Planet beyond Pluto?

Recently, there has been news that astronomers have found another planet beyond Pluto. At the moment, astronomers think the new planet is about 10 Earth masses and has a period of 20,000 years.

The orbit is highly elliptical with a perihelion somewhere between 200 to 300 AU and an aphelion between 600 and 1200 AU. We can also assume that since the planet is out beyond the orbit of Pluto, it has a density similar to that of Pluto, about 2000 kg/m³.

What do these assumptions mean? It could explain why it took so long for astronomers to find this planet, if it is really there. Using Kepler's laws, specifically, P²=a³, where P is the period of the orbit in Earth years and a is the semi-major axis in AU, we can determine the semi-major axis of the planet. With P at 20,000 years, we find that a is about 740 AU.

Using the density of Pluto and the mass of the new planet (10 Earth masses = 6x1025 kg), we can estimate that the radius of Planet 9 is about 41,500 km. For a perihelion of 200 AU, the angular diameter is only 0.01" (arcseconds) which is typical of a star. It is also possible that the planet has a low albedo, so it is very dim.

So, while it is possible that there is a planet beyond Pluto, it took this long to find it because it is so far away.

New Horizons

Today, July 14th, 2015 will go down as a major milestone in humanity exploration of the cosmos. After 85 years of pondering what Pluto actually looks like, we know and will learn more in the upcoming days and months.

New Horizons made its closest approach to Pluto at 11:50 UTC, allowing us to see it for the first time with clarity. By now, many of you have probably seen the images of Pluto with its heart-shaped surface feature, which was actually hinted at by Hubble images taken between 2002 and 2003.

If you look at the 180° face, a hint of the heart-shaped feature seen below may now be apparent.

This image of Pluto from New Horizons’ Long Range Reconnaissance Imager (LORRI) was received on July 8, and has been combined with lower-resolution color information from the Ralph instrument.

Via NASA/New Horizons

 

And from images taken on July 11, a composite of Pluto with its companion Charon.

A portrait from the final approach. Pluto and Charon display striking color and brightness contrast in this composite image from July 11, showing high-resolution black-and-white LORRI images.

Via NASA/New Horizons



For more information, follow New Horizons on Twitter and here, Alan Stern, Principle Investigator for New Horizons.

Also, visit the New Horizons page for updated images as they are posted.

 

Is Pluto a Planet?

In the past, I know I've been adamant to say that Pluto is not a planet. You can read them again here, here, and here. However, I've thought about things and may be coming around to categorizing Pluto as a planet.


Before, I had always said that there are two types of planets: terrestrial planets, like Earth, and Jovian planets, like Jupiter. But what if we redefined a planet to have a third type: dwarf planets. These are bodies in the solar system that are too large to be asteroids or comets but are not necessarily big enough to have swept all the smaller bodies from its orbit (meaning it could have co-orbital bodies), have densities somewhere between Jovian planets and terrestrial planets, and can be found anywhere in the solar system.


I did discuss dwarf planets in more detail in a previous post, when discussing Pluto (see first link above), so I may have unwittingly decided already that Pluto is a planet. In fact, if we make Pluto a planet again (which probably will happen sooner rather than later), we will also have add a few more planets to our solar system.

Trans-Neptunian Objects (objects orbiting the Sun outside of Neptune's orbit)

Image Credit:

NASA/Hubble

Eris and Makemake will definitely be added. Haumea, Sedna, and 2007 OR10 may as well. It all depends on where the lower limit of a dwarf planet's size will be if the IAU determines that.

This is what I love about science. It's a fluid subject that can easily change based on new information. I've changed my mind about Pluto (for now), and there may be other things I might change my mind about.

Update on Pluto

A while back, I posted an argument about why Pluto is not a planet. Recently, there has been a movement to reinstate Pluto as one of the planets in our solar system. Let's delve into this a bit further.


From the previous post, here are the IAU definitions of a planet and a dwarf planet.

• A planet is a spherical body that orbits the Sun and has cleared its orbit of other objects, i.e. it does not share an orbit with other bodies (not including moons).
• A dwarf planet is a spherical body that orbits the Sun but has not cleared its orbit of other objects. They may co-orbit with other bodies. Many of the Trans-Neptunian Objects, Kuiper Belt Bodies, Oort Cloud comets may have the same semi-major axis as other objects, therefore are not planets.

