the distance in.

 

That band is called the 'habitable zone' and roughly corresponds to the planet having liquid water on part of its surface.

 

But also they want the mass of the planet to be like earth, or a few times earth mass, so that it will hold it's atmosphere. A reasonably thick dense atmosphere is important. Helps the water stay liquid. Shields, protects etc. etc.

 

So Kepler spacecraft will be watching for transits of a not too small and not too large planet disk in front of the star, and it will watch for this to occur on a time period basis that says that the distance to the star is right. In other words, HZ and the right size. If you are curious and want to understand the inference better, try asking.

 

Kepler will orbit the sun at the trailing Lagrange point, if I remember correctly. It will not orbit the earth.

 

It can watch 100,000 stars at once because it has a pretty nice CCD (charge coupled device).

Edited March 20, 2009 by Martin

[Airbrush]

According to wikipedia.org:

 

"The random probability of a planetary orbit being along the line-of-sight to a star is the diameter of the star divided by the diameter of the orbit. For an Earth-like planet at 1 AU transiting a solar-like star the probability is 0.465%, or about 1 in 215."

 

Does that mean they will need to look at 215 stars before there is a high probability that they are looking at a proper angle to see an Earth-like planet pass in front of the star?

 

"In addition, the 1 in 215 probability means that if 100% of stars observed had the exact same diameter as the Sun, and each had one Earth-like terrestrial planet in an orbit identical to that of the Earth, Kepler would find about 465 of them."

 

http://en.wikipedia.org/wiki/Kepler_mission

[Martin]

Airbrush, I haven't reviewed the numbers since I saw them a while ago, but I think you have it right.

 

To sum up, this is a "cost effective" way to look for earthlike planets and also get statistical estimates of how common they are.

 

It is expensive in time and resources to measure the wobble of a star because you have to do very precise doppler shift measurements on various spectral (color) lines in the starlight from different chemical elements and you need to commit precise instrument time over several years.

 

The first exoplanets were (at least mostly) found by wobble, measured at groundbased observatories.

 

So this Kepler mission approach is going to look at a patch of 100,000 stars all at once. All it needs to keep track of the light intensity of one star is one pixel. A cheap commitment of resource.

It can report back inside of 2 or 3 years if there was a periodic dimming. A quick return on investment.

 

The trade-off is that only 1/200 of planet systems would be oriented right so that the planet transits the disk of the star and briefly cuts down the light.

 

So you do a large sample like 100,000 and you figure that you will only get 1/200 of the planets that are really there. But that's OK. It is a simple practical method.

 

I think that's the idea (and also I'm just repeating what I think you already figured out.) We don't actually know how it will work!

 

I think the idea makes sense, so I am looking forward to see what success they have.

 

BTW do you know how they estimate the mass of a star starting from things like the color of the light (first using a curve called Hertzsprung-Russell, or "the main sequence"). Then using a mass-luminosity relation calibrated by observing binaries etc. It's rather neat.

 

You might look it up. Once you know the mass of the star, and the orbit period (days or years) of the planet then one of Kepler Laws tells you the distance from the star to the planet. And then knowing brightness of the star and the distance you can infer the temperature etc.

 

so the keystone to inferring what the planet is like is to estimate the mass of the star (and determine the length of the "year", which is what this spacecraft will do).

 

The method is fundamentally so elegant that it is hard not to suspect that humans were evolved by nature specifically to find habitable planets. What other purpose could evolution possibly have had in mind?

Edited March 26, 2009 by Martin

[Airbrush]

Great to hear that stuff Martin.

 

So in at least 3 years they should know something about such planets. But how long will it take for them to know if this method will even work? If they find this method will work, would that appear in the news?

 

After detecting "semi-earths" in HZ, when will they know more about the actual planets? Will that take more advanced technology?

 

"The method is fundamentally so elegant that it is hard not to suspect that humans were evolved by nature specifically to find habitable planets. What other purpose could evolution possibly have had in mind?"

 

My sentiments exactly!

Edited March 26, 2009 by Airbrush

[Martin]

Kepler is now on station and is taking pictures.

http://www.nasa.gov/centers/ames/news/releases/2009/09-43AR.html

 

It will constantly watch a patch of sky about 10 degrees by 10 degrees wide-----100 square degrees.

 

In this patch there are about 100,000 stars which have been identified as good candidates for finding "habitable zone" planets.

 

The occasion on which a new telescope takes its first pictures of the stars is called "first light". Here are the "first light" pictures by Kepler:

http://www.nasa.gov/mission_pages/kepler/multimedia/20090416.html

Edited April 20, 2009 by Martin

[Airbrush]

Thanks for the Kepler update Martin. For us city slickers who don't know how big the Big Dipper is in the sky. Can anyone describe how large 10 degrees by 10 degrees is in the sky using full moon diameters?

 

When I examined the hi res photo of the field of stars I was surprised that there was such a high density of stars in a patch of sky. Next we want to see a wall-sized poster of that field of stars.

[Martin]

Can anyone describe how large 10 degrees by 10 degrees is in the sky using full moon diameters?

..

 

I believe the full moon is about half a degree wide.

Edited April 23, 2009 by Martin

[Arch2008]

Here’s a sort of minute by minute account of what Kepler has found so far:

http://planetquest.jpl.nasa.gov/missions/keplerMission.cfm

I’m sure that we are going to get some numbers up fairly soon.

It's kind of a 'watched pot' that is really going to boil.

[Airbrush]

I believe the full moon is about half a degree wide.

 

Then the Kepler field of vision is about 20 X 20 moons.

 

I wonder what depth of field it covers? Of the 100,000 candidate stars, what is the distant to the closest? And the furthest?

 

Thanks for the link Arch. That article gives a good idea of the Kepler field of vision, roughly the size of your open hand at arm's length, and Hubble sees an area about the size of a grain of sand at arm's length.