- 4265 - BIG PLANET - way too big for its star? We have found a planet that is way too big for its star. Scientists love outliers. Outliers are nature’s way of telling us what its boundaries are and where its limits lie. Rather than being upset when an outlier disrupts their understanding, scientists feed on the curiosity that outliers inspire.
------------- 4265 - BIG PLANET - way too big for its star?
- It’s true in this
case of a new discovery of a massive planet orbiting a small star. That goes
against our understanding of how planets form, meaning our planet formation
model needs an update.
-
- A Neptune-mass
exoplanet orbiting a low-mass star, “LHS 3154”, is an M-dwarf, or red dwarf
star. It’s only 0.11 times as massive as the Sun, which is a normal mass for a
red dwarf.
-
- But what’s
surprising is the size of the planet orbiting the star. The planet is called
“LHS 3154b”, and it’s a monster compared to most planets orbiting red dwarfs.
It has at least 13.2 Earth masses. That places it in the same range as Neptune,
which has 17 Earth masses. LHS 3154b is also in a very close orbit, taking only
3.7 days to orbit the star.
-
- This exoplanet in
close orbit around a very low-mass star challenges all our formation
models. We wouldn’t expect a planet this
heavy around such a low-mass star to exist.
-
- It’s all about
stars and their protoplanetary disks.
When a star forms, it starts as a protostar in the center of a solar
nebula. As the star forms, a rotating disk of gas and dust called a
protoplanetary disk forms around the star. Dense knots form in the disk, and
this is how planets and planetesimals form.
-
- It’s a detailed
process and one we don’t entirely understand. But what scientists do know, or
thought they knew, is that the more mass there is in the disk, the more massive
the planets that can form. And the mass in the disk scales steeply with the
mass of the star.
-
- LHS 3154b and its
star don’t conform. There simply
shouldn’t have been enough mass in the protoplanetary disk for the planet to
form. But it’s out there, so now we
need to reexamine our understanding of how planets and stars form.
-
- It took a special
instrument to spot the massive planet.
The “Habitable Zone Planet Finder” or HPF, a spectrograph built at Penn
State. HPF is designed to detect planets orbiting cool stars that might have
liquid surface water. Small planets can be very difficult to detect around
large, bright stars like our Sun because the Sun’s light overpowers everything
else.
-
- But around smaller
cooler stars, planets close enough to have liquid water are much easier to
find. If the star is colder, then a
planet will need to be closer to that star if it is going to be warm enough to
contain liquid water. If a planet has a close enough orbit to its ultracool
star, we can detect it by seeing a very subtle change in the color of the
star’s spectra or light as it is tugged on by an orbiting planet.
-
- LHS 3154 is one of
the smallest stars ever found. It has only 11% of the mass of the Sun, and a
star needs to have 8% of the Sun’s mass to maintain fusion. It’s called a VLM star or “Very Low Mass”
star. Stars like LHS 3154 are hard to spot because they’re so small and so dim.
For that reason, there aren’t very many VLM stars in exoplanet surveys.
-
- HPF in 2020 found
signs of a planet around the star, tugging slightly on the star and giving it
the telltale wobble that can signal the presence of a planet. But M-dwarfs are
known to flare quite a bit which can be a false positive, so the astronomers
watched for a while. Once they saw that the signal was constant, they knew
they’d found a planet.
-
- M-dwarfs are known
to host lots of planets, but they’re typically much smaller than the huge
planets we see in our Solar System and around other stars similar to our Sun.
LHS 3154b is rare, an outlier, and that means there’s work to do explaining how
it formed there.
-
- Based on current
survey work with the HPF and other instruments, an object like the one
discovered is likely extremely rare.
LHS 3154b should have a heavy planetary core, according to the
measurements. But current models predict that the protoplanetary disk should
not have had enough material for it to form.
-
- A protoplanetary
disk contains both gas and dust. The ratio between the two helps explain what
mass the star will have and what masses the planets will have. The existence of
LHS 3154 b around the M-dwarf suggests that the dust-to-gas ratio of the disk
needs to be ten times higher than how scientists understand it.
-
- There are two
theories of how planets form. One is the core-accretion theory, where matter
forms a clump which accumulates more and more matter until a planetary core is
formed. The other is the gravitational instability model. It explains how
massive planets form in massive disks. Rather than accretion by pebbles, gas
collapses gravitationally in the disk to eventually form gas giants like
Jupiter and Saturn.
-
- Simulations of the
core-accretion mechanism couldn’t produce any planets as massive as LHS 3154b,
and simulations of the gravitational collapse mechanism couldn’t produce any
planets as small as LHS 3154b. Both
potential formation mechanisms require protoplanetary disks that have
substantially greater dust masses than are typically observed around very
low-mass stars.
-
- It’s possible that
protoplanetary disks, at least in some cases, can still accumulate matter from
the molecular cloud that the star formed from. So there’s basically another
reservoir of material for planets to form from. That’s one potential
explanation.
-
- Or, it’s possible that protoplanetary cores form
sooner than thought, within 1 million years after the host protostar. At that
young age, protoplanetary disks are expected to be more massive than at later
times. That could allow enough material to accrete rapidly, forming a gas
giant.
-
- A third
possibility is that we’re not accurately seeing what’s going on. If dust grows
into large pebbles around a star, infrared observations can struggle to see
it. Pebbles of that size would not be
detected by the millimeter observations used to estimate the overall dust
masses, causing them to underestimate.
-
- Whatever the
eventual explanation is, the discovery shows that the HPF is doing what it was
built to do. If it can find more of these outliers, we’ll be on our way to
figuring out more important details of the planet-forming process.
-
-
December 10, 2023
BIG PLANET - way too big for its star? 4265
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