Artist's conception of the cold distant Sedna. The sun is a tiny point of light 8 billion miles away from the red planetoid. A hypothesized tiny moon appears nearby.
Artist's conception of the Inuit goddess Sedna, who rules over the seas (from the collection of M.E Brown).
The view from Sedna with everything identified
See the nice cover article in Discover Magazine about Sedna and the rest of the outer solar system
Does Sedna have a moon?
On 15 March 2004, astronomers from Caltech, Gemini Observatory, and Yale University announced the discovery of the coldest, most distant object known to orbit the sun. The object was found at a distance 90 times greater than that from the sun to the earth -- about 3 times further than Pluto, the most distant known planet.
The discovery was made on the Samuel Oschin Telescope at the Palomar Observatory east of San Diego on 14 November 2003 by the team of Mike Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale).
Because of its frigid temperatures, the team has named the object Sedna, after the Inuit goddess of the sea from whom all sea creatures were created.
Sedna is the most distant solar system object ever discovered. It is twice as far from the sun as any other solar system object and three times farther than Pluto or Neptune. Standing on the surface of Sedna, you could block the entire sun with the head of a pin held at arm's length.
Even more interestingly, the orbit of Sedna is extreme elliptical, in contrast to all of the much closer planets, and it takes 10,500 years to circle the sun.
Here is an image of the orbit and position compared to all the
known solar system objects (click for bigger version)
The sun is in the middle of the swarm of solar system objects. You can see that Sedna is at 90 AU (1 AU is an Astronomical Unit, the distance between the earth and the Sun, about 150 million kilometers, or 93 million miles).
Don't miss the fabulous video, put together by Robert Hurt at the Spitzer Science Center, showing a zoom out from the earth to Sedna to the Oort cloud (Robert is also responsible for the artist's rendition of Sedna at the top).
In our discovery images, we see only a point of light. We can't directly measure the size of Sedna from this point. The light that we see has travelled from the sun, been reflected off the surface of Sedna, and come back to us where we can see it in the images like the discovery images below. So a small icy object and a large coal-covered object, for example, would both look about the same brightness in the discovery images, because both objects could reflect about the same amount of sunlight.
We can measure Sedna's size using a thermal telescope, which
measures the heat coming from the surface. We know how far away Sedna
is, so we know that the surface temperature is about 400 degrees below
zero Farenheit. A large object of that temperature will give off much
more heat than a small object of that temperature (just light a match
and a bonfire are the same temperature, but a bonfire keeps you
much warmer at night because it is so much bigger).
In collaboration with Frank Bertoldi at the MPIfR Bonn, we used the 30
meter diameter IRAM telscope, and in
John Stansberry at the University of Arizona and Bill Reach at the
Science Certer, we used the Spitzer
Sedna was too small to be detected in either. This tells us that
Sedna is at most about 1800 km in diameter: about halfway in size
Pluto and the largest known Kuiper belt object Quaoar.
Even though all we know for certain is that Sedna is smaller than 1800
we have evidence which suggests that the size might be pretty close to
number. We are virtually certain that the size is larger than the 1250
of Quaoar, though this object has shown many unexpected
we can't completely rule out a smaller size.
Is Sedna a planet?
NO, at least not by our definition. Astronomers have
to agree on a precise definition of "planet", but we have a suggestion
for a definition below which is both historically and scientifically
motivated. By our definition, Sedna is not a planet. Nor is Pluto. But
the other 8 are.
Astoundingly, no precise scientific definition of the word "planet" currently exists. It is rare for scientists to have to define a word that is already in common usage and that everybody from school children on up already understand. How does one then go about constructing a scientific definition of such a word after the fact?
In such cases, we believe that it is important to be both true to the historical and popular perception of the meaning of the word while being scientifically descriptive, accurate, and meaningful. We will use these points -- historically valid and scientifically meaningful -- as the criteria on which to judge potential definitions of the word "planet." We have identified 4 major ideas for the definition of the word "planet" (though the most common have never been written down to our knowledge):
Unfortunately, this definition completely fails the historical sanity check. Historically, where does the criterion to be round come from, except for the near coincidence between the historical definition of planet and the transition size from round to not round? At no time in previous history has any discussion of whether or not an object is round been part of the discussion of whether or not it should be called a planet. Ceres was initially considered to be a planet, but not because it is round (which was unknown at the time), but because it was the only object known to exist between Mars and Jupiter. When other asteroids of similar sizes were found at nearly the same location it was decided to call them all members of the asteroid belt, rather than planets.
Roundness is an important physical property, and gravity is the dominant force in the solar system, so perhaps it is important to have a special word which describes the class of objects in the solar system which are round. But simply because all historical planets are round does not at all mean that it is good science to define all round objects to be planets. A much better idea is to use a different word to descibe these objects. Spheroids? Gravispheres? Actually, we prefer the word "planetoid" as a new word to descibe round objects orbiting the sun. All planets are planetoids. Not all planetoids are planets.
