One of the best things about being able to see thousands of exoplanetary systems is that we’re able to track them in different stages of development.
Scientists still have so many questions about how planets form, and comparing notes between systems of different ages is one way to answer them.
A new paper recently published in Nature by John Livingston of the National Astronomical Observatory of Japan and his co-authors details one particularly interesting system, known as V1298, which is only around 30 million years old, and hosts an array of four “cotton candy” planets, which represent some of the earliest stages of planet formation yet seen.
V1298 is located in the Taurus constellation about 350 light years away. In human terms, if the Sun was a middle-aged adult, it would be a 5 month-old baby.
But it still has four planets spinning around it, which a press release supporting the paper calls “cotton candy” planets.
They are extremely large (about the size of Jupiter), but not very massive – their low-density giving rise to the comparison to a favorite low-density carnival snack.
One planet in particular, which is still about 5 times the size of Earth, is equivalent in density to a marshmallow, while the least dense planet is actually equivalent to cotton candy, with a density of 0.05 g/cm2.
While we’ve known about these planets for a while, the paper represents a significant update to our estimates of their mass.
Previous estimates had them being around 200-300 times what their updated mass estimates are.
That appears to be because of the youthfulness of their star. Young stars like V1298 are covered in sun-spots and constantly flaring, creating activity that mimics the signals exoplanet hunters use to look for planets.
Using their typical technique of radial velocity to measure the weight of these planets led to inaccurate conclusions because of V1298’s high level of activity.
To account for this, the authors used data from numerous telescopes, including Kepler, TESS, Spitzer, and the Las Cumbres Observatory, collected over the course of 9 years, to use another method for planetary weight estimation known as transit-timing variations (TTVs).
This watches for a planet to transit in front of its host star, but models slight deviations in the timing of those transits to account for the gravitational impact of other nearby planets.
One added advantage of this technique is that the study “recovered” one particular planet in the system, which was previously “lost” as researchers hadn’t been able to accurately determine its orbital period (which is about 48.7 days).
With a better grasp on the actual weights of these new-born planets, it’s best to place the discovery in the wider context of exoplanet evolution. There are two distinct types of exoplanets that seem to be the most common in the galaxy – “Super-Earths” and “Sub-Neptunes”.
Super-Earths tend to be rocky, with a radius less than 1.5 times that of our home planet, whereas “Sub-Neptunes” are around 2.0 times the radius of Earth but are much more gaseous.
Both types are typically found on very close-in orbits, even closer than Mercury in our own system, though that might simply be because the fast orbital periods of these planets are easiest to track given our relatively limited telescope time.
The plethora of planets at V1298 provides a model of how those two distinct types of exoplanets can evolve in older, more mature systems.
Most astronomers agree that the planets surrounding V1298 will lose their atmosphere and “shrink” as they get older. This process is thought to take place either by photoevaporation, where radiation from a planet’s host star blasts the atmosphere away, or by a process where the heat from the planet’s own core pushes the atmosphere away, both of which can take billions of years.
However, according to the paper, there’s a third process that might be the main driver in the early stage of planet formation.
Known as “boil-off”, this happens once a protoplanetary disk, which acts like a “lid” to keep the pressure of a planet’s atmosphere in check, is blown away. This allows the atmosphere of a planet to burst outwards, similar to how steam would rapidly escape from a pressure cooker if you removed the lid.
V1298 isn’t the first system we’ve seen these processes in – Kepler-51 is another famous one with examples of “cotton candy” planets. However, it is more than 10 times older than V1298, and some scientists think the “boil-off” process would have ended already at the age Kepler-51 is. So having the insight into a much earlier stage of the process via V1298 is invaluable.
That value is shown by the fact that the paper was accepted for publication in Nature, one of the world’s premier academic journals.
The fact that it took 9 years of data collection contributed to that importance, hinting that this paper will go down as a milestone in exoplanetary history.
There might be data on more, even younger star systems, hiding in the decades of exoplanet data sets with planets that will hold even more clues to their formation. But for now, this snapshot of V1298 is the best we’ve got.
Written by Andy Tomaswick/Universe Today.
