Anthonares

In a press conference this morning, astronomers announced the discovery of a planet with a likely mass of 5.5 times that of the Earth orbiting a red dwarf star some 21,500 light years away. This is not the lowest mass exoplanet discovered thus far, but it is the first one that is both near Earth mass and near Earth orbital distance. I’m sure you’ve all heard the news, but now read on and hear the science (I’ll note that the Bad Astronomer did a science explanation too, but mine has pictures and graphs in it, so it must be better! Okay, also Centauri Dreams has done a summary as well, but again, mine has more pictures and graphs, plus an original analysis in the Context section below.). Also, I made a cool graph of all of the exoplanets we’ve found so far, that’s below too. So, here’s a second bonus published research synopsis in as many weeks:

Citation (online at CiteULike.org):
Beaulieu, J.-P. and 72 others (2006). Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing. Nature 439(7075), pp. 437-440.

Synopsis:
As part of an experiment monitoring thousands of individual stars near the galactic center (called OGLE), a temporary brightening event occurred. The target star was a giant star very near the galaxy core that over a course of approximately 40 days increased in brightness by a factor of 3 before dimming back to its original level (see the left-upper inset in the figure on the left). The cause of this sudden increase in brightness was a microlensing event (see below). The lensing star took approximately 40 days to traverse the path between the Earth and the target giant star.

When astronomers looked at the data, they noticed the characteristic shape of a lens event with one small bump on the receeding limb. The primary graph on the left shows the lens event in detail, overlying observations from six different telescopes on top of a solid black line indicating their computer models of the event. The only way to get their computer models to have the small deviation on the receeding limb (the right-hand part of the brightness curve) was to include a very small partner to the main lensing star. The dashed yellow curve visible on the top-right inset is a model of the event without a companion planet. This planet, they calculated, had a mass approximately 1/10,000 the mass of the primary lensing star with a separation that depends also on the mass of the star.

Unfortunately, the mass ratio and mass-dependent separation were all that the lensing event told the research teams. To determine the actual mass of the planet and how far it was from the star it orbits, they turned to statistical models of the inner galaxy. These statistical models describe how many stars of a given mass are expected in the inner galaxy. The model told the astronomers that there was a 95% probability that the lens star was a “Main-Sequence” or normal type star, a 4% probability that it was a white dwarf, and a 1% probability that it was a neutron star or a black hole. The most likely type is a red-dwarf, and as the figure on the right shows, the most likely mass of the planet is 5.5 Earth Masses (top left), with a likely orbital radius of 2.6 AU (1 AU is the Earth-Sun distance). Also, their statistical model indicated that the star is probably 21,500 light years from Earth.

Don’t fail to notice the large grey areas in the figure on the right. Those indicate the region that the astronomers are 95% certain contains the mass, orbital radius, lens distance, and orbital period. That region is quite large. Indeed, it’s probable that the mass of the new exoplanet, excitingly dubbed OGLE-2005-BLG-390Lb, is NOT 5.5 Earth masses, and that it is probably not 21,500 light years from Earth. But, those are the most likely values. Before we send starships to explore the system, we’d better pin down the orbital distance to better than 2-4 AU, though!

At this orbital distance around a much dimmer star than our own, OGLE-2005-BLG-390Lb is probably much much cooler than our own planet. As the artist’s rendition depicts above, the planet is probably rocky and frozen, with a thin atmosphere (image credit: European Southern Observatory). I’m not sure how the atmosphere, rocks, and ice were arrived at, but it’s probably something to do with assumptions about what is most likely.

Context:
Don’t get me wrong, this is some fantastic science despite the large uncertainties in exact properties and distances. Never before have we found a planet so close in Earth’s mass and orbital distance. The OGLE project has now detected three planets, the first two were announced last year and have greater than Jupiter masses. To keep score, today’s announcements bring us to 181 confirmed exoplanet detections with several still under review or pending verification. I’ve compiled the database of detections to generate the graph on the left (Data source for graph).

The vast majority of the exoplanets we have found so far were detected by looking at temporary decreases in brightness of a star around which planets orbit, or slight shifts in its velocity. See the article on Wikipedia linked above for a good discussion of these methods and others. These spectral methods (detected by measuring the spectrum, or at least the brightness, from a star) favor detection of massive planets, but can detect smaller planets if they are very close to the host star. These close-orbiting planets would be bakingly hot, on the order of 1,500 degrees celsius. Life would have a hard time existing in such places, though perhaps the dark side of these tidally-locked planets would be habitable.

By looking at very slight variations in the timing of signals from pulsars, we’ve detected very tiny planets, ones even smaller than Mercury. These planets, even though they’re small, are orbiting neutron stars and are very unlikely to harbor life. The supernovae that formed those neutron stars would probably have entirely destroyed any planets so close, so what’s there now probably came to be captured or formed at some later time.

The planets detected by microlensing are very much more promising, because they can be both small and at reasonable distances from their host stars. Astronomers consider microlensing to be the method by which Earth-sized planets will first be detected. Unfortunately, we cannot see anything of the atmospheres or surfaces of these planets, so we could not tell a Venus or Mars from an Earth. But, perhaps in the next decade (or maybe longer), NASA will launch the Terrestrial Planet Finder that will image the atmosphere and spectrum from a planet detected with this method.

General Explanations:
Gravitational microlensing is a specific type of a phenomenon called gravitational lensing. The most photogenic example of gravitational lens is the so-called Einstein’s Cross (image credit: David Darling). This image shows four copies of a distant quasar lensed by an intervening galaxy. The quasar is nearly exactly behind the galaxy, thus producing the almost-symmetric Cross.

Lensing is caused by light bending as it passes near massive objects. Because light is bent near massive objects, diverging light rays can be made to converge, thus focusing the light source. The geometry of a gravitational lens event is a massive object lying between us and a very distant light source. The massive lensing object must be very far away as well, as the light bending is very slight.

Gravitational microlensing is a specific type of lensing that occurs when an object moves in between the us and the distant object before moving on again. The NSF has a good video that explains microlensing. Thus by monitoring the brightness of distant objects, if one suddenly appears to flare up it may be the beginning of a microlensing event. As the lensing object moves away, the brightness returns to normal. Another NSF video explains detection methods using microlensing. We may never actually see the lensing object itself, as is the case in the research summarized above. Nevertheless, we can tell many of its properties.