Revisiting exoplanets and dark matterby Jeff Foust
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| “The selection of TESS has just accelerated our chances of finding life on another planet within the next decade,” said Seager. |
Astronomers will soon have another tool to search for exoplanets. On Friday, NASA announced it had selected the Transiting Exoplanet Survey Satellite (TESS) as its next Explorer-class astronomy mission. TESS, a small spacecraft scheduled for launch in 2017, will perform an all-sky survey of the nearest and brightest stars to look for exoplanets, including those similar to the Earth.
The selection suggested that the search for exoplanets is still more important—or, at least, more feasible—than trying to characterize them. The other finalist for the Explorer mission selection was another exoplanet mission, called Fast Infrared Exoplanet Spectroscopy Survey Explorer (FINESSE), which would study the atmospheres of a number of exoplanets to determine their composition and temperature.
The TESS team believes that their mission will be able to detect thousands of exoplanets, with a particular emphasis on those relatively close to the Earth and thus preferred targets for follow-on studies. “The selection of TESS has just accelerated our chances of finding life on another planet within the next decade,” said Sara Seager, an MIT planetary scientist who is part of the TESS team, in a press release about the selection.
At first glance, TESS sounds a lot like an existing NASA mission, Kepler, which has detected over 2,700 potential exoplanets by looking for periodic decreases in the brightness of stars caused when planets pass in front, or transit, them. However, as TESS team members explained last year, when the Explorer competition was still in progress, the two missions have somewhat different goals. Kepler is looking continuously as a single part of the sky to determine what scientists call “eta Earth,” the fraction of Sun-like stars that have Earth-like planets. TESS is instead looking across the entire sky with an emphasis on the nearest stars, looking for exoplanets that could be followed up by other missions, including the James Webb Space Telescope (see “Exoplanet explorers”, The Space Review, January 9, 2012).
TESS’s selection comes as many in the exoplanet community watch, with some concern, the status of Kepler. Launched in March 2009, the spacecraft completed its primary mission late last year and is now on an extended mission, continuing observations of the same region of sky and detecting more exoplanets. An issue with the spacecraft’s reaction wheels, though, could end, or at least significantly alter, that mission.
Kepler launched with four reaction wheels, used to accurately point the spacecraft. Last July, one of the four wheels, designated wheel #2, failed. Kepler only needs three wheels for normal operations, so that failure did not disrupt the mission significantly. However, a second wheel has shown elevated levels of friction that some fear could be a sign of its impending failure.
In early January, after detecting the increased friction from wheel #4, the project put the spacecraft into a “wheel rest” mode for ten days, interrupting observations in the hopes that the wheels’ lubricants would redistribute and reduce the friction. Kepler resumed normal operations in late January, but the increased friction from wheel #4 persists. “The wheel rest period of January 17-January 27 appears to have had no beneficial impact on alleviating the elevated friction in reaction wheel #4,” Kepler mission manager Roger Hunter said in an update posted on the Kepler website on March 29.
| “It would be a different flavor of mission” if another Kepler reaction wheel fails, Sobeck said. “It would not be Kepler as we know it, but it still might provide useful science.” |
That increased friction, though, doesn’t mean a failure is imminent. “The way it presents itself is very different” from wheel #2, said deputy project manager Charlie Sobeck in an interview last week. Wheel #2 became “somewhat chaotic” in its levels of friction starting about six months before its failure, he said. By comparison, wheel #4 has jumped between a baseline level of friction and an elevated level since its launch. After emerging from its wheel rest mode, that level of friction jumped to a higher level before returning to the earlier elevated level, which he said suggested the wheel could be in a “tristable” mode, going among those three levels without become chaotic like wheel #2.
To try to alleviate the wear and tear on the reaction wheel, engineers have implemented a number of mitigation measures, Sobeck said, including operating the wheel at faster speeds and warmer temperatures to increase the effectiveness of the wheel’s lubricant. “If you imagine the lubrication like snow, we’re now skiing on top of it rather than plowing through it,” he explained. There are no plans to do another “wheel rest” period, which he said had a low probability of success to begin with.
