Gravitational Lensing vs. Relativity ?

[ihwip]

Let's say that a single star 10 ly away was about to transit a star several hundreds of ly away. Obviously we could benefit greatly from this event due to gravitational lensing. Our scientists detect that the star will transit the other star and lense it in 20 years.

Then comes SETI. They want to send a signal during the lensing in hopes that it will reach intelligent life. Pretend for this scenario that we were able to keep a signal from degrading despite traveling those distances.

The question is, how do we determine when and where to send the signal? Do we have to send it 10 years early so that it makes it to the lensing point which is then focused toward the star? Do we send it immediately in hopes that it gets there on time because the light has already taken 10 years to reach Earth etc? Do we send the signal to where the star appears now, where it will be 10 years from now or where it will be 20 years from now?

I hope this question is intelligible.

 

[MeteorWayne]

You would have to send it to where the star would be 10 years from now. The problem is that you probably wouldn't know that the target exists, unless a previous lensing event had made it visible to you, which is VERY unlikely

 

[thnkrx]

Gravitational lensing is one of the methods by which extra-solar planets have been detected, though (the 'HAT' program). I do recollect a couple of instances where the star with the planet occluded first one star and then another.

However, all the planets dscovered with this technique to date have been 'hot jupiters'.

 

[MeteorWayne]

I could be wrong, but I believe most of the Hot Jupiters have been detected with the radial velocity technique, with a small percentage by occultation.

I'm not aware of any hot jupiters that have been detected through gravitational lensing, though there certainly could be a few.

In fact, microlensing is most effective for planets at large distances from the star, which would certainly disqualify them from the label "Hot Jupiets) which refer to massive planets very close to their star.

 

[yevaud]

Apparently there are several found via Lensing.

http://www.iop.org/EJ/article/0004-637X ... .text.html

 

[MeteorWayne]

From the list I use most often ( http://exoplanet.eu/catalog.php ), only 10 of 431 planets have been found through microlensing, and I doubt many were hot jupiters. Will have to check, and look at your link, Yev.

The closest semimajor axis listed is 0.62 AU, which I guess if it's a hot star could qualify.

Hmmm, Not likely since the star is only 0.06 solar mass.
The second closet is 0.85 AU, but the star is only 0.35 solar mass. So nowhere near as hot as the sun.
The next closest is 2.1 AU...

MW

 

[SpaceTas]

I'll have a go at answering the thought experiment posed in the original post.

micrlensing occurs when a foreground object passes close to the line of sight to a background star, as seen by the observer.
The gravity of the foreground object acts as a gravity lens magnifying the image of the background star making it appear brighter. As the alignment gets closer and closer the apparent brightness increases, then fades again as the alignment widens. Peak amplifications of 1000 have very occasionally been observed from Earth.

The observer does not need a transit, the observer doesn't even need to see the light from the foreground object.

So the light being amplified is that of the background star, and it was emitted at the look back time corresponding to the distance of the background star from the observer. So our ETI would need to be at the background star well before the alignment actually happens ie difference in look back times between the lens object and the background star. Which in thie example is "hundreds of light years - 10 light years".

For real microlensing events: most of the background stars are in the galactic bulge 8 kpc (26-28 thousand light years) and the foreground lens stars are at about half that distance (just happens that way because of distribution of stars in The Galaxy). So our ET would need to anticipate the micro-lensing event by about 12,000 to 24,000 years.

 

[SpaceTas]

There are 12 published planet detections via microlensing, including a 2 planet system. There are several more detections that are being modeled now and will appear in the next year--ish. All are cold planets. The lowest mass planet found is <=3.3 Earth mass; new data is revising its mass.

Microlensing detection is biased toward low mass host stars (just that these are the most common stars) and toward cold planets. The peak sensitivity is near the Einstein radius of the foreground lens star. For 1 solar mass star this is about 2 AU; closer in for lower mass. The sensitivity falls quickly inside the Einstein radius, but falls off more slowly beyond. So microlensing is good at finding cold planets. From the ground the low mass limit is about 2-3 Earth mass, from about 2 AU out to free-floating planets. It has already found Jupiter analogs. These are easy

Amateur astronomers with CCD equipped telescopes can help via micrFUN
http://www.astronomy.ohio-state.edu/~microfun/

 

[thnkrx]

Whoops...my bad. I apparently got the detection programs mixed up...

