NASA’s early afternoon press conference lived up to expectations, announcing two new planetary discoveries in the Neptune-mass range. The planet detected around the star 55 Cancri is one of four now known to orbit there, making 55 Cancri the most complete extra-solar planetary system we’ve found.
Nobody knows whether the new 55 Cancri planet is rocky or gaseous, but its mass — roughly 18 times that of the Earth — and its location close to its parent star have many speculating that it formed through the accumulation of smaller rocky bodies. Barbara McArthur of the University of Texas, Austin and colleagues found the planet by studying observations made at the Hobby-Eberly Telescope at McDonald Observatory in West Texas.
55 Cancri continues to loom large among extrasolar planetary systems. To me, the most interesting of its planets is the fourth, a gas giant that orbits roughly at the same distance that Jupiter does from the Sun. With almost 140 extrasolar planets now found, the variety of planetary systems seems bewildering; finding a system that roughly parallels our own would be in its own way reassuring, and would raise hopes of finding Earth-like worlds in the habitable zones surrounding such stars. We may have to wait for the Terrestrial Planet Finder mission to actually get an image of such a planet, but both Kepler and the Space Interferometry Mission should give us a good idea of what’s likely to be there.
The second planet, depicted in the artist’s rendition above, is a stunner because it orbits an M-class red dwarf called Gliese 436. With seven out of ten planets in the galaxy known to be red dwarfs, the discovery of planets around even a few seems to imply that the population of planets in the galaxy is enormous. Proxima Centauri is itself a red dwarf, and at 260,000 AU (4.2 light years) the closest known star to our Sun. Paul Butler and Geoffrey Marcy of the Carnegie Institute of Washington and University of California, Berkeley made the Gliese 436 discovery, which is the second planet known to orbit a red dwarf. The planet orbits at roughly 4.1 million kilometers (2.6 million miles) from its parent star, and is roughly 20 times Earth in mass.
NASA has a nice animation of the 55 Cancri planetary system here, and you can read more about both discoveries at the Planet Quest Web site.
Lots of background on mu Arae’s (second) discovered planet in this press release from the European Southern Observatory. The discovery was made possible by the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrograph attached to ESO’s 3.6-meter La Silla telescope in Chile. The planet previously discovered at mu Arae is roughly Jupiter-size and orbits the star every 650 days. The new one, much closer in, has the mass of Uranus, and thus straddles the line between rocky and gas giant worlds. To quote the press release (text in bold as given by ESO):
The mass of this planet places it at the boundary between the very large earth-like (rocky) planets and giant planets. As current planetary formation models are still far from being able to account for all the amazing diversity observed amongst the extrasolar planets discovered, astronomers can only speculate on the true nature of the present object. In the current paradigm of giant planet formation, a core is formed first through the accretion of solid “planetesimals”. Once this core reaches a critical mass, gas accumulates in a “runaway” fashion and the mass of the planet increases rapidly. In the present case, this later phase is unlikely to have happened for otherwise the planet would have become much more massive. Furthermore, recent models having shown that migration shortens the formation time, it is unlikely that the present object has migrated over large distances and remained of such small mass.
This object is therefore likely to be a planet with a rocky (not an icy) core surrounded by a small (of the order of a tenth of the total mass) gaseous envelope and would therefore qualify as a “super-Earth”.
Well, ‘super-Earth’ is an unfortunate term, given the temperatures that must prevail on the surface here, but finding rocky planets is big news no matter how close they orbit their primary. The star mu Arae is located in the southern constellation Ara (the Altar) and at 5th magnitude is bright enough to see with the naked eye.
Keep your eyes on NASA next Tuesday. SpaceRef.com is carrying a NASA press release that says the agency will announce the ‘discovery of a new class of planets’ next Tuesday at 1 PM EDT. The press conference will be carried live on NASA television or you can check the Webcast here. Exactly what this is escapes me, but I suspect we’re getting into the era of small planet detections, as per the story immediately below. Geoff Marcy and Paul Butler, planet finders extraordinaire, are among those who will appear on the NASA panel.
A planet fourteen times the size of the Earth may not be all that small, but it’s tiny compared to the bulk of planets discovered around other stars, most of which are so-called ‘hot Jupiters’ — gas giants in tight orbits around their parent star. The European Southern Observatory’s La Silla telescope has now found a much smaller planet, and one that may well be rocky, a kind of ‘super Earth,’ as some researchers have said. But the new planet orbits the star mu Arae (about fifty light years from Earth) so tightly that it completes its orbit in less than ten days, making for temperatures that probably top 1000 degrees Fahrenheit.
You can read more about the mu Arae planet at Space.com, whose story quotes Alan Boss of the Carnegie Institution in Washington as saying, “It’s much closer to our solar system than anything we’ve found so far…I’m still somewhat stunned that they have such good data.” The star mu Arae is similar in size and brightness to the Sun, and the new planet seems to have roughly the mass of Uranus (it may, in fact, have been the rocky core of a gas giant that never fully formed). Making the discovery more strikingly like the Solar System is the fact that mu Arae seems to have two other planets, both apparently gas giants. Portuguese researcher Nuno Santos led the team that made the discovery.
As exciting as all this is, keep in mind that we’re soon going to be doing much better. NASA is launching two missions within the next decade to track down Earth-like planets. The Space Interferometry Mission in particular is worth your attention. It’s going to survey 200 nearby stars looking for planets with wet, rocky surfaces. The other mission, called Kepler, will use a photometer and the ‘transit method’ to find planets that pass in front of their home stars. We are mere years away from knowing how widely dispersed Earth-like worlds are, and which stars offer the most tempting targets for our first interstellar probes. As Berkeley planet-hunter Geoff Marcy told an interviewer last year: “With SIM, we’re going to survey about 200 nearby stars. The interesting problem we face is that we don’t know how many planets it’s going to find. It might turn out that every star has an Earth or a Venus or a Mercury or a Mars — a smallish planet heretofore undetectable. That’s the great part about SIM — we don’t know the answer ahead of time. We don’t know if we’ll find three planets or hundreds. We have to do the experiment to find out.”
Over 135 planets have now been discovered by studying their effects on the star they orbit, which produce a pronounced wobble. But so far, only a few have been found by using the transit method, detecting the periodic dimming of a star as a planet passes between it and the Earth. Now a planet called TrES-1 has been located by the Trans-Atlantic Exoplanet Survey (TrES), which uses small, inexpensive telescopes to find planets around bright stars. You can read about the TrES discovery through this press release from the University Corporation for Atmospheric Research.
Obviously, finding planets through the transit method means locating those stars whose orbital plane is lined up properly for the Earth-bound observer to see the crossing (and you have to be careful to eliminate eclipsing binaries). But rare as these may be, the beauty of discovering such planets is that we can now make some direct observations of planetary atmospheres, and we’ll get a good reading on the mass and size of any planets so observed. The team discovering TrES-1 included scientists from the Astrophysical Institute of the Canaries, the National Center for Atmospheric Research, the Harvard-Smithsonian Center for Astrophysics, Lowell Observatory, and the California Institute of Technology. The team was led by NCAR’s Timothy Brown, who built the optical system for the project at Tenerife in the Canaries. The actual discovery was the work of graduate student Roi Alonso Sobrino. Nice work!