Are you rocky or are you gassy? Astronomers unlock the mysteries of super-Earths

An artist’s impression of a stellar system with three
super-Earths. Credit: ESO.

A star about 100 light years away in the Pisces
constellation, GJ 9827, hosts what may be one of the most
massive and dense super-Earth planets detected to date
according to new research led by Carnegie’s Johanna Teske.
This new information provides evidence to help astronomers
better understand the process by which such planets form.

The GJ 9827 star actually hosts a trio of planets, discovered
by NASA’s exoplanet-hunting Kepler/K2 mission, and all three
are slightly larger than Earth. This is the size that the
Kepler mission determined to be most common in the galaxy with
periods between a few and several-hundred-days.

Intriguingly, no planets of this size exist in our Solar
System. This makes scientists curious about the conditions
under which they form and evolve.

One important key to understanding a planet’s history is to
determine its composition. Are these super-Earths rocky like
our own planet? Or do they have solid cores surrounded by
large, gassy atmospheres?

To try to understand what an exoplanet is made of, scientists
need to measure both its mass and its radius, which allows them
to determine its bulk density.

When quantifying planets in this way, astronomers have noticed
a trend. It turns out that planets with radii greater than
about 1.7 times that of Earth are have a gassy envelope, like
Neptune, and those with radii smaller than this are rocky, like
our home planet.

Some researchers have proposed that this difference is caused
by photoevaporation, which strips planets of their surrounding
envelope of so-called volatiles—substances like water and
carbon dioxide that have low boiling points—creating
smaller-radius planets. But more information is needed to truly
test this theory.

A whiteboard video in which lead author Johanna Teske
explains what her results constraining the masses of three
super-Earths mean for our understanding of exoplanets.
Credit: Carnegie Institution for Science

This is why GJ 9827’s three planets are special—with radii of
1.64 (planet b), 1.29 (planet c) and 2.08 (planet d), they span
this dividing line between super-Earth (rocky) and sub-Neptune
(somewhat gassy) planets.

Luckily, teams of Carnegie scientists including co-authors
Steve Shectman, Sharon Wang, Paul Butler, Jeff Crane, and Ian
Thompson, have been monitoring GJ 9827 with their Planet
Finding Spectrograph (PFS), so they were able to constrain the
masses of the three planets with data in hand, rather than
having to scramble to get many new observations of GJ 9827.

“Usually, if a transiting planet is detected, it takes months
if not a year or more to gather enough observations to measure
its mass,” Teske explained. “Because GJ 9827 is a bright star,
we happened to have it in the catalog of stars that Carnegie
astronomers been monitoring for planets since 2010. This was
unique to PFS.”

The spectrograph was developed by Carnegie scientists and
mounted on the Magellan Clay Telescopes at Carnegie’s Las
Campanas Observatory.

The PFS observations indicate that planet b is roughly eight
times the mass of Earth, which would make it one of the
most-massive and dense super-Earths yet discovered. The masses
for planet c and planet d are estimated to be about two and a
half and four times that of Earth respectively, although the
uncertainty in these two determinations is very high.

This information suggests that planet d has a significant
volatile envelope, and leaves open the question of whether
planet c has a volatile envelope or not. But the better
constraint on the mass of planet b suggests that that it is
roughly 50 percent iron.

“More observations are needed to pin down the compositions of
these three ,” Wang said. “But they do seem like
some of the best candidates to test our ideas about how
super-Earths form and evolve, potentially using NASA’s upcoming
James Webb Space Telescope.”

Explore further:

Three ‘super-Earth’ exoplanets orbiting nearby star

More information: Magellan/PFS Radial Velocities of GJ
9827, a late K dwarf at 30 pc with Three Transiting
Super-Earths, arXiv:1711.01359 [astro-ph.EP]