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Current Science at NOAO

Through a Stellar Lens, Darkly:
Limits on Distant Jupiters

Astronomers have known for some time that the gravitational fields of large galaxies can function like a giant "lens," bending and re-focusing the path of light from more distant objects behind the galaxy, as seen from Earth. This unusual coincidence forms a cluster of images of the distant object that appear to surround the lensing galaxy, often revealing some new, interesting properties of the distant object.

Stars within our own galaxy and its neighboring galaxies can also serve as a lens. However, since they are so much closer to Earth and have much less mass than a galaxy, the angle of light bending is much smaller. Therefore, instead of an image, all we can detect, for now at least, is a temporary increase in brightness of the more distant stellar object as a lensing star passes briefly between it and Earth.

Example light curves for lensing stars and a lens with a planet.

Example light curves for lensing stars (top) and a lens with a planet (bottom).

Very recently, astronomers have begun using these rare, one-in-a-million chance lens alignments both within the Milky Way and neighboring galaxies such as the Large Magellanic Cloud and Small Magellanic Cloud to study the distribution of a bizarre theoretical substance called dark matter - and, as a bonus, to look for a tiny telltale bump in the bright light signal curve that would indicate the existence of planets or solar systems around the lensing star!

For the past five years, NOAO's Cerro Tololo Inter-American Observatory (CTIO) has been part of a network of small ground-based telescopes called the Probing Lensing Anomalies NETwork (PLANET) that has done some critical follow-up measurements looking for evidence of Jupiter-sized planets around other stars, based on alerts from several larger sky survey programs.

The worldwide PLANET collaboration involves the 1-meter diameter YALO telescope at CTIO in Chile, similarly sized telescopes at the South African Astronomical Observatory and the University of Tasmania in Australia, and a .6-meter telescope at Australia's Perth Observatory. These telescopes work together as a coordinated network to track the brightness of lensing events intensively for several weeks after they occur, until any temporary signal from a planet would have disappeared.

"PLANET is an excellent way to get 'big science' out of NOAO's smaller telescopes in an era where national funding agencies are more and more concerned with only the largest apertures," says Dr. Robert Blum of CTIO. "We provide access and infrastructure at a superior site, and the collaboration members are free to apply creative ways of getting their science done."

While none of the 40 events investigated in detail so far by PLANET show clear evidence for being a so-called extra-solar planet, the study does put some very interesting limits on what fraction of stars might have such planets, and in what range of orbits that they might exist in.

The key measurement of distance to understand in this effort is an Astronomical Unit or AU, which equals 93 million miles, the distance from Earth to the Sun. Jupiter orbits the Sun at 51/2 times this distance, or 5.5 AU.

"The data from PLANET suggests that less than 50 percent of the stars doing the lensing seem to have Jupiter-mass planets at 5.5 AU," says Dr. Darren DePoy, a team member from Ohio State University, which built the new detector instruments used by PLANET. "We were a bit surprised by this answer."

Assuming the lensing stars are representative of stars like the Sun, this means that, more often than not, a Sun-like star does not have a Jupiter-like planet in the same region where our Solar System does. Even though some people might view this as a negative result, there are other ways to consider it.

Given that our Sun is often labeled as an average star, the idea that less than half of similar stars have a direct analogue to Jupiter is "really exciting, because this is the first time that we've made an actual observation that says our Solar System is unusual!" DePoy explains

It may also have implications for theories about the way that dust, rock and ice is divided up in the birth of a planetary system. The distribution in our Solar System, from rocky inner planets like Mercury, Venus and Earth to gas giants like Jupiter and Saturn to icy bodies like Pluto, has been replicated in computer models. But results like those from PLANET suggest that it might not be as simple as that.

"We're also able to tell that less than 20 percent of the stars we're looking at have Jupiter-like planets between 1-4 AU," DePoy adds, which could be another important constraint on future searches for extra-solar planets.

PLANET's attempts to see a bump in the light curve of starlight brightened by the lens effect of another star's gravity differs from the approach used to find nearly all the roughly 50 extra-solar planets detected by astronomers so far. Most of these have been found by measuring the tiny change in radial velocity of the parent star towards and away from Earth due to the gravitational tug of a large planet as it circles the star. (One has been detected by looking for the dimming of starlight as the planet passes in front of the star, as viewed from Earth.)

Many of these proposed extra-solar planets have been found very close to the parent star, on the order of the orbit of Mercury (.39 AU) or even closer. Scientists believe that this lessens the chance that any life might have developed on them, given the intense radiation and lack of water in such close-in orbits.

NASA's Origins Program is planning some high-tech spacecraft for the future that might produce some direct sightings or spectrographic measurements of planets around nearby stars. But PLANET is somewhat unique, for now, in its ability to directly probe the region where Earth-like planets might exist.

Other interesting results from PLANET include observations of limb-darkening at the edge of distant stars, an effect seen on the Sun whereby its edges appear less bright than the center. The lensing events allow the this effect to be investigated on distant stars even though they appear point-like in our sky, because the details of the micro-lensing light curves are sensitive to the physical scale of the source star. "Seeing this effect beyond the Sun tells us that we do know a bit about how stars work," DePoy notes.

PLANET has also produced some precise mass ratios for binary stars, an important yet tricky estimate to make using other methods.

The PLANET project will end in the fall of 2000 unless further funding is found, but it has already demonstrated a technique that surely will be used again in the future.

"It's kind of cool to use the gravity of a distant star as a probe of even more distant objects," DePoy notes. "It's really been an interesting test of a whole new kind of technique."

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