Beyond the Edge of the Solar System: The Inner Oort Cloud Population

A dwarf planet with the most distant orbit known found beyond the observed edge of our Solar System.

The known Solar System can be divided into three parts: the rocky
planets like Earth, which are close to the Sun; the gas giant planets,
which are further out; and the frozen objects of the Kuiper belt,
which lie just beyond Neptune's orbit. Beyond this, there appears to
be an edge to the Solar System where only one object, Sedna, was known
to exist. New work from Scott S. Sheppard (Carnegie Institution for
Science) and Chad Trujillo (Gemini Observatory) report the discovery
of a second object, dwarf planet 2012 VP113, found beyond this edge.
It is now published in Nature. 

Figure 1: The discovery images of 2012 VP113 (affectionately
nicknamed "Biden" because of the VP in the provisional name).
It has the most distant orbit known in our Solar System.
Three images of the night sky, each taken about 2 hours apart, were
combined into one.  The first image was artificially colored red,
second green and third blue.  2012 VP113 moved between each image as
seen by the red, green and blue dots.  The background stars and galaxies
did not move and thus their red, green and blue images combine to show
up as white sources. (Click on Image or HERE To See More Images. ) 

In 2003, the dwarf planet Sedna was discovered beyond the Kuiper Belt
edge and it was not known if Sedna was unique, like Pluto once was
thought to be before the Kuiper Belt was discovered.  "With the
discovery of 2012 VP113 it is now clear Sedna is not unique and is
likely the second known member of the hypothesized inner Oort cloud,
from where some of the comets may originate", says Trujillo.

2012 VP113's closest orbit point to the Sun brings it to about 80
times the distance that the Earth is from the Sun, a measurement
referred to as an astronomical unit or AU. For context, the rocky
planets and asteroids exist at distances ranging between .39 and 4.2
AU. Gas giants are found between 5 and 30 AU, and the Kuiper belt
(composed of thousands of icy objects, including Pluto) ranges from 30
to 50 AU.  In our solar system there is a distinct edge at 50 AU, of
which only Sedna was known to stay significantly beyond at 76 AU for
its entire orbit.  "The search for more of these distant inner Oort
cloud objects should continue, as they can tell us a lot about how our
solar system formed and evolved", says Sheppard.

Sheppard and Trujillo used the new Dark Energy Camera (DECam) on the
NOAO 4 meter telescope in Chile for discovery.  DECam has the largest
field-of-view of any 4 meter or larger telescope, giving it
unprecedented ability to search large areas of sky for faint objects.
The Magellan 6.5 meter telescope was used to determine the orbit of
2012 VP113 and obtain detailed information about its surface

From the amount of sky area searched, Trujillo and Sheppard determine
that about 900 objects with orbits like Sedna and 2012 VP113 are out
there with sizes larger than 1000 km and that the total population of
the inner Oort cloud is likely bigger than the Kuiper Belt and main
asteroid belt.  "Some of these inner Oort cloud objects could rival
the size of Mars or Earth.  This is because most of the inner Oort
cloud objects are so distant that even very large ones would be too
faint to detect with current technology", says Sheppard.

Both Sedna and 2012 VP113 were found near their closest approach to
the Sun, but they both have orbits that go out to hundreds of AU at
which point they would be too faint to discover.  In fact, the
similarity in the orbits found for Sedna, 2012 VP113 and a few other
objects near the edge of the Kuiper Belt suggests that an unknown
massive perturbing body may be shepherding these objects into these
similar orbital configurations.  Sheppard and Trujillo suggest a Super
Earth or an even larger object at hundreds of AU could create the
shepherding effect seen in the orbits of these objects, which are too
distant to be perturbed significantly by any of the known planets.

a) .....b)

Figure 2: a) Orbit diagram for the outer solar system.  The Sun and
Terrestrial planets are at the center.  The orbits of the four giant
planet Jupiter, Saturn, Uranus and Neptune are shown by purple solid
circles.  The Kuiper Belt (including Pluto) is shown by the dotted
light blue region just beyond the giant planets.  Sedna's orbit is
shown in orange while 2012 VP113's orbit is shown in red.  Both
objects are currently near their closest approach to the Sun
(perihelion).  They would be too faint to detect when in the outer
parts of their orbits.  Notice that both orbits have similar
perihelion locations on the sky and both are far away from the giant
planet and Kuiper Belt regions.  b) Plot of all the known bodies in
the outer solar system with their closest approach to the Sun
(Perihelion) and eccentricity.
(Click on the Image or HERE For More Orbit Information. ) 
There are three competing theories for how the inner Oort cloud might
have formed. As more objects are found, it will be easier to narrow
down which of these theories is most likely accurate.  One theory is
that a rogue planet could have been tossed out of the giant planet
region and this planet could have perturbed objects out of the Kuiper
Belt to the inner Oort cloud on its way out.  This planet could have
been ejected or still be in the distant solar system today.  The
second theory is that a close stellar encounter could put objects into
the inner Oort cloud region while a third theory suggests inner Oort
cloud objects are captured extra-solar planets from other stars that
were near our Sun in its birth cluster.

The outer Oort cloud is distinguished from the inner Oort cloud
because in the Outer Oort cloud, starting around 1500 AU, the gravity
from other nearby stars starts to perturb the orbits of the objects,
causing objects in the outer Oort cloud to have their orbits change
drastically over time.  This creates many of the comets we see as
objects that were perturbed out of the Outer Oort Cloud.  Inner Oort
cloud objects are not highly affected by the gravity of other stars
and thus have more stable and thus primordial orbits.



This work was partially funded by NASA. This project used data obtained with DECam, which was constructed by the Dark Energy Survey collaborating institutions. Observations were partly obtained at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, operated by the Foundation of Universities for Research in Astronomy, under contract with the National Science Foundation. This paper includes data gathered with the Carnegie 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.



Scott S. Sheppard Carnegie Institution for Science 5241 Broad Branch Rd. NW Washington, DC 20015 phone 202-478-8854 email: ssheppard at (replace "at" with "@")

Chad Trujillo Gemini Observatory 670 North A`ohoku Place Hilo, Hi 96720 phone 808-974-2566 email: trujillo at (replace "at" with "@")