'The ultimate goal of astronomers who look for planets orbiting other stars is to find a planet like Earthâ€”the same mass, orbiting at the same distance from a sun-like star.
Weâ€™re not there yet. But new observations are getting us closer and closer to that goal. Today, a team of scientists has reported the discovery of the lowest mass planet yet (view a news video of the discovery, or read about it here). The planet, called OGLE-2005-BGL-390Lb, is about 5 times the mass of the Earth, orbiting a star about 25,000 light years away. Thatâ€™s halfway to the center of the Galaxy! So we wonâ€™t be visiting it any time soon. :-)
The planet is not exactly Earth-like; itâ€™s still pretty hefty at five time Earthâ€™s mass. Also, the parent star is only about 1/5 the Sunâ€™s mass, making it red, dim, and cool. The planet orbits the star at three times the distance Earth orbits the Sun. So its distance, together with the cool temperature of its parent sun, means the planet is very cold, far too cold to sustain life as we know it.
The planet is way too far away to be seen directly. In fact, the way the astronomers found this planet is a little strange. They used a technique called "gravitational microlensing". Part of Einsteinâ€™s great contribution to science was the idea that gravity can bend space. Space is more than just a thing in which objects sit; Einsteinâ€™s math shows that space is itself a thing has properties, like shape. Gravity distorts the shape of space, similar to the way a heavy weight sitting on a bed warps the shape of the surface of the bed. If you take a ball and roll it on the bed, its path wonâ€™t be straight, but it will curve as it passes the depression caused by the weight.
Massive objects have gravity, and that gravity bends space. As light passes through that space, it can bend. If thatâ€™s not weird enough for you (and really, it should be), gravity can also act like a lens, amplifying light as well.
If a massive object is sitting between you and a distant light source, then the gravity of the object can make the light appear brighter. This is the method used to find the planet.
Astronomers look toward regions of the sky where there are lots of distant stars -- like toward the center of our Milky Way Galaxy. In general, the stars give off a steady stream of light. But if a massive object happens to pass between us and one of those stars, weâ€™ll see a jump in brightness. The amount of the brightening can tell astronomers how massive the object is.
|Astronomers observe regions with lots of stars to look for microlensing events. Here is the view toward the center of our Milky Way galaxy... 10 million stars can be seen in the original image (click for higher-resolution and more info).|
Astronomers have dedicated projects looking for planets using this method. Two were involved in this case: OGLE, for Optical Gravitational Lensing Experiment, and PLANET, for Probing Lensing Anomalies Network, RoboNET, and MOA, for Microlensing Observations in Astrophysics (I love those first two names; I have a weakness for good puns).
In this case, the astronomers saw two peaks of brightness from the star. The first was big, and the second smaller. The first was from the parent star of the planet, and the second from the planet itself. From the way the distant star got brighter (how long it took, and how much brighter it got), the team was able to determine the mass of both the planet and its parent star.
This technique has been around a while, and astronomers have used it to look for planets, stars, black holes, even exotic dark matter out between the galaxies. Itâ€™s very different than the usual technique of planet hunters, which is to look for changes in a starâ€™s spectrum as a planet orbits it. That technique works better for big, massive planets orbiting their star very close in. This lensing technique can work for any size planet, including a terrestrial one-- a planet the size of the Earth. Itâ€™s very possible â€“ even likely â€“ that the first truly Earth-like planet will be found this way.
As a kid I always thought that the first Earth-like planet we find would be nearby, like around Alpha Centauri or some other well-known star. But in fact, it may be some distant world, orbiting an unnamed star tens of thousands of light years away.
Maybe as a kid I would have been disappointed knowing that. Iâ€™m not now! Just knowing thereâ€™s a planet like that out there is incredibly exciting. And if we know of one far away, then that makes the search for nearby ones all that much sweeter.'