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune all count as solitary individuals by this definition. Pluto and Quaoar do not. Pluto is clearly a member of the Kuiper belt population, as can be seen from the fact that there are objects in the same vicinity slightly smaller than Pluto (Quaoar, 2004 DW, Varuna), and then even a larger number slightly smaller than that, and then on down.
What about Sedna? Sedna is currently the only object known in its orbital vicinity, but we strongly suspect that there will be many others found out there with time. We thus feel it is more reasonable to classify Sedna as a member of a large population (the inner Oort cloud of objects) rather than a solitary object. This classification saves us from having to go back and reclassify Sedna in a decade when we find more objects!
Since there is a clear scientific distinction between solitary individuals and members of large populations it is instructive to come up with words to describe these objects. The large populations can each be described by the particular population (asteroid belt, Kuiper belt, inner Oort cloud, Oort cloud). What about the solitary individuals? Isn't the best word to describe them "planet"?
Let's examine this definition in more details. First, it is certainly scientifically motivated and well-founded. But so was the "gravisphere" definition above. Is there any historical basis for saying that a planet is a solitary individual that is not a member of a large population? Yes! As mentioned earlier, historically Ceres and the first few asteroids were initially classified as planets. Only when it became known that there were many many asteroids in similar orbits was it decided that they should no longer be classified as planets. Historically, there is a clear distinction between planets and populations. Any definition which fails to make this distrinction is in strong trouble on historical grounds. This simple look at history shows that Pluto is completely analogous to Ceres. Pluto was initially thought to be a solitary individual. Over time we found more objects in the vicinity and realized instead that it is a member of a large population. Historically, then, Pluto, too, should no longer be considered a planet.
We are thus left with a final concept of the word planet. Every object in the solar system quite naturally can be classified as either a solitary individual or a member of a large population. The individuals are planets. The populations are not. This definition fits the historical desire to distinguish between asteroids and planets, and this definition fits all of the requirements of scientific motivation.
Even this definition is not perfect. People will always be able to imagine (and perhaps even find) pathological scenarios in which the above classification scheme fails. In contrast, the first three definitions are much more rigorous and will never need refining. We don't find this aspect of the first definitions an advantage, however. As we learn more about our solar system our language -- both popular and scientific -- should change to fit our knowledge. We think that our proposed classification scheme will suffice for everything that is found in our solar system, but we would like nothing better than to find some object which defies everything that we currently think we know and forces us to completely rethink fundamental questions like "what is a planet."
We know the orbit fairly well. After finding Sedna in November 2003, we were able to trace it back in archival data to 2001. With this nearly 3 year arc, we know that the perihelion (closest approach distance) is most likely to be within about 7 AU of our 76 AU perihelion estimate. With a perihelion of 76 AU, Sedna has a 60% farther closest approach than any other solar system object. We expect that the orbit will be improved in coming weeks as people search though archival data.
NO. Sedna never enters the region of the Kuiper belt. The Kuiper belt is an icy asteroid belt just beyond Neptune. Extremely strong evidence shows that it has a rather sharp edge at 50 AU. Sedna never comes close than 76 AU. Calling Sedna an inner Oort cloud object makes much more sense.
There are some KBOs that go very far from the sun like Sedna does, but they all have closest approach at about 35-45 AU. Sedna is special because it doesn't come any closer than 75 AU to the sun. We believe that this is because of the effects of passing stars, as described above.
A second speculative explanation for Sedna's orbit is that a larger body, perhaps Mars-sized or larger could exist at around 70 AU in a circular orbit and could have caused Sedna to get thrown into its strange orbit. If such a planet existed, we would likely have already found it in our survey, though there are still a few places left to hide.
We have been conducting an ongoing survey of the outer solar
system using the Palomar QUEST
camera and the Samuel Oschin
Telescope at Palomar
Southern California. This survey has been operating since the fall of
2001, with the switch to the QUEST camera happening in the summer of
2003. To date we have found around 40 bright Kuiper belt objects.
To find objects, we take three pictures of a small region of the night sky over three hours and look for something that moves. The many billions of stars and galaxies visible in the sky appear stationary, while satellites, planets, asteroids, and comets appear to move. Objects in the inner Oort cloud are extremely distant and so move extremely slowly.
These are two slightly differently processed views of the same
3 discovery images. The total area of sky shown in the bottom image is
equivalent in size to the head of a pin held at arm's length.
Incidentally, that is how big the Sun would appear from Sedna.
It is moving quite slowly and is faint, much slower and fainter than the recently discovered 2004 DW, which we also found.
Vast areas of the sky have to be searched before something this unusual is found. Our search for new objects will continue for the next few years.
Sedna is about 20.5 magnitudes in R, considerably fainter than 2004 DW and Quaoar. It is beyond the reach of almost all amateurs astronomers (though, interestingly, the first confirmation of the existence of Sedna was made at Tenagra Observatory, an extremely high-end amateur telescope run by Michael Schwartz in southern Arizona).
