Imagine being in a place where the winds are so strong that they move at the
speed of sound. That's just one aspect of the atmosphere on XO-3b, one of a
class of exoplanets (planets outside our solar system), known as hot
Jupiters. The eccentric orbit of the planet also leads to seasonal
variations hundreds of times stronger than what we experience on Earth. In a
recent paper, a McGill-led research team, provides new insight into what
seasons looks like on a planet outside our solar system. The researchers
also suggest that the oval orbit, extremely high surface temperatures (2,000
degrees C- hot enough to vaporize rock) and "puffiness" of XO-3b reveal
traces of the planet's history. The findings will potentially advance both
the scientific understanding of how exoplanets form and evolve and give some
context for planets in our own solar system.
Hot Jupiters are massive, gaseous worlds like Jupiter, that orbit closer to
their parent stars than Mercury is to the Sun. Though not present in our own
solar system, they appear to be common throughout the galaxy. Despite being
the most studied type of exoplanet, major questions remain about how they
form. Could there be subclasses of hot Jupiters with different formation
stories? For example, do these planets take shape far from their parent
stars—at a distance where it's cold enough for molecules such as water to
become solid—or closer. The first scenario fits better with theories about
how planets in our own solar system are born, but what would drive these
types of planets to migrate so close to their parent stars remains unclear.
To test those ideas, the authors of a recent McGill-led study used data from
NASA's retired Spitzer Space Telescope to look at the atmosphere of
exoplanet XO-3b. They observed eccentric seasons and measured wind speeds on
the planet by obtaining a phase curve of the planet as it completed a full
revolution about its host star.
Looking at atmospheric dynamics and interior evolution
"This planet is an extremely interesting case study for atmospheric dynamics
and interior evolution, as it lies in an intermediate regime of planetary
mass where processes normally neglected for less massive hot Jupiters may
come into play," says Lisa Dang, the first author of a paper published
recently in The Astronomical Journal, a Ph.D. student at McGill University's
Department of Physics. "XO-3b has an oval orbit rather than the circular
orbit of almost all other known hot Jupiters. This suggests that it recently
migrated toward its parent star; if that's the case, it will eventually
settle into a more circular orbit."
The eccentric orbit of the planet also leads to seasonal variations hundreds
of times stronger than what we experience on Earth. Nicolas Cowan, a McGill
professor explains: "The entire planet receives three times more energy when
it is close to its star during a brief sort of summer, than when it is far
from the star."
The researchers also re-estimated the planet's mass and radius and found
that the planet was surprisingly puffier than expected. They suggest and
that the possible source of this heating could be due to leftover nuclear
fusion.
Excess warmth and puffiness due to tidal heating?
Observations by Gaia, an ESA (European Space Agency) mission, found that the
planet is puffier than expected which indicate its interior may be
particularly energetic. Spitzer observations also hints that the planet
produces much of its own heat as XO-3b's excess thermal emission isn't
seasonal—it's observed throughout the year on XO-3b. It's possible that the
excess warmth is coming from the planet's interior, through a process called
tidal heating. The star's gravitational squeeze on the planet oscillates as
the oblong orbit takes the planet farther and then closer to the star. The
resulting changes in interior pressure produce heat.
For Dang, this unusual hot Jupiter provides an opportunity to test ideas
about which formation processes may producer certain characteristics in
these exoplanets. For example, could tidal heating in other hot Jupiters
also be a sign of recent migration? XO-3b alone won't unlock the mystery,
but it serves as an important test for emerging ideas about these scorching
giants.
Reference:
Lisa Dang et al, Thermal Phase Curves of XO-3b: An Eccentric Hot Jupiter at
the Deuterium Burning Limit, The Astronomical Journal (2021).
DOI: 10.3847/1538-3881/ac365f
Tags:
Space & Astrophysics