I love astronomy! The discovery of thousands of exoplanets has made it only more exciting. You often hear about the really weird planets in the news. You know, things like low density puffballs, hot Jupiters, rogue planets, planets that orbit their star in hours, and even a Jupiter mass planet that is one huge diamond! As neat as these discoveries are, I also want to know how Earth fits in.
Do all of these weird planets indicate that Earth-like planets are unusual? Or does the planet-making process tend to produce lots of planets like Earth? I’ll take a statistical look at the distribution of exoplanets that astronomers have discovered to see how Earth fits in. The probability of finding an Earth twin depends on what kinds of planets are most common. Exciting data suggests there will be thrilling Earth twin finds.
Our Sample of Exoplanets
Thousands of exoplanets have been discovered. This gives us a great sample to study but unfortunately it’s not necessarily a representative sample. The methods of exoplanet discovery tend to increase the probability that specific types of exoplanets are discovered. Therefore, our sample data are likely to be biased by over representing these types of exoplanets.
The radial-velocity and transit methods are the two key methods for detecting exoplanets. The data I use are from the Planetary Habitability Laboratory’s exoplanet catalog.
The radial-velocity method finds the wobble that exoplanets produce in the star it orbits. Larger wobbles are easier to detect. Consequently, exoplanets that are more massive and closer to their star are easier to detect because they produce the largest wobbles. That’s why this method found a lot of hot Jupiters.
When an exoplanet crosses between the star it is orbiting and Earth, the amount of light that reaches Earth decreases. Instruments like the Kepler space telescope can detect this reduction in light and deduce the presence of exoplanets around that star. The orbit of the exoplanets must align with Earth’s vantage point of the host star for this method to work.
Kepler must detect at least three transits before it flags it as a candidate exoplanet. The additional transits help eliminate other possible reasons for the light reduction and allow astronomers to estimate how long it takes the exoplanet candidate to orbit its star. However, these candidates have to be verified by a direct observation.
Because Kepler failed after collecting data for 4 years, the exoplanets it discovered are biased towards those that orbit nearer to the star because of their shorter orbits. The nearer planets were able to fit in three transits within 4 years. It’s not expected that Kepler’s data will reveal exoplanets further out than 1 AU from its star.
Comparison of Exoplanet Characteristics by Method of Discovery
The distribution of the mass and distance for confirmed exoplanets are displayed in the histogram below. Percent refers to the percentage within the specific method.
The graphs show that most discovered exoplanets are less massive and closer to their star. Even though the radial-velocity method favors detecting more massive planets, it still discovered a higher percentage of less massive planets. Kepler didn’t detect exoplanets more than 1 AU from the star. However, the radial-velocity method continues to find planets further out.
While we most likely don’t have a representative sample, we can get a general picture based on these graphs. The majority of exoplanets discovered by both methods are close to the star and not extremely massive. That’s where Earth fits in, so perhaps we’re not complete oddballs!
How Earth Fits into the Distribution of Exoplanets
Let’s assess the distribution of confirmed exoplanets to see where the Earth falls for several important characteristics. The green bar shows Earth’s position.
The graph shows that the range of exoplanet mass is quite large! It goes from very low mass to thousands of Earth masses. Jupiter is the most massive planet in the solar system and it weighs in at 317 Earth masses. Less massive planets, like Earth, are much more common. Let’s take a closer look.
We’ve zoomed in to focus on planets with masses of up to 100 Earth masses. This allows us to see that Earth is actually less massive than the most frequent value on the graph. Super-Earths are more common than exoplanets with 1 Earth mass. Super-Earths are rocky planets that have 2-5 Earth masses. It’s possible that Earth is a bit undersized for the typical rocky planet.
Moving on to the mean distance that exoplanets are from their stars, we see that Earth is further out than the typical exoplanet. We know why this is the case. Red dwarf stars are the most common type of star and they account for 80% of all observed stars. Red dwarfs are small, cool stars that have tightly clustered planetary systems. Earth is unusual because it doesn’t orbit a red dwarf star.
Orbital eccentricity gages how close an exoplanet’s orbit is to a perfect circle. More eccentric orbits are more oval in shape, which causes severe climate changes due to the varying distance to the star.
Orbital eccentricity ranges from zero for a perfect circle to just less than one for the most eccentric orbit possible. Earth and the other planets in the solar system are very close to perfect circles. The graph shows that most confirmed exoplanets have this property too.
Collectively, the graphs show that, while there is a wide range of properties for these characteristics, Earth tends to fall near the more common values. The exception is that Earth does not orbit a red dwarf star.
Searching for an Earth Twin
These exoplanet distributions are interesting. However, we really want to know whether an exoplanet is like Earth and is habitable. Enter the Earth similarity index (ESI)!
ESI values range from 0 to 1. Earth has a value of 1. ESI is calculated based on estimate properties of the exoplanet, such as surface temperature, radius, and density among others. For comparison, Venus has an ESI of 0.78 and Mars is 0.64.
I’ve graphed all confirmed exoplanets and unconfirmed Kepler candidates with an ESI greater than 0.8. The blue bubble represents Earth for comparison.
The bubbleplot displays 23 exoplanets with an ESI of at least 0.8. Five of the exoplanets are greater than 0.9! The maximum ESI is 0.93. This graph reconfirms the earlier graph which showed that Earth is less massive than most rocky planets. On the bubbleplot:
- 78% are super-Earths.
- 17% are Earth-sized.
- 5% are less massive than Earth.
In terms of habitability, super-Earths might have the advantage for a variety of reasons.
You probably noticed that the exoplanets form two groups. I’ll refer to them as Earth cousins and Earth twins.
On the bottom-left, you’ll find the Earth cousins. These exoplanets are cousins because, even though they score high on the ESI, they orbit red dwarf stars. Because red dwarfs are cooler, these exoplanets must orbit their star more closely to be habitable, which is why their years are all less than 200 days.
On the top-right, you’ll find the Earth twins. They score high on the ESI and their stars are similar to the sun.
I’ve included both confirmed and unconfirmed exoplanet candidates on the bubbleplot. All of the green confirmed bubbles are Earth cousins. All 14 Earth twins remain unconfirmed so far. Based on the confirmation rate so far, we can expect that 88% of these candidates will be confirmed by direct observation. That’s a dozen Earth twins plus or minus waiting to be confirmed. Exciting!
These Earth twins are just the exoplanets for which we have data currently. As we saw in the exoplanet distributions, the planet making process creates a wide variety of planets but it tends to make planets with characteristics like Earth more frequently than other types of planets. That context is the reason why astronomers estimate a total of 40 billion habitable, Earth-sized planets in the Milky Way galaxy!
The image of the radial velocity method is created by Rnt20 and the image of the planet transit method is created by Nikola Smolenski. Both images are used under this Creative Commons license.