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In 1990, no planet outside of our solar system was known. Now not only do we know thousands, but we also have an almost embarrassing number of ways to find them.
You may have heard of transit mode, where a planet passes in front of its star, and we see a drop in the amount of starlight, like a mini-eclipse. This is the most successful method so far, and gives us the radius of the planet. You may also be familiar with the radial velocity (or reflex) method, where a planet gravitationally pulls its orbiting star, which we can see as a Doppler shift in its light. This gives us the mass of the planet. There is also direct imagery, where we literally see the planet in a picture. It works best for young systems.
But there is another more subtle method that works quite well in a limited number of circumstances.
Suppose a planet is discovered orbiting a star via transits. An undetected planet is also orbiting the star, in an orbit close enough to the first planet for it to interact gravitationally. If, just before a transit, the second planet is in front of the star, it pulls the first planet forward a bit in its orbit, and the transit will occur a little earlier. If the second planet is behind the first in its orbit, it will retreat to the first planet and the transit will be a bit late.
This can be measured, and this is called the transit time variation (or TTV) method. A lot can be determined on the planets this way! And in fact, it was used to discover an interesting planet orbiting a nearby star., that’s what it really is.
The star is called K2-146. It’s about 260 light years away, so pretty much. It’s a red dwarf: a faint, cold star, only about a third the mass and width of the Sun, and just over 1% as bright. It has been observed by the Kepler exoplanet research spacecraft within months in 2015, and a planet was quickly discovered orbiting it.
The planet, K2-146b, orbits in just 2.6 days at a distance of about 4.1 million kilometers – Earth orbits the Sun 150 million kilometers away, so this planet is close. Even though the star is faint, this planet is probably much hotter than Earth due to its proximity to the star.
The rapid orbital period meant that many transits were observed even during the short observation period that Kepler saw it … and several astronomers noted that it exhibited noticeable variations in its transit times. No other planet has been seen in transit, however.
… Until the end of 2017 and again in mid-2018. It was at this point that Kepler (although renamed K2 at that time) observed the star again, and this rather long gap was actually a godsend in this case, as the third planet inferred from the variations time has also started to pass through the star!
This Planet K2-146c is approximately 5.4 million kilometers away and takes 4.0 days to circle the star. Here’s the fun part: the two planets orbit very close to the star but not quite front view from Earth. During the previous campaign, the second planet barely grazed the star from our perspective, and the transit was too shallow to be detected. However, as the two planets orbit and shoot at each other, their orbital orientation may change, and what was once a grazing transit gets deeper a year later! So it took a while, but the planet finally raised its hand.
Both planets are larger than Earth, around 28,000 and 30,000 km in diameter – Earth is approximately 13,000 km in diameter. This would make them either super-Earths (rocky planets but bigger than us) or mini-Neptunes (rather gas giants with a thick atmosphere). Which one is it?
You need mass for this, and variations in transit times allow astronomers to get it. Using data from all of Kepler’s observations, astronomers found that TTVs altered the transit times of the two planets by up to 3 hours. Using these numbers, they could run the equations backwards to get the masses of the planets needed to create these TTVs.
They discovered that the planets have masses 5.7 and 7.5 times that of Earth. This makes them both rough at 2/3 the density of the Earth, so they are either aquatic worlds (more water and less iron make them denser), or they have thick atmospheres. At the moment, it is not possible to say. Perhaps follow-up observations with the James Webb Space Telescope could provide atmospheric information.
It is all quite astonishing. Planet c was not even visible at first, but astronomers could deduce that it was there, and although it barely passed through the star, they were able to determine a lot of things, including, Critically, its size and mass – from there you can get the density, a possible composition, and even a little bit about its atmosphere. And the same for planet b too: normally you need radial velocity measurements to get mass, but here only the effect of the two planets on each other was enough to figure it out. Astonishing.
There are many varieties of planetary systems, but we also have many ways of discovering and characterizing them. The more we have to look at something in different ways, the more we learn. Diversity is strength.
Huh. There may also be a life lesson in there.
