Space Friday: Exoplanets, Gravitation Waves, and Chiral Molecules
Eyeballing an Exoplanet
As astronomers search the sky for other solar systems, most discoveries are made either by transit photometry (the very successful method used by NASA’s Kepler spacecraft) or by spectroscopic detection of a wobble in the star’s motion caused by the gravitational pull of any circling planets (also known as the radial velocity method). These detections are all inferences, or implied by the evidence, and more than 3,200 exoplanets have been found, many benefitting cross-corroboration between methods. Fewer than two dozen have been directly observed, which sounds simple but is actually quite difficult. It requires a very large orbit that takes the planet far enough from its star so that its image isn’t overwhelmed by the star’s glare. By the same token, it helps if the planet is orbiting a star that isn’t quite so bright to begin with.
Last March, a team of astronomers announced that they had successfully imaged yet another exoplanet. Combining images taken by three giant telescopes in Chile, Spain, and Hawaii, astronomers directly imaged a planet orbiting a star in the constellation Orion the Hunter. Located about 1,200 light years away, the star, known as CVSO 30, is a T-Tauri star. Only about 2.5 million years old, this is a type of young star which has not yet begun generating energy like the Sun via thermonuclear fusion reactions, and still generates its heat and light by gravitational contraction. At this time, it emits a powerful solar wind (the “T-Tauri wind”)—sort of a “final tantrum” before the young sun finally grows up and starts fusing hydrogen into helium like a proper star. If such a star is going to have any planets, they must form before the T-Tauri wind blows away the leftover material from the star’s formation, precluding further planet formation.
In the case of CVSO 30, a planet was discovered via transit photometry in 2012. Dubbed CVSO 30b, this planet is about four times the mass of Jupiter and orbits its star at a distance of less than a million miles, taking only 11 days to do so. Now, it turns out that there’s a second planet—CVSO 30c—and in fact, we can see planet “c.” In contrast to its closer-orbiting counterpart, the newly-discovered world has nearly five times the mass of Jupiter and circles the star once every 27,000 years at an enormous distance more than 200 times the distance from the Sun to Neptune. That vast distance provides plenty of angular separation between the planet and the star so that the former could be observed, and also makes CVSO 30 an exotic system, with one of its two known planets very close to the star and the other very distant, each discovered by a different method. —Bing Quock
Astronomers Spot a “Super-Tatooine”
Astronomers estimate that up to half the stars in the Universe are part of binary systems, where two stars are close enough to each other that they orbit a common center of gravity. We see plenty of examples of this in the sky (not to be confused with optical double-stars, where two stars are simply aligned along the same line of sight). Astronomers once thought that planets couldn’t exist in stable orbits in a binary system, thinking that any planets would be caught in a gravitational tug-of-war that would eventually eject them into space or send them crashing into one of the stars. That thinking changed, starting in 1993, as astronomers began detecting worlds circling binary systems—as long as their orbits are farther than the distance between the two stars, the orbits are stable. To date nearly two dozen have been found, including Kepler-16b, which was unofficially dubbed “Tatooine” (if you’re not a “Star Wars” fan, that’s the planet that Luke Skywalker called home, where each day ended with a glorious double-sunset). Since that discovery, nearly every circumbinary planet found has been compared to the fictional Tatooine.
Now, astronomers have announced the discovery of the largest and widest-orbiting circumbinary exoplanet yet found, and it’s being called a “super-Tatooine.” Located 3,700 light years away in the constellation Cygnus the Swan, Kepler-1647b, as the planet is known, is roughly the size and mass of Jupiter, orbiting its star once every three years. This puts its orbital distance at 2.7 times Earth’s distance from the Sun, which is farther out than that of any other circumbinary planet known. Still, the planet is located within the habitable zone surrounding the Kepler 1647 system. This zone is the range of distances where liquid water might be able to exist on an Earthlike body. Since Kepler-1647b is the size of Jupiter, it’s very likely a gaseous world without a solid surface. However, like the giant planets of our own solar system, each of which has at least a dozen moons, it too might be surrounded by a family of rocky satellites, upon whose surfaces habitable conditions could conceivably develop, provided many other favorability factors fall into place. —Bing Quock
Second Gravitational Wave Detection
Four months after announcing the first detection of gravitational waves, this week the large team of more than 1,000 scientists working with the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors—located in Livingston, Louisiana, and Hanford, Washington—have announced a second observation of a black hole merger causing ripples through the fabric of space-time. And while the first gravitational wave observation was a doozy—the merger of two black holes the size of 29 and 36 solar masses—this second, more subtle event, with the masses measuring between eight and 14 times that of the Sun, is more representative of the black holes we observe in the Universe. “In a way this is nice and reassuring—it means we may be targeting the same population that we can observe with traditional astronomy,” says Lisa Barsotti, of MIT.
