Techciple
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By John Timmer | Published 2 days ago
Two of the planets in the very crowded HD 10180 system.
Astronomers have pulled off an interesting magic trick and made two planets appear. Since they appeared in a planetary system that already had seven, it gives that planetary system a total of nine, making it the most planet-rich system we're aware of (since our own has only eight). It may further its lead in the future, as well, as the authors conclude that there's not enough data yet to identify an Earth-sized planet.
How does one make a planet appear? In this case, it's the result of their method of discovery. Most planet candidates these days are coming from Kepler, the space-based telescope that watches for planets that orbit in front of their host star from Earth's perspective. This results in a small fraction of the star's light being blocked out on a periodic basis, with the size of the fraction being roughly proportional to the size and orbit of the planet.
Prior to Kepler, most exoplanets were discovered by looking for their gravitational influence on the host star. Planets don't actually orbit around an immovable host star; they both orbit around their collective center of mass. It's just that the star is so much more massive, the center of mass is typically within its radius. Nevertheless, that's enough to pull the star slightly in various directions as the planets orbit from one side of the star system to another. That motion is enough to allow the Doppler effect to shift the light of the star to slightly different wavelengths.
Detecting these Doppler shifts is easy when there's just one planet, since they should change with a period that corresponds the the planet's orbit. As the number of planets grow, however, the signals get increasingly complex. On one orbit, a set of planets could all pull together, creating a large shift. A few orbits later, those same planets may be scattered across the system, largely canceling each other out. Periodic changes within the star itself can also complicate the analysis.
The system in question here, HD 10180, has been observed by one of the most sophisticated planetary detection systems on our own planet, HARPS, the High Accuracy Radial velocity Planet Searcher. It's a very good star for this sort of observation, since it's bright, relatively nearby, and doesn't seem to experience much in the way of periodic variations. (In fact, the researchers found that the noise in the system was so low they had difficulty figuring out how best to include it in their model.)
Researchers had already looked over the HARPS data, doing a statistical analysis to look for periodic signals in the total of the signal seen in the star. When they found one, they subtracted it from the signal and tried again. This kept working until they had piled up six obvious planets, and the data contained hints of a seventh that didn't quite make the statistical cut. By that point, all that was left to sift through was noise.
A lone Finn, Mikko Tuomi, working at the University of Hertfordshire, decided that this wasn't quite the right way to do the analysis. Each signal you extract runs the risk of taking parts of other signals with it. Instead, he reasoned, the way to go about this is to force the algorithm to model the entire system as if it contained N planets, and then keep increasing N, checking how well the model fit the data at each step. Using this method, the probability got higher as N approached seven planets—then kept going up until it hit nine. At this point, there were no hints of periodic signals left in the data.
To provide some indication of whether nine planets could happily get along in a compact system, they performed some basic modeling of stable orbits. Not only were the nine planets in relatively stable locations, but there were a couple of gaps that might also fit additional bodies. They estimate that these could be up to twelve times as massive as the Earth without creating a detectable signal in the existing data. So, as we examine HD 10180 further, there's a chance that more planets will make their presence known.
If the results hold up, the inner portion of the system is very crowded. Five of the planets orbit faster than Mercury does, indicating they are quite close in to the star; another two take less than a year to orbit. The remaining two are further out. Two of the planets are less than two times the mass of Earth, and another is about five times. Most of the remainder are Neptune-sized, ranging from 12 to 25 times Earth's mass. The most distant object is massive, roughly 66 times the mass of the Earth.
A nine-planet system is pretty impressive, as is the fact that we've gone from a few hints of super-Jupiters to having thousands of exoplanet candidates in just a few short years. But things are progressing so rapidly that I wouldn't be surprised to see this mark eclipsed before too long.
Astronomy & Astrophysics, 2012. DOI: Not yet available.
Photograph by herts.ac.uk
Two of the planets in the very crowded HD 10180 system.
Astronomers have pulled off an interesting magic trick and made two planets appear. Since they appeared in a planetary system that already had seven, it gives that planetary system a total of nine, making it the most planet-rich system we're aware of (since our own has only eight). It may further its lead in the future, as well, as the authors conclude that there's not enough data yet to identify an Earth-sized planet.
How does one make a planet appear? In this case, it's the result of their method of discovery. Most planet candidates these days are coming from Kepler, the space-based telescope that watches for planets that orbit in front of their host star from Earth's perspective. This results in a small fraction of the star's light being blocked out on a periodic basis, with the size of the fraction being roughly proportional to the size and orbit of the planet.
Prior to Kepler, most exoplanets were discovered by looking for their gravitational influence on the host star. Planets don't actually orbit around an immovable host star; they both orbit around their collective center of mass. It's just that the star is so much more massive, the center of mass is typically within its radius. Nevertheless, that's enough to pull the star slightly in various directions as the planets orbit from one side of the star system to another. That motion is enough to allow the Doppler effect to shift the light of the star to slightly different wavelengths.
Detecting these Doppler shifts is easy when there's just one planet, since they should change with a period that corresponds the the planet's orbit. As the number of planets grow, however, the signals get increasingly complex. On one orbit, a set of planets could all pull together, creating a large shift. A few orbits later, those same planets may be scattered across the system, largely canceling each other out. Periodic changes within the star itself can also complicate the analysis.
The system in question here, HD 10180, has been observed by one of the most sophisticated planetary detection systems on our own planet, HARPS, the High Accuracy Radial velocity Planet Searcher. It's a very good star for this sort of observation, since it's bright, relatively nearby, and doesn't seem to experience much in the way of periodic variations. (In fact, the researchers found that the noise in the system was so low they had difficulty figuring out how best to include it in their model.)
Researchers had already looked over the HARPS data, doing a statistical analysis to look for periodic signals in the total of the signal seen in the star. When they found one, they subtracted it from the signal and tried again. This kept working until they had piled up six obvious planets, and the data contained hints of a seventh that didn't quite make the statistical cut. By that point, all that was left to sift through was noise.
A lone Finn, Mikko Tuomi, working at the University of Hertfordshire, decided that this wasn't quite the right way to do the analysis. Each signal you extract runs the risk of taking parts of other signals with it. Instead, he reasoned, the way to go about this is to force the algorithm to model the entire system as if it contained N planets, and then keep increasing N, checking how well the model fit the data at each step. Using this method, the probability got higher as N approached seven planets—then kept going up until it hit nine. At this point, there were no hints of periodic signals left in the data.
To provide some indication of whether nine planets could happily get along in a compact system, they performed some basic modeling of stable orbits. Not only were the nine planets in relatively stable locations, but there were a couple of gaps that might also fit additional bodies. They estimate that these could be up to twelve times as massive as the Earth without creating a detectable signal in the existing data. So, as we examine HD 10180 further, there's a chance that more planets will make their presence known.
If the results hold up, the inner portion of the system is very crowded. Five of the planets orbit faster than Mercury does, indicating they are quite close in to the star; another two take less than a year to orbit. The remaining two are further out. Two of the planets are less than two times the mass of Earth, and another is about five times. Most of the remainder are Neptune-sized, ranging from 12 to 25 times Earth's mass. The most distant object is massive, roughly 66 times the mass of the Earth.
A nine-planet system is pretty impressive, as is the fact that we've gone from a few hints of super-Jupiters to having thousands of exoplanet candidates in just a few short years. But things are progressing so rapidly that I wouldn't be surprised to see this mark eclipsed before too long.
Astronomy & Astrophysics, 2012. DOI: Not yet available.
Photograph by herts.ac.uk