As to these current definitions, we can see that Pluto is not a planet. All the arguments I made are in the previously linked post. A nice thing about science is that new information can change our understanding of nature and the universe. Science is a fluid subject. Our perceptions can alter. So that is why it may be important to reinvestigate the idea of a planet.


If we were to redefine what makes a planet, we should be clear on what is and what is not a planet. Pluto is smaller than seven moons in our solar system, including our Moon. However, Mercury is also smaller than the two largest moons, Ganymede (orbiting Jupiter) and Titan (orbiting Saturn). We can all agree that Ganymede, Titan, the three other Galilean satellites, Triton (orbiting Neptune), and our Moon are NOT planet, they are moons. They orbit around planets which in turn orbit around the Sun. Pluto, only orbits the Sun (though it can be argued that it also orbits around the common center of mass of its system (Pluto, Charon, and its other orbital companions).


If the IAU does change the definition of a planet, it will have to get rid of the idea of co-orbiting bodies that are a significant fraction of the largest body's mass and radius. Remember, Charon is about 11.6% the mass of Pluto and has a radius about half that of Pluto. Our Moon is only 1.2% the mass of Earth and has a radius just over a quarter of the Earth. Looking at all the large satellites of the gas giant planets shows that all of them are significantly smaller in comparison to their parent planet than our Moon is to the Earth.


So if the definition is changed, what other objects in our solar system will have to be redesignated as a planet? Pluto is obviously the first. Eris will also have to redefined as it is a larger body than Pluto. After that, it depends on what the lower limit the IAU wants to use. Makemake may become a planet, Ceres may as well, though if Ceres does get redefined, than all the trans-Neptunian objects larger than Ceres will have to be classified as planets. Not only that, we will have to add a third type of planetary body along with terrestrial and Jovian. This will have to be something in between, though that type is not really a bridge between terrestrial and Jovian.


At the moment, I personally like the definition we have for planets. It's clear, concise, and makes a lot of sense. But as I said before, science can change and our understanding of what is going on can help us make more informed conclusions. Only if the IAU changes the definition of a planet, only then will Pluto, Eris, and some of the other dwarf planets/minor planets in our solar system become full-fledged planets.


We will learn more about Pluto once New Horizons reaches the Plutonian system in July of 2015. Then we will know more about Pluto and its sisters and may be able to make more informed conclusions about what they are.

Planets beyond Pluto?

A recent article I came across via Twitter suggested that there could be at least two planets outside Pluto's orbit (again, Pluto is not a planet!) based on gravitational perturbations on Trans-Neptunian Objects.


Just a couple of highlights:

• Astronomers think that their could be at least two planets beyond Pluto's orbit causing these changes.
• The TNOs have high inclinations (>20°) with a large dispersion of semi-major axes (150 AU to 525 AU). Based on observational analysis, they should orbit in the plane of the solar system with semi-major axes of 150 AU.

Click the link above for the full article.

New Horizons

On December 6, New Horizons "woke up" for the last time. It is on its last 162 million mile journey to an object in our solar system that has never been visited by Earth spacecraft until now.


Pluto will be visited for the first time and we will have the best resolution images of Pluto ever by mid-May. By July of 2015, New Horizons will finally arrive at the Pluto system and we will also have close-up images of five of its major moons (or co-orbital bodies): Charon, Nix, Styx, Hydra, and Cerebus.

New Horizons Instrumentation

Image Credit:

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Read more about the mission at NASA.gov

Kuiper Belt

The Kuiper Belt is the region of the Solar System out beyond Neptune where the majority of our short-period comets are found. This is the region where we find the dwarf planets Pluto and Eris. The objects in this region besides being comets and dwarf planets are sometimes referred to as plutinos, KBOs (cubinos or Kuiper Belt Objects), or TNOs (Trans-Neptunian Objects). The belt is generally a torus centered on the Sun populated mostly by icy bodies as the heavier material generally settled in the interior part of the Solar System.

The comets that come from this region are called short-period comets as they have elliptical orbits with large eccentricities and periods of less than 200 years. They are disturbed in their orbits by large passing bodies, like Neptune that can send the object into in the inner solar system.

The Kuiper Belt is named after Gerard Kuiper as he proposed that there must be a disk that has icy remnants from the formation of the Solar System, much like there is a rocky disk in the inner Solar System containing leftovers from the birth of the Sun and planets, the Asteroid Belt. Disks found around other stars indicate that Kuiper Belt like features may be a consequence of planetary formation around stars.