In March 2004, the location of Sedna is easily found in the evening sky to the southwest just after sunset. It is almost directly below Mars, and forms a triangle with the very bright Venus. The following sky chart was accurate for mid-March 2004 and is only left in place for an historical reference.
We don't know. Because it's surface is relatively bright, from the thermal observations (see above), we might expect it to have water ice or methane ice like Charon and Pluto have. But observations from the Gemini Telescope and (in collaboration with Chris Koresko at JPL) the Keck telescope suggest that this is not true. From observations at the 1.3-m SMARTS telescope in Chile, we do know that Sedna is one of the most red objects in the solar system -- almost as red as Mars. Why? We're currently baffled.
When we first announced the discovery of Sedna, we noted that
evidence suggested that there is a moon around Sedna. Soon after, we
acquired the images below with the
Hubble Space Telescope . Much to
our suprise no moon is visible!
Why did we think we would see a moon?
The evidence for the existence of a moon is circumstantial, but nonetheless compelling. The story is a little complicated, though, and it goes like this:
We have found that Sedna systematically gets a little brighter and a little fainter every 20 days or so (more complete information can be found here ). We think this is because there are bright and dark spots on the surface of Sedna and Sedna is rotating once every 20 days or so.
Most planets and asteroids rotate much more quickly. The earth rotates in 24 hours, Jupiter and Saturn rotate in about 10 hours, many asteroids rotate in just a handful of hours. Why is Sedna so different?
The answer perhaps can be found by thinking about Pluto. Pluto, too, has an unusually slow rotation: about 6 days. For many years this slow rotation was a mystery until it was realized that Pluto has a large moon, Charon, which revolves around Pluto once every six days. We now understand that Pluto once rotated more quickly, but Charon's tug on Pluto has, over time, slowed the rotation of Pluto until now Pluto finally rotates as slowly as Charon revolves around it.
The same process could explain why Sedna rotates unusually slowly. If Sedna has a large moon which revolves around it with a 20 day or so orbital period that moon would have slowed Sedna's initially faster rotation and given the unusually slow rotation seen today.
(As an interesting aside, this also happens on the earth! The moon is gradually slowing the earth's rotation over time. Over a typical person's lifetime, the earth day gets longer by about one one-thousandth of a second.)
Why is no moon visible?
We can think of 4 possibilities for why we do not see a moon around Sedna.
Understanding which of the 2 above possibilities is correct will be possible from additional observations of Sedna. The two types of observations that we would most like to see are:
Technology is the reason. Clyde Tombaugh discovered Pluto in 1930 using photographic plates, which let you look at a very wide piece of the sky, but they are not nearly as sensitive as the CCD's that we use now. (A CCD is what you will find inside most digital cameras.) The new, large objects listed above tend to be just faint enough that they would be out of range of all the older surveys for moving objects done after Tombaugh's. Today, CCD's are getting large enough and computers are getting fast enough that it is significantly easier to find these types of planetoids than it was even 5 years ago. We use a 172 Megapixel camera mounted on a robotic telescope to find these things. Even about 5 years ago, such cameras were not available, and the computing power to analyze these cameras was not quite there either.
It is very likely that there are more inner Oort cloud objects like Sedna. We have looked at only 15% of the sky before finding Sedna. As we continue to look at the sky, we may find a few more objects like Sedna. But this is only the beginning. Kepler's law states that an object on a very elliptical orbit like Sedna spends most of its time farthest from the Sun. Thus, for every Sedna we find near closest approach, there should be many more very far from the Sun that we can't see because they are so far away and faint. Also, Sedna is rather large, about 1/2 to 3/4 the size of Pluto. Most solar system populations like the Kuiper belt objects and the asteroids actually have many more smaller objects than large objects. So, for every Sedna we find that is large, there should be many more that are small that we missed because they were faint. Although it is very difficult to make predictions from one object, it seems very likely that the inner Oort cloud will have thousands of times more objects than just Sedna. It is likely that there is more mass in the inner Oort cloud than in the Kuiper belt and the asteroid belt combined.
2003 VB12 was the official temporary designation of the International Astronomical Union (IAU) Minor Planet Center, based on the year (2003) and date (14 Nov = the 22nd 2-week period of the year thus V=the 22nd letter of the alphabet. after that it is sequential based on the discovery announcement) of discovery. Once the orbit of 2003 VB12 is known well enough (probably 1 year), we will reccomend to the IAU Committee on Small Body Nomenclature -- which is responsible for solar system names -- that it be permanently called Sedna (this has now happened, see above) . Our newly discovered object is the coldest most distant place known in the solar system, so we feel it is appropriate to name it in honor of Sedna, the Inuit goddess of the sea, who is thought to live at the bottom of the frigid arctic ocean. We will furthermore suggest to the IAU that newly discoverd objects in this inner Oort cloud all be named after entities in arctic mythologies.
You can find out more about the legend of Sedna from many websites and books, including the ones listed here.
Our search for outer solar system objects is supported by funding from the NASA Planetary Astronomy program.