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Possible “alien moon” detected, but never to be seen again

April 10, 2014
Courtesy of NASA Jet Propulsion Laboratory
and World Science staff

As­tro­no­mers have spot­ted the first pos­si­ble signs of an “ex­o­moon,” or a moon cir­cling a plan­et out­side our so­lar sys­tem. But it is impos­si­ble to con­firm its pres­ence, and the ob­ject will nev­er be seen again, they add.

Still, the find­ing is con­sid­ered a tan­ta­liz­ing first step to­ward lo­cat­ing oth­er ex­o­moons. The dis­cov­ery was made by watch­ing a chance en­coun­ter of ob­jects in our gal­axy, said the NASA-funded re­search­ers.

“We won’t have a chance to ob­serve the ex­o­moon can­di­date again,” said Da­vid Ben­nett of the Uni­vers­ity of Notre Dame, Ind., lead au­thor of a new pa­per on the find­ings ap­pear­ing in the As­t­ro­phys­i­cal Jour­nal. “But we can ex­pect more un­ex­pected finds like this.”

The astronomers worked with programs known as the joint Japan-New Zea­land-American Mi­crolens­ing Ob­serva­t­ions in As­t­ro­phys­ics and the Prob­ing Lens­ing Anoma­lies NET­work, us­ing tele­scopes in New Zea­land and Tas­ma­nia. 

The tech­nique, called gravita­t­ional mi­crolens­ing, takes ad­van­tage of chance align­ments be­tween stars. When a fore­ground star passes be­tween us and a more dis­tant star, the clos­er star can act like a mag­ni­fy­ing glass to fo­cus and bright­en the light of the more dis­tant one. These bright­ening events usu­ally last about a month.

If the fore­ground star—which as­tro­no­mers call the “len­s”—has a plan­et cir­cling it, the plan­et will act as a sec­ond lens to bright­en or dim the light even more. By scru­ti­niz­ing these bright­ening events, as­tro­no­mers can fig­ure out the mass or weight of the fore­ground star rel­a­tive to its plan­et.

In some cases, how­ev­er, the fore­ground ob­ject could be a free-float­ing plan­et, not a star. Re­search­ers might then be able to meas­ure the mass of the plan­et rel­a­tive to its or­bit­ing com­pan­ion: a moon. While as­tro­no­mers are ac­tively look­ing for ex­o­moons—for ex­am­ple, us­ing da­ta from NASA’s Kep­ler mis­sion—so far, they haven’t found any.

In the new stu­dy, the na­ture of the fore­ground, lens­ing ob­ject is­n’t clear. The larg­er body weighs an es­ti­mat­ed 2,000 times as much as its com­pan­ion. That means the pair could be ei­ther a small, faint star cir­cled by a plan­et weigh­ing the equiv­a­lent of about 18 Earth­s—or a plan­et more mas­sive than Ju­pi­ter cou­pled with a moon weigh­ing less than Earth.

But as­tro­no­mers have no way of tell­ing which of the two sce­nar­i­os is cor­rect.

“One pos­si­bil­ity is for the lens­ing sys­tem to be a plan­et and its moon, which if true, would be a spec­tac­u­lar dis­cov­ery of a to­tally new type of sys­tem,” said Wes Traub, the chief sci­ent­ist for NASA’s Exoplan­et Ex­plora­t­ion Pro­gram of­fice at NASA’s Je­t Pro­pul­sion Lab­o­r­a­to­ry, Pas­a­de­na, Calif., who was not in­volved in the stu­dy. “The re­search­ers’ mod­els point to the moon so­lu­tion, but if you simply look at what sce­nar­i­o is more likely in na­ture, the star so­lu­tion wins.”

The an­swer lies in learn­ing the dis­tance to the cir­cling du­o. A light­er pair clos­er to Earth will pro­duce the same kind of bright­ening as a heav­ier pair far­ther away. But once a bright­ening is over, it’s very hard to take ad­di­tion­al meas­urements and de­ter­mine the dis­tance. The true ident­ity of the ex­o­moon can­di­date and its com­pan­ion, a sys­tem dubbed MOA-2011-BLG-262, is ex­pected to re­main un­known.