 

[ramparts]

Yeah, the whole point about microlensing is that it's able to find super-Earths where other contemporary methods like radial velocity haven't been able to. Should change as more Kepler data come in, though...

 

[MeteorWayne]

That's a good point ramparts. While specifically designed to detect transits, it should also provide excellend data on microlensing events.

 

[SpaceTas]

All 3 methods (transits, Doppler, micro-lensing) have found SuperEarth mass planets. The current low limit is near 2-3 Earth mass. Kepler is designed to go beyond this limit and find Earth mass planets in earth like orbits (habitable zone).

The microlensing surveys find transiting planets; limited to Hot Jupiters.
While in principle the Kepler mission (and other transit surveys) could find micro-lensing events, this is unlikely. Toward the galactic bulge the odds of finding a micro-loensing event is 1:1million. So you need to survey 1 million stars to have 1 showing microlensing. Only a small fraction of these events every show a planetary signature. Kepler "only" surveys about 100,000 stars, in a direction with a lower star density. Kepler is also a targeted search; the 100,000 stars are chosen to be the brighter and hence closer stars. These don't make good background stars for micro-lensing because there are just fewer stars between us and the star. Overall Kepler is unlikely to spot a micro-lensing event.

If Kepler does measure a microlensing event; it will be a fabulous data set.

 

[MeteorWayne]

All 3 methods (transits, Doppler, micro-lensing) have found SuperEarth mass planets. The current low limit is near 2-3 Earth mass. Kepler is designed to go beyond this limit and find Earth mass planets in earth like orbits (habitable zone).

The microlensing surveys find transiting planets; limited to Hot Jupiters.
While in principle the Kepler mission (and other transit surveys) could find micro-lensing events, this is unlikely. Toward the galactic bulge the odds of finding a micro-loensing event is 1:1million. So you need to survey 1 million stars to have 1 showing microlensing. Only a small fraction of these events every show a planetary signature. Kepler "only" surveys about 100,000 stars, in a direction with a lower star density. Kepler is also a targeted search; the 100,000 stars are chosen to be the brighter and hence closer stars. These don't make good background stars for micro-lensing because there are just fewer stars between us and the star. Overall Kepler is unlikely to spot a micro-lensing event.

If Kepler does measure a microlensing event; it will be a fabulous data set.

Sorry, not correct. Microslensing is most sensitive to planets too far from their star to be hot Jupiters.

 

[SpaceTas]

The microlensing surveys find transiting planets; limited to Hot Jupiters.

MeteorWayne: In your last post you seem to have mis-interpreted this statement in my post. As stated in earlier post(s) micro-lensing finds cold planets. To detect the micro-lensing events a survey of many millions of stars is done once to several times per night from 2 sites. From this survey stage many transiting planets are found. These transiting planets are limited to Hot jupiters eg OGLE-TR-113b (TRansiting system) as opposed to say OGLE-05-169Lb (L for lens star, b for planet) a micro-lensing found planet.

 

[MeteorWayne]

I think we are not speaking the same language with transiting planets and microlensing. To my understanding transiting planets are detected by a drop in brightness when the planet eclipses the sun, or the sun eclipses the planet. Gravitational mircrolensing detects objects not aligned so as to transit, and causes a unique shaped increase in brightness of the star caused by the planet's microlensing effect due to relativistic bending. Transiting planets may also cause microlensing, but only events that are detected only by microlensing alone are listed in that category here if I'm reading it right: http://exoplanet.eu/catalog.php



Gravitational microlensing from Wiki:
http://en.wikipedia.org/wiki/Gravitational_microlensing

Detection of extrasolar planets

Gravitational microlensing of an extrasolar planet. If the lensing object is a star with a planet orbiting it, this is an extreme example of a binary lens event. If the source crosses a caustic, the deviations from a standard event can be large even for low mass planets. These deviations allow us to infer the existence and determine the mass and separation of the planet around the lens. Deviations typically last a few hours or a few days. Because the signal is strongest when the event itself is strongest, high-magnification events are the most promising candidates for detailed study. Typically, a survey team notifies the community when they discover a high-magnification event in progress. Followup groups then intensively monitor the ongoing event, hoping to get good coverage of the deviation if it occurs. When the event is over, the light curve is compared to theoretical models to find the physical parameters of the system. The parameters that can be determined directly from this comparison are the mass ratio of the planet to the star, and the ratio of the star-planet angular separation to the Einstein angle. From these ratios, along with assumptions about the lens star, the mass of the planet and its orbital distance can be estimated.


