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<title>Wise Observatory - microlensing limits on terrestrial planets
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<h2>First limits on terrestrial-mass extrasolar planets
from microlensing <br>
<font size=-1>1/Sep/2004</font>
<p>
<img src=../icons/triangle_red.gif>
<a href=http://www.sciencemag.org/cgi/content/full/305/5688/1264>
<i>Science article </a>
<img src=../icons/triangle_red.gif>
<a href="http://www.haaretz.co.il/hasite/pages/ShArtPE.jhtml?itemNo=471425&conrassID=2&sbContrassID=2&sbSubContrassID=0)">Ha'aretz article</a></i>
</h2>
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A precondition for life outside the Solar System
is the existence of "terrestrial" planets around other stars, i.e.,
planets that are Earth-like in terms of mass and separation from
their parent star. Over 100 extrasolar planets have been detected
over the past decade, almost all by means of the "radial velocity"
technique, in which the small wobble of the parent star induced by 
the planet is measured. However, the radial velocity method is sensitive
only to approximately Jupiter-mass, or larger, planets, i.e., 
hundreds of times more massive than the Earth. 
Now, an international team of astronomers, using telescopes in <b>Israel</b>,
<b>New Zealand</b>, and <b>Chile</b>, have demonstrated for the first time 
the viability of a different technique, gravitational microlensing, 
for searching for terrestrial extrasolar planets. 
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<td>
<img src=microlens1.gif width=650 >
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During a microlensing event, two
distant stars happen to line up almost exactly, one behind the other, as
viewed from
Earth. According to Einstein's Theory of General Relativity, the image
of the background star is distorted by the gravitational field of the
foreground star, which acts as a "gravitational lens", into an "<b>Einstein
ring</b>". While the ring is too small to be resolved as such by existing 
telescopes, the phenomenon also involves amplification of the light from
the background "source" star. The amplification progresses with time
until it reaches a maximum at the closest projected separation of the two
stars, and then decreases symmetrically. If the "lens star" happens to
have planets around it, these will also contribute to the gravitational
lensing effect, and will be seen as a asymmetric perturbation in the
amplification light curve. The larger the amplification of the
event, the greater the sensitivity to planets around the lens star.
Under suitable conditions, Earth-mass planets
can be detected this way.  
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<p>
In an <a href=http://www.sciencemag.org/cgi/content/full/305/5688/1264>article published in Science</a> the researchers show measurements of the microlensing
event with the largest amplification reported to date. The light
from the source, a star near the center of the Milky Way, at a distance
of about 30,000 light years, was amplified by a factor of more than 500 
over its normal brightness by the lensing effect of an intervening star.
The high-amplification part of the event, during which the sensitivity to
planets is highest, was covered only by the
observations at 
<b><a href=http://wise-obs.tau.ac.il>Wise Observatory</a></b>, 
during daytime in <b>New Zealand</b> and <b>Chile</b>.
Observations at the other observatories were nonetheless essential, first of
all for discovering the onset of the event (microlensing events are
extremely rare, and can be found only by routinely monitoring the
brightnesses of millions of stars), but also for establishing the
"baseline" brightness of the source star.
<p>
<center><img src=world_obs.jpg></center>
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<hr noshade size=3>
<p> 
The observed <b>light curve</b> of the event appears extremely smooth and
symmetric, with no evidence for the presence of planets around the lens star. 
Indeed, the researchers show in their article that, if Earth-mass
planets existed around the lens star in an annulus between 2-3 Earth-Sun
distances, they would have been detected at about 50% probability.
This is the first time that such a statement can be made about any star
other than the Sun. In addition, more massive,  Jupiter-like, planets,
can be completely excluded over a very large region around the lens star.
In terms of such giant planets, this is indeed a "lone star".
But the main importance of this work has been the demonstration, in
practice, of the viability of microlensing for detecting Earth-like
planets. If such planets around other stars are common, they will be
discovered in the foreseable future by means of similar upcoming
high-amplification microlensing events.
<p>
<center><img src=ml_lc1.gif></center>
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<hr noshade size=3>
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<ul>
<li>
<a href=http://wise-obs.tau.ac.il>Wise Observatory</a>
 team members in this effort were:<br> 
<b>
<img src=../icons/triangle_red.gif>
<a href=http://wise-obs.tau.ac.il/institute.html#maoz>Prof. Dan Maoz</a>,
</b>
and graduate students 
<b>
<img src=../icons/triangle_red.gif>
<a href=http://wise-obs.tau.ac.il/astro-depart/index.html#eran>Eran Ofek</a>
</b>,
<img src=../icons/triangle_red.gif>
<b><a href=http://wise-obs.tau.ac.il/astro-depart/index.html#lipkin>
Yiftah Lipkin</a></b>
and <img src=../icons/triangle_red.gif>
<b>
<a href=http://wise-obs.tau.ac.il/astro-depart#avishay>Dr. Avishay
Gal-Yam</a></b> (now in Caltech)
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<li>Links to the other members of the collaboration:
<b>
<img src=../icons/triangle_red.gif>
<a href=http://www.physics.auckland.ac.nz/moa/index.html>MOA</a> 
<img src=../icons/triangle_red.gif>
<a href=http://www.bulge.princeton.edu/~ogle>OGLE</a> 
</b>
</ul>
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