Solar System

Solar system’s distant ice-rocks come into focus
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.


World Science