June 21st: Astronomy Word of the Week : Trojan

By on June 21, 2012 in

Date: June 21st, 2012

Title: Astronomy Word of the Week : Trojan

Podcaster: Dr. Christopher Crockett

Organization: United States Naval Observatory

Link: http://christophercrockett.com

Description: Four of the planets have captured families of asteroids and comets in clouds that lead and trail their orbits around the sun.  The astronomy word of the week is “Trojan”.

Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.

Today’s Sponsor: This episode of “365 days of Astronomy” is sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.


Trojan asteroids reside in clouds that either lead or trail a planet as it orbits the sun.  They collect at points of gravitational stability where the forces from the sun, the planet, and orbital acceleration are all in balance.   These asteroid swarms are collections of the solar system’s detritus, possibly from the earliest days of its formation, and provide us a peek in to the complex motions that govern the rhythm of the planets.

Around every planet are five stable points in space – called Lagrangian points – where the gravitational influence of the sun and the planet are in balance.  Place an object – an asteroid, a telescope, a person – at any of these points and it will remain there.  Two of these points, called L4 and L5, sit sixty degrees ahead of and behind the planet on its orbit, respectively.  Like water spiraling in a drain, passing asteroids and comets can become trapped and end up indefinitely circling these otherwise empty regions.  The asteroids that get caught are called Trojans.

Of the eight major planets in our solar system, only four – Jupiter, Neptune, Mars, and even Earth – are known to host Trojan asteroids.  Of these four, Jupiter is king.  Astronomers know of over 5200 Jupiter Trojans and estimate that the giant planet may shepherd hundreds of thousands of asteroids around the sun.

Jupiter is also where the first such asteroid was found.  In 1906, German astronomer Max Wolf identified an asteroid in the same orbit as Jupiter near the planet’s leading Lagrange point.  Wolf named the asteroid Achilles, a choice that cemented a tradition of naming these asteroids after heroes of the Trojan War.  Those asteroids ahead of Jupiter are named after individuals on the Greek side of the conflict, while those trailing the gas giant come from the Trojan side.  Of course, both camps have enemy spies in their midst: two asteroids named before the Greek/Trojan naming convention became official.

How Jupiter came to have these swarms of asteroids isn’t entirely clear.  The leading hypothesis has Jupiter capturing planet-building material during the solar system’s formative years.  As Jupiter quickly grew in size, it drew all manner of planetesimals towards it.  Some became moons of the giant planet while others were thrown out of the solar system along a gravitational slingshot.  And some coalesced into the two Trojan clouds following Jupiter around the sun.

While Trojan asteroids historically refer to Jupiter’s brethren, other planets have their own small collections.  Mars has four tiny companions: one leading and three trailing.  Neptune has eight, each roughly the size of Puerto Rico.  Saturn and Uranus have none; Jupiter’s influence prevents any captured asteroids from sticking around for very long.

In 2010, the Wide Field Infrared Survey Explorer (WISE) – a space-based infrared telescope launched in 2009 – imaged the first Earth Trojan.  Temporarily named 2010 TK7, the 300 m diameter body moves in a complicated spiraling orbit around Earth’s leading Lagrange point, never coming closer to us than about 20 million km.  At that distance, it remains over fifty times farther than the moon.

2010 TK7 went undetected for so long because Earth Trojans are exceedingly difficult to find.  The locations of their orbits means they are only visible to ground-based telescopes during the day.  But to a space-based telescope, it is always dark.  Plus, with an infrared telescope, it is easier to find small objects by the heat they radiate rather than by the sunlight they reflect.  Finding tiny neighbors in space is exactly what WISE was built to do.  As mission scientists continue to pore over the telescope’s images, we may yet uncover a whole family of previously hidden Earth Trojans.

A planet orbiting a star is one case where you can collect Trojans; so is a moon orbiting a planet.  This second scenario plays out around Saturn which has a unique family of Trojan moons.  Two of its satellites, Tethys and Dione, each have tiny Trojan satellites following them on shared orbits around the ringed planet.  Tethys claims the moons Telesto and Calypso, while Dione shepherds Helene and Polydeuces around their home world.

The moons of Saturn hint at even richer possibilities.   If Saturn’s satellites can host Trojan moons, why can’t a planet host Trojan planets?  Rather than asteroid swarms around the Lagrange points, whole worlds could find themselves in the Trojan orbits of another larger planet.  We have yet to find any such arrangement in the 625 planetary systems discovered in our galaxy, but the cataloguing of other solar systems is just beginning.  The zoo of exoplanets continues to surprise us with its diversity.    We may yet find Trojan planets circling about a distant sun.

The solar system is far more rich and complex than appears at first glance.  We are not just eight planets orbiting a star, but a diverse hierarchy of planets, moons, asteroids, and comets, caught in a variety of streams, swarms, and clouds marching to the clockwork rhythm of Newton’s Laws.  The Trojan asteroids are another peek into that complexity: comets and asteroids caught in the net of gravitational stability around the Lagrange points of the solar system.  Earth has one; it may have many more.  What stories can they tell us about the history of our planet, our star, and our place in the galaxy?

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.

About Christopher Crockett

Christopher Crockett is a University of California, Los Angeles graduate student currently working as a predoctoral fellow at Lowell Observatory. His research involves searching for planets and brown dwarfs around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.

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