Category: Gravitational Lensing

As the search for extrasolar planets continues, researchers are finding systems more and more like our own Solar System. And today researchers announced another significant find: a system with two planets smaller than Jupiter and Saturn. It’s almost starting to sound like home.

The report, due to be published in the February 15th edition of the journal Science discusses a series of observations made back on March 28, 2006. An experiment, known as the Optical Gravitational Microlensing Equipment (OGLE), detected the telltale signal of a microlensing event on a star 5,000 light-years away.

In case you weren’t up in the latest techniques for planetary discovery, a lensing event happens when two stars line up perfectly in the sky from our perspective on Earth. The closer star acts as a natural lens, magnifying the light from the more distant star.

The curve of light coming from the event is very specific, and astronomers know when they’re seeing a microlensing event, compared to something else like a nova or a variable star.

But there are special situations, where the light from the star brightens normally, but then has an additional distortion. The gravity from planets orbiting the closer star can actually create this additional distortion. And from this, astronomers can calculate their size (amazing!). Only 4 planets had been discovered this way so far.

Okay, enough back story.

The OGLE group announced their potential lensing event, and astronomers around the world sprung into action, gathering data for the entire time that the stars were lined up.

Researchers first calculated that there was a Saturn-sized planet orbiting the star, and then another group found that there had to be a Jupiter-sized planet as well.

“Even though we observed the micolensing effect of the Saturn for less than 0.3 percent of its orbit, the observations simply could not be explained without accounting for the orbit,�? said David Bennett, a research associate professor of astrophysics from the University of Notre Dame.

Unfortunately, viewing this planetary system was a one-time event. We’ll probably never see this star line up again, so there’s no way to perform any followup observations.

Original Source: University of Notre Dame News Release

Astronomers have several techniques to discover planets. But one of the least used so far, gravitational microlensing, might be just the right technique to find planets in the habitable zone of nearby dwarf stars.

The first way astronomers find planets is with the radial velocity technique. This is where the gravity of a heavy planet yanks its parent star around so that the wobbling motion too and fro can be measured.

The second technique is through transits. This is where a planet dims the light coming from its parent star as it passes in front. By subtracting the light from when the planet isn’t in front of the star, astronomers can even measure its atmosphere.

The third way is through gravitational microlensing. When two stars are perfectly lined up, the closer star acts as a natural lens, brightening the light from the more distant star. Here on Earth, we see a star brighten in a very characteristic way, and then dim down again. A blip in the change of brightness can be attributed to a planet.

Unlike the other two methods, microlensing allows you to reach out and see planets at tremendous distances – even clear across the galaxy. The problem with microlensing is that it’s a one-time opportunity. You’re never going to see those stars line up in just the same way again.

But Rosanne Di Stefano and Christopher Night from the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA think there’s another way microlensing could be used. In their research paper entitled, Discovery and Study ofNearby Habitable Planets with Mesolensing, the researchers propose that many stars have a high probability of becoming a lens.

Instead of watching the sky, hoping to see a lensing event, you watch specific stars and wait for them to pass in front of a more distant star.

These high-probablility lenses are known as mesolenses. By studying a large number of dwarf stars, they expect that many of them should pass in front of a more distant star as often as once a year. And if pick your targets carefully, like dwarf stars moving in front of the Magellanic Clouds, you might get even more opportunities.

Unlike other methods of planet detection, gravitational lensing relies on light from a more distant star. It is therefore important to ask what fraction of nearby dwarfs will pass in front of bright sources and so can be studied with lensing. Within 50 pc, there are approximately 2 dwarf stars, primarily M dwarfs, per square degree.

For less massive red dwarf stars, you should be able to see them at a distance of 30 light years, and for Sun-mass stars out to a distance of 3,000 light years. These stars are close enough that if a planet is detected in the habitable zone, followup techniques should be possible to confirm the discovery.

They calculated that there are approximately 200 dwarf stars passing in front of the Magellanic Clouds right now. And many of these will have lensing events with the stars in the dwarf galaxies.

Instead of monitoring specific stars, previous surveys have just watched tens of millions of stars per night – hoping for any kind of lensing event. Even though 3,500 microlensing candidates have been discovered so far, they tend to be with stars at extreme ranges. Even if there were planets there, they wouldn’t show up in the observations.

But if you pick your stars carefully, and then watch them for lensing events, the researchers believe you should see that brightening on a regular basis. You could even see the same star brighten several times, and make follow-up observations on its planets.

And there’s another advantage. Both the radial velocity and transit methods rely on the planet and star being perfectly lined up from our vantage point. But a microlensing event still works, even if the planetary system is seen face on.

By using this technique, the researchers think that astronomers should turn up lensing events on a regular basis. Some of these stars will have planets, and some of these planets will be in their star’s habitable zone.

Original Source: Arxiv