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Solar system’s distant ice-rocks come into focus

Sept. 14, 2010
Courtesy of the Harvard-Smithsonian
Center for Astrophysics
and World Science staff

Be­yond where Nep­tune—of­fi­cially our so­lar sys­tem’s fur­thest plan­et—cir­cles the Sun, there float count­less faint, icy rocks.

They’re called trans-Nep­tu­ni­an ob­jects, and one of the big­gest is Plu­to—once clas­si­fied as a plan­et, but now des­ig­nat­ed as a “d­warf plan­et.” This re­gion al­so sup­plies us with comets such as fa­mous Com­et Hal­ley. 

Now, as­tro­no­mers us­ing new tech­niques to cull the da­ta ar­chives of NASA’s Hub­ble Space Tel­e­scope have added 14 new trans-Nep­tu­ni­an ob­jects to the known cat­a­log. Their meth­od, they say, promises to turn up hun­dreds more.

“Trans-Neptunian ob­jects in­ter­est us be­cause they are build­ing blocks left over from the forma­t­ion of the so­lar sys­tem,” said Ce­sar Fuentes, form­erly with the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics and now at North­ern Ar­i­zo­na Uni­vers­ity. He is the lead au­thor of a pa­per on the find­ings, to ap­pear in The As­t­ro­phys­i­cal Jour­nal.

As trans-Nep­tu­ni­an ob­jects, or TNOs, slowly or­bit the sun, they move against the star­ry back­ground, ap­pearing as streaks of light in time ex­po­sure pho­tographs. The team de­vel­oped soft­ware to scan hun­dreds of Hub­ble im­ages for such streaks. Af­ter prom­is­ing can­di­dates were flagged, the im­ages were vis­u­ally ex­am­ined to con­firm or re­fute each disco­very.

Most TNOs are lo­cat­ed near the eclip­tic—a line in the sky mark­ing the plane of the so­lar sys­tem, an out­growth of the fact that the so­lar sys­tem formed from a disk of ma­te­ri­al, as­tro­no­mers say. There­fore, the re­search­ers search­ed for objects near the eclip­tic.

They found 14 bodies, in­clud­ing one “bi­na­ry,” that is, a pair whose mem­bers or­bit each oth­er. All were more than 100 mil­lion times faint­er than ob­jects vis­i­ble to the un­aided eye. By meas­ur­ing their mo­tion across the sky, as­tro­no­mers cal­cu­lat­ed an or­bit and dis­tance for each ob­ject. Com­bin­ing the dis­tance, bright­ness and an es­ti­mat­ed re­flec­ti­vity al­lowed them to cal­cu­late the ap­prox­i­mate size. The new­found TNOs range in size from an es­ti­mat­ed 25 to 60 miles (40-100 km) across.

Un­like plan­ets, which tend to orbit very near the ecliptic, some TNOs have or­bits quite tilted from that line. The team ex­am­ined the size dis­tri­bu­tion of ob­jects with both types of or­bits to gain clues about how the popula­t­ion has evolved over the past 4.5 bil­lion years.

Most smaller TNO’s are thought to be shat­tered re­mains of big­ger ones. Over bil­lions of years, these ob­jects smack to­geth­er, grind­ing each oth­er down. The team found that the size dis­tri­bu­tion of TNOs with flat ver­sus tilted orbits is about the same as ob­jects get faint­er and smaller. There­fore, both popula­t­ions have si­m­i­lar col­li­sion­al his­to­ries, the re­searchers said.

The study ex­am­ined only one-third of a square de­gree of the sky, so there’s much more ar­ea to sur­vey. Hun­dreds of ad­di­tion­al TNOs may lurk in the Hub­ble ar­chives at high­er eclip­tic lat­i­tudes, said Fuentes and his col­leagues, who plan to con­tin­ue their search. “We have prov­en our abil­ity to de­tect and char­ac­ter­ize TNOs even with da­ta in­tend­ed for com­pletely dif­fer­ent pur­pos­es,” Fuentes said.

* * *

Beyond the zone where Neptune, officially considered our solar system’s furthest planet, circles the Sun, there float countless faint, icy rocks. They’re called trans-Neptunian objects, and one of the biggest is Pluto—once classified as a planet, but now designated as a “dwarf planet.” This region also supplies us with comets such as famous Comet Halley. Now, astronomers using new techniques to cull the data archives of NASA’s Hubble Space Telescope have added 14 new trans-Neptunian objects to the known catalog. Their method, they say, promises to turn up hundreds more. “Trans-Neptunian objects interest us because they are building blocks left over from the formation of the solar system,” said Cesar Fuentes, formerly with the Harvard-Smithsonian Center for Astrophysics and now at Northern Arizona University. He is the lead author of a paper on the findings, to appear in the The Astrophysical Journal. As trans-Neptunian objects, or TNOs, slowly orbit the sun, they move against the starry background, appearing as streaks of light in time exposure photographs. The team developed software to analyze hundreds of Hubble images hunting for such streaks. After promising candidates were flagged, the images were visually examined to confirm or refute each discovery. Most TNOs are located near the ecliptic—a line in the sky marking the plane of the solar system, an outgrowth of the fact that the solar system formed from a disk of material, astronomers say. Therefore, the researchers searched near the ecliptic. They found 14 objects, including one “binary,” that is, a pair whose members orbit each other. All were more than 100 million times fainter than objects visible to the unaided eye. By measuring their motion across the sky, astronomers calculated an orbit and distance for each object. Combining the distance, brightness and an estimated reflectivity allowed them to calculate the approximate size. The newfound TNOs range an estimated 25 to 60 miles (40-100 km) across. Unlike planets, which tend to have very flat orbits, some TNOs have orbits quite tilted from the ecliptic. The team examined the size distribution of objects with both types of orbits to gain clues about how the population has evolved over the past 4.5 billion years. Most smaller TNO’s are thought to be shattered remains of bigger ones. Over billions of years, these objects smack together, grinding each other down. The team found that the size distribution of TNOs with low- versus high-inclination orbits is about the same as objects get fainter and smaller. Therefore, both populations (low and high inclination) have similar collisional histories. The study examined only one-third of a square degree of the sky, so there’s much more area to survey. Hundreds of additional TNOs may lurk in the Hubble archives at higher ecliptic latitudes, said Fuentes and his colleagues, who plan to continue their search. “We have proven our ability to detect and characterize TNOs even with data intended for completely different purposes,” Fuentes said.