In the fu­ture, how­ev­er, it may be pos­si­ble to ob­tain these dis­tance meas­urements dur­ing lens­ing events, the as­tro­no­mers said. 

For ex­am­ple, NASA’s Spitzer and Kep­ler space tele­scopes, both of which re­volve around the sun in Earth-trailing or­bits, are far enough away from Earth to be great tools for a tech­nique known as par­al­lax-dis­tance. Its bas­ic prin­ci­ple can be ex­plained by hold­ing your fin­ger out, clos­ing one eye af­ter the oth­er, and watch­ing your fin­ger jump back and forth. A star, when viewed from two tele­scopes spaced far apart, will al­so seem to move. When com­bined with a lens­ing event, the par­al­lax ef­fect al­ters how a tel­e­scope will view the re­sult­ing mag­nif­ica­t­ion of star­light. Though the tech­nique works best us­ing one tel­e­scope on Earth and one in space, such as Spitzer or Kep­ler, ground-based tele­scopes on dif­fer­ent sides of our plan­et can al­so be used.


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Astronomers have spotted the first possible signs of an “exomoon,” or a moon circling a planet outside our solar system. But it is impossible to confirm its presence, and the object will never be seen again, they add. Still, the finding is considered a tantalizing first step toward locating other exomoons. The discovery was made by watching a chance encounter of objects in our galaxy, said the NASA-funded researchers. “We won’t have a chance to observe the exomoon candidate again,” said David Bennett of the University of Notre Dame, Ind., lead author of a new paper on the findings appearing in the Astrophysical Journal. “But we can expect more unexpected finds like this.” The study is led by projects known as the joint Japan-New Zealand-American Microlensing Observations in Astrophysics and the Probing Lensing Anomalies NETwork, using telescopes in New Zealand and Tasmania. The technique, called gravitational microlensing, takes advantage of chance alignments between stars. When a foreground star passes between us and a more distant star, the closer star can act like a magnifying glass to focus and brighten the light of the more distant one. These brightening events usually last about a month. If the foreground star—which astronomers call the “lens”—has a planet circling it, the planet will act as a second lens to brighten or dim the light even more. By scrutinizing these brightening events, astronomers can figure out the mass or weight of the foreground star relative to its planet. In some cases, however, the foreground object could be a free-floating planet, not a star. Researchers might then be able to measure the mass of the planet relative to its orbiting companion: a moon. While astronomers are actively looking for exomoons—for example, using data from NASA’s Kepler mission—so far, they haven’t found any. In the new study, the nature of the foreground, lensing object isn’t clear. The larger body weighs an estimated 2,000 times as much as its companion. That means the pair could be either a small, faint star circled by a planet weighing the equivalent of about 18 Earths—or a planet more massive than Jupiter coupled with a moon weighing less than Earth. But astronomers have no way of telling which of the two scenarios is correct. “One possibility is for the lensing system to be a planet and its moon, which if true, would be a spectacular discovery of a totally new type of system,” said Wes Traub, the chief scientist for NASA’s Exoplanet Exploration Program office at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., who was not involved in the study. “The researchers’ models point to the moon solution, but if you simply look at what scenario is more likely in nature, the star solution wins.” The answer lies in learning the distance to the circling duo. A lower-mass pair closer to Earth will produce the same kind of brightening event as a more massive pair located farther away. But once a brightening event is over, it’s very hard to take additional measurements of the lensing system and determine the distance. The true identity of the exomoon candidate and its companion, a system dubbed MOA-2011-BLG-262, is expected to remain unknown. In the future, however, it may be possible to obtain these distance measurements during lensing events, the astronomers said. For example, NASA’s Spitzer and Kepler space telescopes, both of which revolve around the sun in Earth-trailing orbits, are far enough away from Earth to be great tools for a technique known as parallax-distance. Its basic principle can be explained by holding your finger out, closing one eye after the other, and watching your finger jump back and forth. A star, when viewed from two telescopes spaced far apart, will also seem to move. When combined with a lensing event, the parallax effect alters how a telescope will view the resulting magnification of starlight. Though the technique works best using one telescope on Earth and one in space, such as Spitzer or Kepler, ground-based telescopes on different sides of our planet can also be used.