The disk itself ranges from 30 AU to 100 AU in radius and contains at least several thousand objects, many of which are over 100 km in diamter.

Pluto and Charon

Pluto has satellites, or more correctly, objects that co-orbit the Sun with Pluto. The largest of these objects is Charon, which is half the size of Pluto. Why do I say it co-orbits around the Sun rather than orbiting around Pluto? It has f\to do with something called the barycenter of the two bodies.

The easiest way to explain barycenter is it is the balance of mass between the two objects. Think of a seesaw (or teeter-totter) with a person on either end. If both people are the same weight, the seesaw is perfectly balanced with the center of mass being at the fulcrum of the seesaw. However, if one person weighs more then the other person, the seesaw is no longer in balance, and the center of mass shifts towards the heavier person. To get the seesaw back in balance, the heavier person needs to move closer to the fulcrum. Therefore the center of mass is closer to the heavier person.

The same thing is true in orbiting objects. In the Earth-Sun system, the center of mass is closer to the Sun. In fact, the Sun is so much more massive, that the barycenter is within the Sun itself. This is true for all planets orbiting the Sun. In the Earth-Moon system, the Earth is about 83 time more massive than the Moon, so therefore the barycenter for the Earth-Moon is about 4700 km from the center of the Earth (about 75% the Earth's radius). In general, you can look at all the planets and their moons and find the same thing: the barycenter for a planet-moon system will lie within the radius of that planet. (The Jupiter-Sun system is an exception. However, the barycenter is just outside the surface of the Sun.)

However, Pluto and Charon are different. Charon is about half the mass of Pluto. Therefore, the barycenter for Pluto and Charon is a third of the total distance between the two planets, well outside Pluto's radius. This gives us another argument against Pluto being a planet. Charon is not truly a satellite of Pluto since their center of mass is not within Pluto. Pluto and Charon are more like co-orbiting bodies around the Sun.

The Discovery Of Pluto

Believe it or not, the discovery of Pluto was by accident. Astronomers had noted anomalies in the orbit of Neptune, and much like they did with Uranus' orbit, thought there was a planet lying outside Neptune's orbit which would give rise to the anomalies.  So astronomers began to look for it.

It wasn't until Kansas-born Clyde Tombaugh (it's only important that I mention he's from Kansas because so am I), using stereo imaging of photographic plates found what he thought was the new planet. However, the new object he found was Pluto.

Lowell Observatory Archives

Clyde Tombaugh

 

But after more measurements, the new planet was just not large enough to have the effect that they saw on Neptune. After some more calculations, Neptune's orbit was confirmed just using the masses of Neptune and Uranus.

So Pluto was there, but was not what was expected. If the orbit of Neptune had been calculated  correctly initially, it might have been a while longer before Pluto were discovered.

Why Pluto is NOT a planet

First of all, I'm just going straight out and telling you. PLUTO IS NOT A PLANET. The International Astronomical Union made that clarification in 2006 and nothing will change that. I'm going to give you the reasons why I think Pluto is not a planet and why the IAU made the correction determination.

Secondly, when I was teaching college astronomy at the University of Pittsburgh, one of the things I also taught my students was that Pluto was not a planet. I last taught in 2005, a full year before the IAU made the announcement. Everything that I will talk about in here were the reasons I gave as to why Pluto is not a planet. Demoting Pluto did not diminish what Clyde Tombaugh accomplished in 1930 when he found Pluto at Lowell Observatory. There will be more of the discovery of Pluto in a later post.

One thing that argues against Pluto being a planet is its inclination to the equator of the Sun. In general, a planet should have a low orbital inclination as the Sun and planets were formed in the same nebula. As the nebula rotates and shrinks, all larger objects should stay in the same general plane. Here are the inclinations of the eight planets, Ceres, and Pluto

• Mercury 3.38°
• Venus 3.86°
• Earth 7.155°
• Mars 5.65°
• Ceres 17.75°
• Jupiter 6.09°
• Saturn 5.51°
• Uranus 6.48°
• Neptune 6.43°
• Pluto 11.88°

As you can see, Ceres (the largest asteroid or a dwarf planet in the Asteroid Belt) and Pluto both have orbital inclinations to the solar equator of more than 10 degrees. Earth has the largest of all the planets, but is inclined three degrees shallower. It would make sense since all the planets formed at the same time as the Sun and are the most massive bodies in the solar system after the Sun, that they would orbit within the equatorial plane of the Sun and not deviate much within that plane.