“When we detected the very first signal, it was so short and loud that you could see it in the data, this nice beautiful track,” says Salvatore Vitale, also of MIT. “With this new event, it’s totally different. It’s not a single bump of power that’s easy to see. It’s totally buried inside the noise.”
The first event, which actually occurred in September 2015, produced a clear peak, or “chirp,” in the data. The second signal, from December 2015, was far subtler, generating a shallower waveform—essentially a faint squeak. Using advanced data analysis techniques, the team determined that indeed, the waveform signaled a gravitational wave. In addition, since the black holes from the second observation were lighter, they moved towards each other less rapidly: the signal lasted about one second, as opposed to 0.2 second for the previous observation. (For information about how LIGO detects these signals, check out our post on the first observation, or this video from MIT.)
Black holes are the final stage in the evolution of the most massive stars. Some of these holes form a pair, orbiting around each other and gradually getting closer while losing energy in the form of gravitational waves, until a point is reached where the process suddenly accelerates, and they end up merging into a single black hole. For the recent observation, the detected signal comes from the last 27 orbits of the black holes before their merger, during which a quantity of energy roughly equivalent to the mass of the Sun was converted into gravitational waves. Based on the arrival time of the signals—with the Livingston detector measuring the waves 1.1 milliseconds before the Hanford detector—the position of the source in the sky can be roughly determined. The event took place 1.4 billion light years from Earth, which means that the gravitational waves travelled through space for 1.4 billion years.
LIGO is currently offline and undergoing upgrades. When it goes back online in the fall, it will be joined by Virgo, a third interferometer located near Pisa, Italy. “The three interferometers together will permit a far better localization in the sky of the signals,” says Fulvio Ricci, of Sapienza University in Rome.
“The first event was so beautiful that we almost couldn’t believe it,” Vitale says. “Now, the fact of having seen another gravitational wave proves that indeed we are observing a population of binary black holes in the Universe. We know we’ll see many of these frequently enough to make interesting science out of them.” –Molly Michelson
Chiral Molecules in Interstellar Space
When looking for life elsewhere in the Universe, it often comes down to chemistry. If the right molecules and matter exist, could life take hold and thrive? We’ve found some evidence of these on Mars and comets within our solar system, but what if we look farther out? A group of scientists, using radio telescopes, went searching for chiral molecules, near the center of the Milky Way in an enormous star-forming cloud of dust and gas known as Sagittarius B2 (Sgr B2).
Like a pair of human hands, certain molecules have mirror-image versions of themselves, a chemical property known as chirality. While not identical, these two different images, or “hands”—also called enantiomers—have the same values of most physical properties, such as boiling and melting temperatures, but differ in their interactions with other chiral molecules. Living things are selective about which “handedness” or enantiomer of a molecule they use or produce. For example, amino acids, proteins, enzymes and sugars, are found in nature in only one “handedness” or enantomier. This is called homochirality.
“Homochirality is one of the most interesting properties of life as we know it,” says Geoffrey Blake of Caltech. “How did it come to be that all living things use one enantiomer of a particular amino acid, for example, over another? If we could run the tape of life again, would the same enantiomers be selected through a deterministic process, or is a random choice made that depends on a tiny imbalance of one handedness over the other? If there is life elsewhere in the Universe, based on the biochemistry we know, will it use the same enantiomers?”
Looking for the answers, the team found the signature of a chiral molecule called propylene oxide (CH3CHOCH2) in Sgr B2. “It’s the first molecule detected in space that has the property of chirality, making it a pioneering leap forward in our understanding of how prebiotic molecules are made in space and the effects they may have on the origins of life," says Brandon Carroll, also of Caltech. “While the technique we used does not tell us about the abundance of each enantiomer, we expect this work to enable future observations that will let us understand a great deal more about chiral molecules, the origins of homochirality, and the origins of life in general.”
The findings are published this week in Science. –Molly Michelson and Matt Blackwell
Image(s): B. Saxton, NRAO/AUI/NSF from data provided by N.E. Kassim, Naval Research Laboratory, Sloan Digital Sky Survey; ESO/Schmidt et al.