Exoplanet Stuff

[lpetrich]

The Sun's gravitational field as a lens: [2002.11871] Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission

One needs to be at least 548 AU away, and getting there will be very difficult. Spacecraft escaping the Solar System  List of artificial objects leaving the Solar System - the fastest departing object is Voyager 1, at 3.6 AU/yr. It will take over 150 years to reach the minimum distance for lensing.


Peering Into the Atmosphere of the Hottest Planet Known

KELT-9b is an extreme world. Clocking in with a dayside temperature of more than 4,500 K (~7,600 °F), it is the hottest planet known — hotter than many stars! This ultra-hot Jupiter orbits at a mere 0.035 AU from its scalding A- or B-type host star, whizzing around its host in just 1.5 days.

The intense radiation bombarding KELT-9b almost certainly takes a toll: this energetic light should dissociate molecules into their component atoms and ionize metals in the hot atmosphere, and it may inflate the envelope of hydrogen gas around the planet to the point where the hot gas escapes.
atmospheric absorption lines

Observed and modeled Hα (top) and Ca II (bottom three) spectral lines in the atmosphere of the ultra-hot Jupiter KELT-9b. [Adapted from Turner et al. 2020]
Turner and collaborators explore the extreme conditions in KELT-9b’s atmosphere with high-resolution transmission spectra taken with the CARMENES instrument on the Calar Alto 3.5-m telescope in Spain.

They detected once-ionized calcium: Ca II.

 

[lpetrich]

Earth’s own evolution used as guide to hunt exoplanets | Cornell Chronicle
noting
High-resolution Transmission Spectra of Earth Through Geological Time - IOPscience

Kaltenegger and her team created atmospheric models that match the Earth of 3.9 billion years ago, a prebiotic Earth, when carbon dioxide densely cloaked the young planet. A second throwback model chemically depicts a planet free of oxygen, an anoxic Earth, going back 3.5 billion years. Three other models reveal the rise of oxygen in the atmosphere from a 0.2% concentration to modern-day levels of 21%.

“Our Earth and the air we breathe have changed drastically since Earth formed 4.5 billions years ago,” Kaltenegger said, “and for the first time, this paper addresses how astronomers trying to find worlds like ours, could spot young to modern Earth-like planets in transit, using our own Earth’s history as a template.”

That's great. The Earth's past is an underappreciated resource in exoplanet studies. That's because the Earth's past surface and atmosphere can be estimated from its geological record, making it a known quantity. This, in turn, expands our catalog of known planetary surfaces and atmospheres.

The Earth, our homeworld, has the best-available record of its past, but we now have plenty of clues about the pasts of the Moon and Mars.

 

[lpetrich]

Catastrophic Collisions in Protoplanetary Disks

Some of the most spectacular images to come out of observatories like the Atacama Large Millimeter/submillimeter Array (ALMA) or the Very Large Telescope (VLT) are detailed views of protoplanetary disks. These disks of gas and dust around young stars aren’t just smooth and featureless; instead, they exhibit arcs, rings, gaps, and spirals. What causes this impressive array of structure?

Scientists have primarily focused on two explanations:

• The structures are caused by the perturbations of massive baby planets interacting with the disk as they orbit.
• The structures are generated by various instabilities within the disk that cause the gas and dust to clump.

A new study has now put forward an alternative explanation: the structures are the result of catastrophic, destructive collisions of planetesimals within the disk.

Catastrophic Events in Protoplanetary Disks and Their Observational Manifestations - IOPscience

 

[lpetrich]

ESA - Cheops observes its first exoplanets and is ready for science

Cheops, ESA’s new exoplanet mission, has successfully completed its almost three months of in-orbit commissioning, exceeding expectations for its performance. The satellite, which will commence routine science operations by the end of April, has already obtained promising observations of known exoplanet-hosting stars, with many exciting discoveries to come.

...
“The pointing is extremely stable: this means that while the telescope observes a star for hours while the spacecraft moves along its orbit, the image of the star remains always within the same group of pixels in the detector,” explains Carlos Corral van Damme, ESA’s System Principal Engineer for Cheops.

“Such a great stability is a combination of the excellent performance of the equipment and of the bespoke pointing algorithms, and will be especially important to fulfill the scientific objectives of the mission. The thermal stability of the telescope and the detector has also proven to be even better than required,” adds Carlos.

The spacecraft then observed a star with a known exoplanet: HD 93396 with KELT-11b.

Mass = 0.2 MJup, radius = 1.4 RJup, sma = 0.063 AU, per = 4.74 d, temp = 1700 K

The measurements made by Cheops are five times more accurate than those from Earth, explains Willy Benz, Principal Investigator of the Cheops mission consortium, and professor of astrophysics at the University of Bern. “That gives us a foretaste for what we can achieve with Cheops over the months and years to come,” he says.

Spacecraft operations have not been affected by the COVID-19 pandemic, happy to say.

