seeing planets (APOD 14 Jun 2005)

[lewishb]

why cant our telescopes use the near earth objects to obstruct the light from a star so we can see its planets Thanks, Lewis

[Nereid]

why cant our telescopes use the near earth objects to obstruct the light from a star so we can see its planets Thanks, Lewis

In principle, they can.

In practice, it won't work.

Why? Either because the NEOs move too quickly or the exoplanets are too faint (or both).

In more detail: the time you would have to image (or detect) an exoplanet is no more than the time the NEO is occulting (blocking) the star's disc AND not blocking the planet. Taking just the first part: assume (for now) that the star's disc is a point; then the time available is the time it takes the NEO to travel the chord the star makes ... which depends on the NEO's apparent size (in arc seconds, say) and its speed (in arcsecs/sec, say).

AFAIK, most NEOs are quite small, and, being close, move quite quickly across the sky .... so the max time available is no more than a minute, and may be as little as a second.

An exoplanet will have an apparent magnitude of what, 15? 25? And the NEO will have an apparent magnitude of what, 10?

So we would be trying to detect an object at least 5 mags fainter than the NEO, in a window of less than a minute (and maybe only a second) ...

[BMAONE23]

The other problem is the ammount of time necessary to gather sufficient light from the Exoplanet source so that it can be imaged. It could require hours of light gathering from the source location. It might be more probable to "pinpoint occult" the star light from the field of view to image any orbiting exoplanets.

[Martin]

I don't understand why this thread was moved. Observing planets in other star systems is a frequent topic in astronomy.

[BMAONE23]

My guess would be that it doesn't reference to an actual APOD with respect to the question. It is apparently a general non APOD specific question.

[Dr. Skeptic]

http://antwrp.gsfc.nasa.gov/apod/ap050614.html

Now it does.

[orin stepanek]

I understand that the Kepler mission should help us to see stellar planets.
http://www.kepler.arc.nasa.gov/
Orin

[iamlucky13]

http://antwrp.gsfc.nasa.gov/apod/ap050614.html

Now it does.

Wait a second...isn't that like telling the cop, "Well I'm not speeding now?"

I think the Kepler mission will be very interesting to watch. It could very easily find thousands of new planets...or it could find almost none (which doesn't mean they aren't there).

However, it doesn't use a direct observation technique to occlude the glare from a host star. It will watch for the planets themselves to pass in front of the stars, thereby causing a small but precise dip in the star's apparent brightness. Stars with planets in orbit in the appropriate plane should do this consistently, with exactly the same period between dips. Hubble recently found a handful of planets this way from only one week of observation.

On the other hand, the Terrestrial Planet Finder is more like what lewishb described. It will hopefully be a follow-on to the Kepler mission, but commitments to it have been continuously delayed because it's a much more ambitious project than any previous observatory. Using a coronograph to block light from the star is one of the methods of filtering the glare that has been proposed for the project. This mission could have huge scientific payoffs.

[Keldor314]

why cant our telescopes use the near earth objects to obstruct the light from a star so we can see its planets Thanks, Lewis

In principle, they can.

In practice, it won't work.

Why? Either because the NEOs move too quickly or the exoplanets are too faint (or both).

In more detail: the time you would have to image (or detect) an exoplanet is no more than the time the NEO is occulting (blocking) the star's disc AND not blocking the planet. Taking just the first part: assume (for now) that the star's disc is a point; then the time available is the time it takes the NEO to travel the chord the star makes ... which depends on the NEO's apparent size (in arc seconds, say) and its speed (in arcsecs/sec, say).

AFAIK, most NEOs are quite small, and, being close, move quite quickly across the sky .... so the max time available is no more than a minute, and may be as little as a second.

An exoplanet will have an apparent magnitude of what, 15? 25? And the NEO will have an apparent magnitude of what, 10?

So we would be trying to detect an object at least 5 mags fainter than the NEO, in a window of less than a minute (and maybe only a second) ...

Imagine a rock the size of your car whizzing past you at 15 miles per second. As it goes past, it happens to occlude the star in question. Now, the time it occludes the star is the length of time it takes the car sized rock to move from where the "front bumper" first covers the star until where the "back bumper" reveals it again. This works out to simply the width of the rock, say 15 feet. Now, if its moving at 15 miles per second, and it blocks the star over a distance of 15 feet, that works out to 1/5280 of a second, a rather short time for a deep exposure.

Now, even the largest NEOs are only a few miles across at most, so even then they will only occlude the star for less than a second.

It is an interesting approach though - you may even be able to get something out of it if you used something larger and more distant (like Pluto, for example) as your occluder.