Should wheel #4 fail, Sobeck said they are looking at options to maintain spacecraft pointing, including a “hybrid” mode that would use the remaining two wheels as well as the spacecraft’s thrusters. However, the pointing in hybrid mode would be less accurate, and may rule out the ability of Kepler to detect transits by exoplanets. “We wouldn’t be able to see Earth-sized planets around Sun-like stars,” he said, adding that they’re looking at whether it would still be able to detect larger planets.
Kepler could, in that case, be repurposed to another mission entirely, including searches for asteroids or extragalactic objects. “It would be a different flavor of mission,” Sobeck said. “It would not be Kepler as we know it, but it still might provide useful science.” If Kepler’s reaction wheels remain healthy, though, he said it could continue exoplanet observations through late 2018, limited only by its propellant levels as well as its communications link as it drifts away from Earth in its heliocentric orbit.
In February, Samuel Ting, an MIT physicist and Nobel laureate, dropped some very strong hints that an instrument on the International Space Station (ISS) he is the principal investigator on was about to reveal some significant findings. Speaking at the annual meeting of the American Association for the Advancement of Science (AAAS) in Boston, Ting said his team was wrapping up a paper on the first results from the Alpha Magnetic Spectrometer (AMS) mounted on the ISS exterior.
Ting, at both a press conference at the AAAS meeting and a presentation the following day, wouldn’t discuss the results, other than that they would show the ratio of positrons to electrons detected by AMS at energies from 0.5 to 350 billion electron volts. “Certainly we’re not going to publish something if it’s not worthwhile,” he said at the press conference (see “Turning ISS into a full-fledged space laboratory”, The Space Review, February 25, 2013).
| “These results are consistent with the positrons originating from the annihilation of dark matter particles in space, but not yet sufficiently conclusive to rule out other explanations,” CERN stated. |
Ting’s comments piqued the interest of astrophysicists, since a measurement of that ratio as a function of energy and direction could provide insights into the nature of dark matter, which comprises 26.8% of the universe, according to data collected by ESA’s Planck mission and revealed by the space agency last month. A steady rise followed by a sharp drop would be consistent with one explanation for dark matter, that it is made of weakly interacting massive particles, or WIMPs, that create positrons when they collide with each other.
Wednesday, Ting revealed the results during a presentation at CERN in Geneva, coinciding with the paper’s publication in the journal Physical Review Letters. The results show an increase in the positron ratio as energies increase, with the rate of that increase slowing towards the higher end of the data set. However, the data end before there is a conclusive dropoff that would be the signature of a dark matter source for the particles.
While the finding was hailed as potential evidence of dark matter, the results are not conclusive. Even project scientists acknowledged that the results could also be explained by positrons created by pulsars scattered through the galactic plane. “These results are consistent with the positrons originating from the annihilation of dark matter particles in space, but not yet sufficiently conclusive to rule out other explanations,” CERN noted in its press release about the results.
| “While intriguing, the AMS signal is not a ‘hint’ of dark matter — it’s an anomaly that needs to be explained,” said Mack. |
Some astrophysicists believe that the AMS results said very little, and were being overhyped by both the AMS project and the media. “[T]here is nothing to suggest that they have detected any evidence whatsoever for particle dark matter,” concluded astrophysicist Ethan Siegel, who argued that the CERN press release was “misleading” and even “deceitful” for overemphasizing dark matter.
“While intriguing, the AMS signal is not a ‘hint’ of dark matter — it’s an anomaly that needs to be explained,” wrote Katherine Mack, a theoretical astrophysicist at the University of Melbourne. Even if the AMS data detected a dropoff in the positron ratio, she noted some models of positron emission from pulsars also have similar drops.
Ting, though, said that AMS could be able to differentiate between the two as the instrument collected more data on higher energy particles. “Over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin,” he said in the CERN statement.
And NASA treated the announcement as further evidence of the quality of the science that the ISS can perform, an issue they have been emphasizing now the station is complete and as discussion begins on whether to extend the life of the station beyond 2020. “It's a remarkable testament that the orbital laboratory could play such an important supporting role in research at the very smallest scale of the physical universe,” NASA administrator Charles Bolden said in a statement. “It’s proof positive the space station is humanity’s greatest achievement in low Earth orbit.”
For astrophysicists, though, the AMS results are proof positive only that we still don’t know with any certainty what comprises a quarter of the universe.
