- 4248 - EXOPLANETS - many new discoveries! Scientists have four methods of detecting exoplanets. The “transit method” is the most prolific, but there’s also the radial “velocity method”, “astrometry”, and, “direct imaging”. Each one works differently, has different strengths and weaknesses, and is better at finding some planets around different types of stars than others.
------------------------- 4248 - EXOPLANETS - many new discoveries!
- Despite the fact
that we’ve discovered thousands of exoplanets.
Some types are harder to find than others. Some of the hardest ones to
find are the ones we most want to find.
-
- We’re mostly
interested in finding terrestrial planets in the habitable zones of nearby
stars. Which of the four methods will deliver the best results as we continue
our search?
-
- Terrestrial
planets in the habitable zone around nearby stars are of great interest and
provide a good sample for further characteristics of their habitability. Gaia’s Catalog of Nearby Stars has 2,234 main
sequence stars within 20 parsecs, or about 65 light years. This sample excluded
brown dwarfs and white dwarfs.
-
- Astronomers
determined what signals each of these 2,234 planets would send that would be
recieved by each of the four exoplanet detection methods: velocity amplitude
for radial velocity, transit probability and depth for the transit method,
stellar displacements for astrometry, and contrast and angular separation for
imaging.Then, predict the highest possible detection number of Earth-like
planets via different methods in the best-case hypothetical scenario.
-
- Why the focus on
nearby stars? No matter what method planet-hunters use, exoplanets are easier
to find the closer they are, and also easier to confirm. They’re also easier to
study with follow-up observations by telescopes like the JWST, which can
characterize exoplanet atmospheres and detect potential bio-signatures.
-
- The habitable zone
is rarely determined precisely because it depends on not only on the stellar
spectrum and stellar activity, but also on the planetary atmosphere. To simplify things, they used extended
habitable zones, which increases the occurrence rate of planets. The inner
boundary (IHZ) is set to “Recent Venus” and the outer boundary (OHZ) is set to
“Early Mars”.
-
-
“The average location is 0.22 and 0.43 “astronomical units”,
respectively,”. That may sound awfully close, but most stars are cool M dwarfs,
and are closer than our Sun’s, which is a much more luminous main sequence
star.
-
- The study concerns
the signals we’d receive from all of these planets with the four detection
methods.
-
- The transit method
has been our most prolific exoplanet detection method, and the probability of
detecting a signal reflects that. However, the transit method only works when
the geometry is right. The transiting planet has to pass between its star and
us. The other methods don’t have the same limitation.
-
- Of all the planets
detected in the simulations, the transit method first detected 24 of them.
However, since the transit method excels at finding planets close to their
stars, all 24 of them were too close to their stars.
-
- The radial
velocity method is most effective. Of
the 252 planets detected within the study’s 20 parsec radius, the RV method
using ESPRESSO is responsible for 221 of them. Four of them are Earth-like.
-
- The astrometry
method is one of the most sensitive ways to detect exoplanets. It relies on
detecting the minute wobbles in host stars created by their orbiting planets.
It’s how the ESA’s Gaia mission works, and in the near future, several more
observatories will use the astrometry method.
-
- There’s a critical
distinction between Gaia’s astrometry method, and astrometry used to find
exoplanets. Gaia measure stars using absolute astrometry, but finding
exoplanets uses relative astrometry. Astrometry measures displacement, which is
measured in microarcseconds.
-
- The researchers
simulated results from direct imaging, a method only in its infancy. It works
best on young planets still emitting infrared that are far from their stars.
There are sub-types of direct imaging, and together they’ve found 30 planets.
-
- Planet hunting
scientists have found six exoplanets within 20 parsecs with direct imaging.
That may not sound great, but the method has some advantages over RV and the
transit method. It’s not sensitive to an exoplanet’s inclination. But compared
to the other methods, direct imaging is more complex because it needs to take
both contrast and angular resolution into account. Direct imaging can use both
optical and infrared light.
-
- In the optical
band, the predicted number of exoplanets is 159, and 92 of them are around G
stars. Infrared direct imaging is even more productive, finding 191 exoplanets,
with 106 of them around G-type stars.
-
- The hunt for
exoplanets shows no signs of slowing down.
Finding Earth-like planets in habitable zones is one of our most
meaningful scientific pursuits. Six
exoplanets with sizes between Earth and Neptune have been in rhythm with each
other since they were born around the same star 4 billion years ago.
-
- All six planets
orbit the same star in resonance with each other, following an unwavering
rhythm that has lasted billions of years.
Because of this peculiar resonance, the outermost planet in the system
completes one full orbit of its star in the same time it takes the innermost
planet to complete six orbits. The remaining four planets follow similar
rhythmic patterns.
-
- All six planets
have orbits that are perfectly tuned to each other in resonance, which is rare.
The star is so bright. It's the brightest star yet discovered to host more than
four transiting planets.
-
- The six extrasolar
planets, or exoplanets, orbit a bright-yellow star located around 100
light-years from Earth in the constellation Coma Berenices. These distant
worlds have widths between that of Earth and the ice giant Neptune, making them
mini-Neptunes or "sub-Neptunes."
-
- Though sub-Neptunes
are the most common planets in the Milky Way, worlds of this size are absent
from the solar system. The fact that the
six planets are still rhythmically linked tells scientists a great deal about
the peaceful existence they have enjoyed thus far. This is because when
collapsing clouds of gas and dust around infant stars create planets, these
proto-worlds are often in resonance, but violent events wipe out this rhythm.
-
- We know that these
resonances form while the planets are forming, so this means that the planets
must have remained pretty much unchanged with no orbital shuffling,
collisions, or mass loss for billions of years.
-
- With the data we
had from TESS and CHEOPS alone, the orbits of the outer three planets were
unknown. But because we saw they were
in a resonance chain, this allowed us to perfectly predict their orbits, with
later observations revealing that our predictions were correct. That's the
first time these resonances have been instrumental in discovering new planets.
-
- The resonance means
the closest world to the star completes an orbit in 9.1 Earth days, the next
planet out orbits in 13.6 days, the third in 20.5 days, the fourth in 30.8
days, the fifth in 41 days, and the outermost planet in 54.7 days. So, for
every orbit of the outer star, the inner star completes six orbits. A 6:1 resonance. The other resonances between
different pairs of planets in the system are 3:2, 3:2, 3:2, 4:3 and 4:3.
-
- The planets are
two to three times the diameter of Earth.
They seem to be made of ice or rock, with low densities that point to
extended atmospheres of hydrogen and helium. There may even be more planets
orbiting HD 110067.
-
- Because all of the
planets pass in front of their star and this star is so bright, these worlds
have been some of the easiest exoplanets to characterize. These kinds of
systems are "worth their weight in gold.
The James Webb Space Telescope will soon be able to observe their
planetary atmospheres, potentially detecting molecules such as methane, which
could indicate underlying oceans.
-
-
November 30, 2023
4248
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