Upper
left: Schematic of the K2-132 system on the main sequence.
Lower left: Schematic of the K2-132 system now. The host star has become
redder and larger, irradiating the planet more and thus causing it to
expand. Sizes not to scale.
Main panel: Gas giant planet K2-132b expands as its host star evolves
into a red giant. The energy from the host star is transferred from the
planet's surface to its deep interior, causing turbulence and deep
mixing in the planetary atmosphere. The planet orbits its star every
nine days and is located about 2000 light years away from us in the
constellation Virgo. Credit: Karen Teramura, UH IFA
Maunakea, Hawaii - Since astronomers first measured the size of an extrasolar planet seventeen years ago, they have struggled to answer the question: how did the largest planets get to be so large?
Thanks
to the recent discovery of twin planets by a University of Hawaii
Institute for Astronomy team led by graduate student Samuel Grunblatt,
scientists are getting closer to an answer.
Gas giant planets are
primarily made out of hydrogen and helium, and are at least four times
the diameter of Earth. Gas giant planets that orbit scorchingly close to
their host stars are known as "hot Jupiters." These planets have masses
similar to Jupiter and Saturn, but tend to be much larger - some are
puffed up to sizes even larger than the smallest stars.
The
unusually large sizes of these planets are likely related to heat
flowing in and out of their atmospheres, and several theories have been
developed to explain this process. "However, since we don't have
millions of years to see how a particular planetary system evolves,
planet inflation theories have been difficult to prove or disprove,"
said Grunblatt.
To solve this issue, Grunblatt searched through
data collected by NASA's K2 Mission to hunt for hot Jupiters orbiting
red giant stars. These stars, which are in the late stages of their
lives, become themselves significantly larger over their companion
planet's lifetime. Following a theory put forth by Eric Lopez of NASA's
Goddard Space Flight Center, hot Jupiters orbiting red giant stars
should be highly inflated if direct energy input from the host star is
the dominant process inflating planets.
The search has now
revealed two planets, each orbiting their host star with a period of
approximately nine days. Using stellar oscillations to precisely
calculate the radii of both the stars and planets, the team found that
the planets are 30 percent larger than Jupiter.
Observations
using the W. M. Keck Observatory on Maunakea, Hawaii also showed that,
despite their large sizes, the planets were only half as massive as
Jupiter. Remarkably, the two planets are near twins in terms of their
orbital periods, radii, and masses.
Using models to track the
evolution of the planets and their stars over time, the team calculated
the planets' efficiency at absorbing heat from the star and transferring
it to their deep interiors, causing the whole planet to expand in size
and decrease in density. Their findings show that these planets likely
needed the increased radiation from the red giant star to inflate, but
the amount of radiation absorbed was also lower than expected.
It
is risky to attempt to reach strong conclusions with only two examples.
But these results begin to rule out some explanations of planet
inflation, and are consistent with a scenario where planets are directly
inflated by the heat from their host stars. The mounting scientific
evidence seems to suggest that stellar radiation alone can directly
alter the size and density of a planet.
Our own Sun will
eventually become a red giant star, so it's important to quantify the
effect its evolution will have on the rest of the Solar System.
"Studying how stellar evolution affects planets is a new frontier, both
in other solar systems as well as our own," said Grunblatt. "With a
better idea of how planets respond to these changes, we can start to
determine how the Sun's evolution will affect the atmosphere, oceans,
and life here on Earth."
The search for gas giant planets around
red giant stars continues since additional systems could conclusively
distinguish between planet inflation scenarios. Grunblatt and his team
have been awarded time with the NASA Spitzer Space Telescope to measure
the sizes of these twin planets more accurately. In addition, the search
for planets around red giants with the NASA K2 Mission will continue
for at least another year, and NASA's Transiting Exoplanet Survey
Satellite (TESS), launching in 2018, will observe hundreds of thousands
of red giants across the entire sky.
"Seeing double with K2:
Testing re-inflation with two remarkably similar planets orbiting red
giant branch stars" has been published in November 27th edition of The Astronomical Journal as and is available online at http://iopscience.iop.org/article/10.3847/1538-3881/aa932d.
Contacts
Sam Grunblatt
skg3@hawaii.edu
Cell: 845-430-4603
Dr. Daniel Huber
huberd@hawaii.edu
Office: 808-956-8573
Dr. Roy Gal
Media Contact
Office: 808-956-6235
Cell: 301-728-8637
rgal@ifa.hawaii.edu
Contacts
Sam Grunblatt
skg3@hawaii.edu
Cell: 845-430-4603
Dr. Daniel Huber
huberd@hawaii.edu
Office: 808-956-8573
Dr. Roy Gal
Media Contact
Office: 808-956-6235
Cell: 301-728-8637
rgal@ifa.hawaii.edu
About W. M. Keck Observatory
The W. M. Keck Observatory telescopes are
among the most scientifically productive on Earth. The two, 10-meter
optical/infrared telescopes on the summit of Maunakea on the Island of Hawaii
feature a suite of advanced instruments including imagers, multi-object
spectrographs, high-resolution spectrographs, integral-field spectrometers, and
world-leading laser guide star adaptive optics systems.
Some of the data presented herein were
obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization
operated as a scientific partnership among the California Institute of
Technology, the University of California, and the National Aeronautics and
Space Administration. The Observatory was made possible by the generous
financial support of the W. M. Keck Foundation.
The authors wish to
recognize and acknowledge the very significant cultural role and reverence that
the summit of Maunakea has always had within the indigenous Hawaiian
community. We are most fortunate to have the opportunity to conduct
observations from this mountain.
Article Summary
A University of Hawaii Institute for Astronomy-led team
discovers unusually large twin exoplanets. Desipite their large sizes,
observations using the W. M. Keck Observatory on Maunakea, Hawaii show
that the planets are only half as massive as Jupiter. The discovery
could help astronomers understand how the largest planets became so
large.
Source: W.M. Keck Observatory
