Where is our Super-Earth?

Often we can become trapped in binary thinking that blinds us to other possibilities that are potentially better answers. The classical example is that when faced with a glass that contains exactly 50% water you divide the world up into the optimists who see a glass that as half-full and the pessimists who see it as half empty. That binary choice is not the only ways of looking at this, because some can break away from that modality by pointing out that the glass is too big.

Why is our Solar-System special?

A question that has been bugging astronomers concerning our solar system is to wonder why it appears to be very unique and special.

It was once thought that most solar systems consisted of small rocky worlds close to the star and then far bigger gas giants further out. Now however we have a lot more than a data sample of just one. Since the 90s we have been gathering more and more examples of other solar systems. When analysed there was a realisation that our solar system is distinctly unique.

Here is how things look. At the top you have what we have in our solar system and below a bar chart showing how common the different sizes out there are. So do you notice anything odd?

kepler_planetsWhen you look at the above what immediately leaps out is the following

  • Having small inner rocky words and huge gas giants further out is not the most common exoplanet type.
  • Super-Earth/mini-Neptune worlds are by far the most common type of exoplanet, but we don’t have any in our solar-system

This rather obviously leads to a fascinating puzzle. Why don’t we have any of the most abundantly common planet types within our own solar system, what makes it so special and unique?

Various ideas have been proposed, for example perhaps our gas giants consumed them, but nothing really satisfactory addresses it.

That however has just changed.

We have not been thinking about this correctly

A new paper by Columbia University astronomers Jingjing Chen and David Kipping has been published that suggests that the way we think about all of this has fooled us into leaping to the false conclusion that our solar-system is unique.

The paper, is entitled “PROBABILISTIC FORECASTING OF THE MASSES AND RADII OF OTHER WORLDS” and the abstract starts as follows …

Mass and radius are two of the most fundamental properties of an astronomical object. Increasingly, new planet discoveries are being announced with a measurement of one of these terms, but not both. This has led to a growing need to forecast the missing quantity using the other, especially when predicting the detectability of certain follow-up observations. We present am unbiased forecasting model built upon a probabilistic mass-radius relation conditioned on a sample of 316 well-constrained objects ….

Let’s take this step by step.

We have observational data for planets in other systems. The way we make those observations enables us to know either the radius or the mass of the exoplanet – never both.

The problem is that if you want to know if a world is actually a body of rock or a gas giant then you need to know both. For example, if you know that the detected body has the mass of four earths then what you don’t know is if it is mostly rock with a thin layer of gas like our Earth is, or if it is instead a huge gas giant with a tiny rocky core. To work out which, it means that you also need to not only know the mass, you also need the radius as well.

What this paper does is to utilise a way of working this out and that then leads to a rather surprising discovery.

Basically the conclusion is this …

We detect a transition in themass-radius relation at2.0+0.7M ,which we associate with the divide between solid, Terran worlds −0.6 ⊕ and Neptunian worlds. This independent analysis adds further weight to the emerging consensus that rocky Super-Earths represent a narrower region of parameter space than originally thought.

In other words the cutoff point between rocky planets and gas planets is about two times the size of earth. Bigger that 2 x Earth means you have a mini Neptune and not a bigger chunk of rock.

The net effect is that the super-earth category is wrong, and is not one of the most common exoplanet types at all …

The large number of 2-10M⊕ planets discovered is often cited as evidence that Super-Earths are very common and thus Solar System’s makeup is unusual (Haghighipour 2013). However, if the boundary between Terran and Neptunian worlds is shifted down to 2 M⊕ , the Solar System is no longer unusual. Indeed, by our definition three of the eight Solar System planets are Neptunian worlds, which are the most common type of planet around other Sun-like stars (Foreman-Mackey et al. 2014).

In other words, what their analysis leads to is that the best way to actually think about things is as follows …

earth_venusThe above breaks down into 4 basic categories as follows …

  • Rocky worlds such as Earth and Mars. These can be as big as two earth masses in size, but no bigger.
  • 2 to 130 earth masses are you have Neptune types with the volatile gas envelopes
  • 130 earth masses up to about 8% of the mass of the sun and you have Jupiter types
  • Anything bigger, and its a star in its own rights

In other words, if you eliminate the super-earth category (2-10 x earth sized) and instead use the above way of looking at it all, then our solar-system ceases to be special. We have the rocky worlds, and then for the next category we have three of those, and also of course a Jupiter for the category after that.

The paper finished with the observation that “It may be, then, that the Earth is the Super-Earth we have been looking for all along.“.

In other words, the way we sized the glass was indeed wrong and by looking at the categories of exoplanets differently we can then appreciate that there is no puzzle, our solar-system is not distinctly unique after all and is not missing something from a super-earth category, because such a category is not a valid way of looking at things.

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