Another reason why Pluto is not a planet is it has a highly eccentric orbit. Objects that form in the same cloud as a star should be in a relatively circular orbit. (There really is no such thing as a perfect circle in science or nature. Variations in conditions can distort objects to make them less than perfect.) The nebula rotated which caused the cloud to collapse into a disk-like shape with the protosun at the center. Therefore, anything forming in that cloud will have a nearly circular orbit. Let's look at the different eccentricities of the same nine objects.

• Mercury 0.206
• Venus 0.007
• Earth 0.017
• Mars 0.093
• Ceres 0.076
• Jupiter 0.049
• Saturn 0.056
• Uranus 0.047
• Neptune 0.009
• Pluto 0.249

Looking at these eccentricities, all of them except Mercury and Pluto have eccentricities less than 0.1. Mercury's orbit is eccentric because of its proximity to the Sun and relativistic effects of the space that Mercury orbits in. Pluto is so much farther out, that relativistic effects do not affect it as much. Curvature of spacetime is a consequence of massive bodies that won't be explained here. You can always google the topic, or if you would like me to explain general relativity, comment below.

The density of Pluto is also a dead giveaway that Pluto is not a planet. 

Terrestrial Planets:

• Mercury 5.427 g/cm³
• Venus 5.243 g/cm³
• Earth 5.514 g/cm³
• Mars 3.934 g/cm³

Jovian Planets:

• Jupiter 1.326 g/cm³
• Saturn 0.687 g/cm³
• Uranus 1.27 g/cm³
• Neptune 1.638 g/cm³

Dwarf Planets:

• Ceres 2.077 g/cm³
• Pluto 2.03 g/cm³
• Eris 2.52 g/cm³ (estimate)
• Haumea 2.6 g/cm³ (estimate)
• Makemake 2.3 g/cm³ (estimate)

Pluto's density is too low to be terrestrial, but too high to be Jovian. Based on its density, we know that Pluto is a combination of both icy material and rocky material, with slightly more ice than rock.

The last argument I can make about Pluto not being a planet is its size relative to multiple moons in our Solar System.

 

Trans-Neptunian Objects (objects orbiting the Sun outside of Neptune's orbit)

Image Credit:

NASA/Hubble

 

If we were to include Ceres on this image, it would be smaller than Orcus (~800 km for Orcus and ~500km for Ceres).

 

The IAU has given a definition for a planet and a dwarf planet. 

• A planet is a spherical body that orbits the Sun and has cleared its orbit of other objects, i.e. it does not share an orbit with other bodies (not including moons).
• A dwarf planet is a spherical body that orbits the Sun but has not cleared its orbit of other objects. They may co-orbit with other bodies. Many of the Trans-Neptunian Objects, Kuiper Belt Bodies, Oort Cloud comets may have the same semi-major axis as other objects, therefore are not planets.

Pluto, the Planet that is NOT a Planet

Rotating Face of Pluto taken by Hubble

Image Credit:

NASA, ESA, and M. Buie (Southwest Research Institute).

Pluto. Never in astronomy has there been a more controversial object that we have discovered in our Solar System, our galaxy, or even the Universe. When it was first discovered, it was thought to be the farthest planet in our Solar System. But as we learned more about it, we were able to determine that it is not, in fact, a planet.

But before we learn why Pluto is not a planet, let's get to know a few things about one of the farthest denizens of our Solar System. For one, it was actually discovered by accident, even though it was actively being searched for by astronomers. Astronomers had used the orbit of Neptune and found that its orbit did not match calculations of how Neptune should go around the Sun. They thought that there was an object orbiting outside Neptune's orbit that caused these anomalies and were looking for it.

Secondly, it has a very eccentric orbit. In fact, its orbit is so eccentric, that for portions of its year, Pluto is actually closer to the Sun than Neptune, but because of its highly inclined orbital plane, Neptune and Pluto are never in danger of colliding.

Pluto is tiny. It is smaller than the largest moons in our Solar System including our Moon. But despite its small size, it does have satellites orbiting around it, including the largest Plutonian satellite, Charon, though it would be more accurate to say that Pluto and Charon co-orbit the Sun.

Pluto in comparison to some moons of the Solar System

Image Credit:

NASA

It is an icy body, much like the objects in the Kuiper Belt and Oort Cloud. There is another Kuiper Belt objects that have a similar composition to Pluto, but it actually larger than it.