NOVEMBER/DECEMBER 2019 •SKYNEWS 11WATER WORLD?The first exoplanet discovered by TESS is Pi Mensae c, depicted here, which orbits
a Sun-like star in the southern hemisphere. It is twice the Earth’s radius and nearly five times the
Earth’s mass, and its bulk density is almost exactly that of pure water. ILLUSTRATION COURTESY NASA/MIT/TESS
rized as a “sub-Neptune,” this object is 2.
times bigger than Earth and at least 20 times
its mass, making it an unusually dense body.
Its solid interior could be shrouded in a thick
mantle of water vapour warmed to a tem-
perature of about 150°C by the parent star.
“It’s not a proper gas giant because it’s too
dense,” says Dragomir, “and it’s not a rocky
planet, so we’re left speculating.”
Taken together, the two members of
the HD 21749 system perfectly illustrate a
curious trend. Hardly any exoplanets found
to date are between 1.5 and 2 Earth radii
in size. One possible explanation for this
so-called radius valley is that it separates
smaller, close-in planets nearly stripped of
their atmospheres (by the host star’s ioniz-
ing radiation) from worlds slightly larger
and distant enough to hold onto a dense
layer of surrounding gas.
The HD 21749 planets lying on either
side of the radius valley excite Seager. “It’s
like comparing two people in the same
family,” she says. “The hope is that if you
can follow up with measurements of the
planets’ atmospheres, you may be able to
figure out why the valley exists.”
PRECIOUS FEW
TESS is not expected to yield a perfect
analogue of our own home—an Earth-
sized planet in an Earth-like orbit around a
Sun-like star. Because the spacecraft spends
less than one month on a search field (ex-
cept where fields overlap), it’s unlikely to
track a world with a 12-month period like
ours. Even if an Earth twin did turn up, it
would be difficult for astrono mers to learn
much from the comparatively shallow tran-
sits such a planet would produce.
A better bet is to troll for candidates
very close to cool stars. Planets in tight
orbits are easier to find (since they transit
more frequently), and their transits are
more pronounced relative to the dull glare
of their host stars. This raises the prospect
of filtering information about the planet
from the combined light of the planet and
star. A faint planetary spectrum, carefully
obtained and analyzed, could possibly yield
the fingerprint of an atmosphere.
Based on the Kepler stats, there might be
hundreds of ultratight exoplanets in side
TESS’s search area. Odds are that only about
10 of them are transiting their stars and are