Exoplanets discovered using microlensing, by year, through 2010-01-13.The first success of this technique was made in 2003 by both OGLE and MOA of the microlensing event OGLE 2003–BLG–235 (or MOA 2003–BLG–53). Combining their data, they found the most likely planet mass to be 1.5 times the mass of Jupiter.[31] As of February 2008, a total of six exoplanets have been detected in microlensing events, including OGLE-2005-BLG-071,[32] OGLE-2005-BLG-390,[33], OGLE-2005-BLG-169,[34], and two exoplanets in OGLE-2006-BLG-109[35]. Notably, at the time of its announcement in January 2006, the planet OGLE-2005-BLG-390Lb probably had the lowest mass of any known exoplanet orbiting a regular star, with a median at 5.5 times the mass of the Earth and roughly a factor two uncertainty. This record is now contested by Gliese 581 c with a minimal mass of 5 times the mass of the Earth in April 2007, where either of these has a fair chance to be the less massive, but OGLE-2005-BLG-390Lb still being the frontrunner. With its higher surface temperature, Gliese 581 c however appears to be the smaller planet.

This method of detecting extrasolar planets has both advantages and disadvantages compared with other techniques such as the transit method. One advantage is that the intensity of the planetary deviation does not depend on the planet mass as strongly as effects in other techniques do. This makes microlensing well suited to finding low-mass planets. One disadvantage is that followup of the lens system is very difficult after the event has ended, because it takes a long time for the lens and the source to be sufficiently separated to resolve them separately.

 

[SpaceTas]

Again MW. Your understanding is close enough, but you somehow are not getting the point that both transiting and micro-lensing planets have been found using the same equipment and data, by the same groups.

Details:
---------
In the process of finding planets via micro-lensing, planets are also found via transiting. 2 modes of discovery for the same set of equipment and data. The two types of discovery are listed separately.

The discovery of micro-lensing planets is a 2 step process.
1st step:
A very large number of stars are repeatedly imaged to search for brightness changes.
This survey data can be used to find micro-lensing events, variable stars, asteroids, or transiting planets.
For transiting planets, small regular drops in brightness can be measured with the survey data, with methods much the same as used by say WASP, CoRoT, or Kepler.

The planets found this way are listed as transiting planets the exoplanet database

2nd step:
Of the thousands of variable brightness objects found by the OGLE and MOA survey telescopes, these groups identify those that are due to microlensing. Some of these events are then monitored closely by an array of telescopes run by various groups (PLANET, micro-FUN, RoboNet, MindStep). When a deviation from the smooth lightcurve caused by s single lens object passing in line with single background source star; then intense monitoring is started, and initial modeling begun.
At this stage we often find binary stars rather than a star+planet binary. In the end only detailed modeling of a full light curve shows weather a planet has been found or not. Often follow up adaptive optics of Hubble imaging is needed to tie down the mass of the planet.

 

[MeteorWayne]

Again MW. Your understanding is close enough, but you somehow are not getting the point that both transiting and micro-lensing planets have been found using the same equipment and data, by the same groups.

I know that. This was my point:

In the process of finding planets via micro-lensing, planets are also found via transiting. 2 modes of discovery for the same set of equipment and data. The two types of discovery are listed separately.

.....

For transiting planets, small regular drops in brightness can be measured with the survey data, with methods much the same as used by say WASP, CoRoT, or Kepler.

The planets found this way are listed as transiting planets the exoplanet database

 

[Couerl]

The question is, how do we determine when and where to send the signal? Do we have to send it 10 years early so that it makes it to the lensing point which is then focused toward the star? I hope this question is intelligible.

Well MW is correct, you'd have to send the signal to where the star would be in 10 years and a good signal wouldn't degrade appreciably at that distance but, my question is why would it matter if the star 10 Ly's away were lensing another star hundreds of Ly's beyond that? It is not as though the signal were suddenly going to hop to that star hundreds of Ly's beyond the closer one, (and perhaps reach it in 10 years?) so I guess I don't follow your SETI extrapolation and beam delivery method too well. What I do think you are leaning towards with your apparent use of the word "focused" is that it might actually help the signal get to the more distant star quicker than it would if it were not lensed, which of course it would not. In fact the star in front might simply block the signal or cause it to veer away from the intended target unless it was extraordinarily well calculated and with a kind of precision that we do not currently possess. Last but not least, if the intended target of the signal is the star hundreds of Ly's away then it would be best to simply shoot the signal directly there.

