I’ve been most fascinated lately by studies of planet formation. Every small detail is like that one letter in the crossword you need to fill all the other boxes in, every discovery a cornerstone that holds together a unique piece of the universe. For example, using just the find that the exoplanet Beta Pictoris b has a very short day of eight hours, astronomers could speculate on how it was formed, what its density could be, and how heavy it could get over time. And it isn’t surprising if a similar tale awaits telling by Kepler 421b, an exoplanet some 1,000 ly from Earth toward the constellation Lyra. Its discovery was reported on July 17, a week ago. And its pièce de résistance is that it has a long year, i.e. orbital period, of 704 days.
Image: Illustrating the transit technique. The Kepler telescope looks for the drop in brightness in its search for exoplanets. The technique applies only when the planet can be seen head on against the background of its star. Credit: http://www2.ifa.hawaii.edu/
To have such a long year, it must be orbiting pretty far from its star – Kepler 421 – which in turn should’ve made it hard to discover. The NASA Kepler space telescope spots exoplanets by looking for the dip in a star’s brightness as the planet moves in front of it, called a transit. Because of Kepler 421b’s high orbital period, it transited its central star only twice in the four years Kepler was looking at it. Together with its orbital eccentricity – i.e. how elliptic its orbit is – Kepler had only a 0.3% chance of spotting it on its way around the star. In fact, 421b has the longest year for any known exoplanet discovered using the transit technique. This means we need to start considering if the M.O. isn’t good enough to spot exoplanets with large orbital periods, a class of planets that astronomers have been looking for. On the other hand, now that 421b has been spotted and studied to some extent, astronomers can form impressions of its history and future.
The frost line
For starters, they were able to deduce the planet’s size based on how much starlight it blocked and the shape of its orbit from how much light it blocked during each full transit. The readings point to 421b being like Uranus, with radius four times Earth’s, density at least 5 g/cc, and an eccentric orbit. Being like Uranus also means a surface temperature of -90 degrees Celsius (183 kelvin). This is plausible because 421b is 1.2 times as far from its star as Earth is from the Sun, and its star is a dimmer orange dwarf.
These wintry conditions are found beyond a star’s frost line, an imaginary line marking the distance beyond which space is cold enough to cause hydrogen-based molecules to condense into icy grains. So planets orbiting beyond this distance are also icy. Kepler 421b is likely the first exoplanet astronomers have found (using the transit technique) orbiting a star beyond its frost line. In other words, this might be our first exoplanet that’s an ice giant – “might” because 421b hasn’t been independently observed yet.
Not surprisingly, the frost line also marks a more significant boundary in terms of planet formation. Though observations made by Kepler are starting to show that the Solar System is a surprisingly unique planetary system, it’s still the one we understand best and use to analogize what we finds in other worlds. Astronomers believe planets in the system formed out of a disk of matter surrounding a younger Sun. The inner Earth-like (telluric) planets formed when rocky matter started to clump together and “fall out” of this disk. The outer gaseous planets, beyond the frost line, formed when icy grains stuck together to form watery planetary embryos.
Image: In this artist’s conception, gas and dust-the raw materials for making planets-swirl around a young star. The planets in our solar system formed from a similar disk of gas and dust captured by our sun. Credit: NASA/JPL-Caltech
The prevailing belief is that planets take at least three million years to form. In the same period, the central star is also evolving – in this case, Kepler 421 is a K-class star becoming brighter – and the amount of material available in the protoplanetary disk is diminishing because planets are feeding off it. Consequently, the frost line is on the move. Calculations by the astronomers who discovered 421b find the exoplanet to be now where the system’s frost line might’ve been three million years ago.
The sedate giant
Right now, we’ve a lot of letters in the crossword. Piecing them together, we can learn the following:
- If a beyond-the-frost-line gas giant is as big as Uranus but not as big as Jupiter, it’s possible that not enough material was available when it started to form, rendering it a latecomer in the system
- The abundance of material required to form Jupiter-sized planets makes smaller worlds likelier than larger ones, and in fact implies worlds like 421b should be less unique than Kepler makes it seem (a 2013 study cited by the discoverers suggests that there might actually be a pile-up of planets transiting at the frost line of their stars)
- If the planet had to have formed behind its star’s frost line, and the frost line was three million years ago where the planet is now, the planet could be around three million years old – assuming it hasn’t moved around since forming
- 421b is very Uranus-like; if it has to be a rocky world, its mass has to be 60 times Earth’s, pointing at an improbably massive protoplanetary disk within one or two AU of a star – something we’re yet to find
#3 warrants a comparison with the Solar System’s history, especially Jupiter’s. Jupiter didn’t form where it is right now, having possibly moving toward and away from the Sun as a result of gravitational interactions with other planets that were forming. During its journeys, its own gravitational pull could’ve tugged on asteroid belts and other free-floating objects, pulling them out of one location and depositing them in another. Contrarily, 421b appears to have been far more sedate, probably not having moved at all due to its youth and isolation. If only it had moved inward, like Jupiter eventually did, its orbital period would’ve been shorter and Kepler would’ve have spotted it easier.
Image: The confusion Jupiter might’ve caused during its journey through a nascent Solar System. Credit: http://www.astro.washington.edu/courses/astro557/GrandTack2.pdf
Another comparison can be made with Beta Pictoris b, the other exoplanet mentioned at the beginning of this piece, the one with the eight-hour-long days. Younger planets spin faster because they still have the angular momentum they acquired while accumulating mass before slowing down in time. Heavier planets also spin faster because they have more angular momentum to conserve. Similarly, we might be able to find out more about Kepler 421b’s past by uncovering its spin rate and getting a better estimate of its mass.
Anyway, a simple piecing together of facts and possibilities tells us – at least me – this much. Astronomers have one more awesome fact to take away: as the finders of 421b write in their pre-print paper, “the first member of this missing class of planets” has been found, and that means more astronomy to look forward to!
Discovery of a transiting planet near the snow line, Kipping et al, arXiv:1407.4807 (accepted in The Astrophysical Journal)
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