 

[SpaceTas]

IF ET want's to take advantage of the "focusing" or amplification of a micro-lensing event, ET needs to put their signal in at the background (source) star and not at the lens star (with possible planet). It is the background (source) star's light that is being amplified.

 

[Imshadi]

Oh my God... so much talk and no one has tried to answer the question!

You would have to send the signal now, because it takes 10y to get there at which time the lensing will happen... the light from the lensing will then take another 10y to get to us, and that's when we'll see the lensing happening so we will see the lensing that (20y from now).

I assume as always that the scientists mean that we will see the lensing 20y from now, because it makes no point to talk about when it's "really" happening. For more reasons than one it only makes sense to talk about when WE will see it happening.

As for the direction, we have to keep in mind that if we point the laser (Yeah! laser! Why not?) at the point where we see the star now, that point is actually where the star was 10y ago. By the time our laser gets there if we fire now it will be twice that distance behind. Assuming we know how the star travels (it's orbit), we point our emitter at twice the distance in the opposite (forward) direction along the star's path from where we see it today (assuming that speed changes along the orbit are negligible... say the star circles around the galactic center and not around another star, so that in 20y the trajectory would seem almost linear and almost at constant speed).

Not clear enough? Let's compare by saying it a littler differently...

Let's say that the star travels a distance of 1 unit every 10 years.

We see it today as it was 10 years ago, at -1 units from where it's supposed to be today even though we can't see it.

We may see it at -1 units but "today" it is at position 0 (but we won't see that for another 10 years)

If we fire today at position 0 (where the star "really" is supposed to be today, not where we see it), in 10 years, by the time the laser gets there, the star will be at plus 1 units!

So, we fire at plus 1 units. That's where the star is going to be in 10 years. And that is 2 units ahead of where we see it today.

If we fire today, the laser will hit the lens in 10 years, but we won't see it happening until the light from that moment makes it back here and that takes another 10 years (for a total of 20 years from now).

I hope this really answers the question.

PS: I wish I could work at a telescope... I would be happy just bringing everybody coffee! But I am no astronomer.

 

[SpaceTas]

Hi Imshadi, have a look at my post 14th and 19th. There's an answer!

Now it seems you've turned the question around from ET signaling Earth; to US Earthlings using the microlensing event to signal ET. Maybe I misinterpreted the original question. One way or another it is a neat question, and a laser is a good signaling device, radio would work as well. In both cases you have to assume ET is looking for such a signal.

You're right; for the original example with a lens star 10 ly away we would need to fire off our signal 10 years ahead of time, so that it is focused by the lens star. An the aim point is where the direction of the lens and background star will be at the time of the lensing ie 10 years ahead.

The signal would be for any ET near the background star;

At the moment our knowledge of the positions of stars, galactic dynamics is not good enough to predict when a micro-lensing event will occur. In most cases we can't even detect the foreground lens star. It takes a minimum of about a day to spot a new event in the data.


More ....
In the reverse case of Earthling astronomers detecting an ET signal: we currently use broad band filters (far red mostly) and so wouldn't notice a laser even it was shinning within the wavelength band of the filter. A laser might just show up as a very weird difference between the brightness measured through 2 filters. Taking spectra could spot a laser. Nobody has looked at micro-lensing events at radio or any non-optical wavelengths.

What I am not sure about; is whether our signal would be boosted by the micro-lensing. Micro-lensing amplifies the light of the background star (or our signal) by creating a larger (very distorted) image of the star. If our signal was point like it may not be amplified: the magnified image of a point is still a point. I'll catch the theory guys down the hall and ask.

Hmmm .....

 

[SpaceTas]

If the angular width of the laser beam is smaller than the lens size (Einstein diameter) then there will be no enhancement via microlensing compared to not lensing. The gravity lens makes things look brighter by capturing diverging light and focus it; as in diagram above. Imaging just a single narrow beam that does not spread out, emitted from the source (background star) passing near the lens star and onto earth. Compare that with a direct beam (no dispersion).

Of course no laser will be perfect, the light will spread out as a very narrow cone. If this cone is wider than the lens (Einstein radius about 2 AU) at lens star then the lens will capture some of the spilled light and so the laser will be enhanced.
So an ET could use a microlensing event to boost a signal.

I hoped to do some calcs on how good the laser would have to be to get a maximum boost (beam width matching lens width) but takes time that isn't available.