Upcoming targets for the spacecraft:

• Hot super-Earth 55 Cancri e - mass = 8 MEar, radius = 1.9 REar, sma = 0.015 AU, per = 0.737 d, temp = 2000 K
• Warm Neptune GJ 436 b - mass = 25 MEar, radius = 4.1 REar, sma = 0.028 AU, per = 2.644 d, temp = 700 K
• A white dwarf

 

[Jimmy Higgins]

Fomalhaut b turns out to not have been an exoplanet after all... damn poser! The Vanilla Ice of exo-planets was original discovered in 2004, back when discovering an Exoplanet was rare and awesome. However, like Vanilla Ice, it's shine has literally faded away and has now been exposed as not actually being a planet at all (or if it was, a Trump like figure was in charge of its stewardship).

What astronomers thought was a planet beyond our solar system has now seemingly vanished from sight. Though this happens in science fiction, such as Superman's home planet Krypton exploding, astronomers are looking for a plausible explanation.

One interpretation is that, rather than being a full-sized planetary object, which was first photographed in 2004, it could instead be a vast, expanding cloud of dust produced in a collision between two large bodies orbiting the bright nearby star Fomalhaut. Potential follow-up observations might confirm this extraordinary conclusion.

"These collisions are exceedingly rare and so this is a big deal that we actually get to see one," said András Gáspár of the University of Arizona, Tucson. "We believe that we were at the right place at the right time to have witnessed such an unlikely event with NASA's Hubble Space Telescope."

Only time will tell if Fomalhaut b will get their own home improvement show on TLC.

 

[skepticalbip]

Fomalhaut b turns out to not have been an exoplanet after all... damn poser! The Vanilla Ice of exo-planets was original discovered in 2004, back when discovering an Exoplanet was rare and awesome. However, like Vanilla Ice, it's shine has literally faded away and has now been exposed as not actually being a planet at all (or if it was, a Trump like figure was in charge of its stewardship).

How did our astronomers miss the obvious? Once the Fomalhaut-bers realized that they had been detected, they turned on their planetary invisibility cloak. The Klingons and Romulans have been using that trick for decades now.

 

[lpetrich]

This massive exoplanet is the 'king' of its solar system - CNN
noting
The Discovery of the Long-Period, Eccentric Planet Kepler-88 d and System Characterization with Radial Velocities and Photodynamical Analysis - IOPscience

That's Kepler-88 d, a planet with 3 times Jupiter's mass that orbits its star with a period of about 4 Earth years. Its orbit eccentricity is 0.41, making its farthest distance over twice its closest distance. The star has two precisely-detected planets, b, at nearly 10 Earth masses and nearly 4 Earth radii, orbiting with a period of nearly 10 Earth days, and c, at a bit over 0.6 Jupiter masses, orbiting with a period of a bit over 22 Earth days.

Planet b was the first to be seen, with transits detected with the Kepler space telescope, and it was observed to go back and forth in its orbit with a period of 1.5 Earth years. This was evidence of an orbital resonance with another planet, and that planet, planet c, was detected with radial-velocity observations done at the Haute-Provence Observatory. Planet d was also detected with radial-velocity observations, this time with the Keck Telescope on Mauna Kea, Hawaii.

Consulting exoplanet.eu the star Kepler-88 has an estimated mass of 1.08 solar masses, and I find orbit distances b: 0.10 AU, c: 0.16 AU, d: 2.5 AU (1.5 to 3.4 AU)

Planet d, though 3 times Jupiter's mass, is likely about as large as Jupiter itself, using planetary-structure calculations.

 

[lpetrich]

Astronomers Find a Beautiful Six-Planet System in Almost Perfect Orbital Harmony
Around star HD 158259.

Periods: b 2.178 d, c 3.432 d, d 5.198 d, e 7.951 d, f 12.028 d

Neighboring periods' ratios: 1.576, 1.515, 1.530, 1.513

Each planet is in a 2:3 resonance with its neighbor, the inner one of each pair orbiting a little more than 3 times when the outer one orbits 2 times.


Two (potentially) exciting new exoplanet discoveries | Space | EarthSky

A new potentially habitable Earth-sized world. The first planet is called Kepler-1649c. It’s 300 light-years from Earth. Astronomers found it in old data from the Kepler space telescope mission. It had been missed when the data was analyzed the first time around by a computer algorithm called Robovetter, but spotted later on when double-checked by scientists in the Kepler False Positive Working Group (FPWG). As it turned out, this this false positive really was a planet after all.

...
Kepler-1649c is almost the same size as Earth, only 1.06 times larger. Orbiting in its star’s habitable zone, where liquid water could exist, it receives about 75% the amount of light that Earth does from the sun, and so its temperature is estimated to be similar to Earth’s as well.

More on this planet: Earth-Size, Habitable Zone Planet Found Hidden in Early Kepler Data | NASA
Its equilibrium temperature is around -12 C, something like the Earth's in its early days.

Kepler-1649c orbits its star in only 19.5 Earth days. This star also has at least one other known planet, Kepler-1649b, which is similar in size but orbits the star at only half the distance of Kepler-1649c. This is similar to how Venus orbits the sun at about half the distance that Earth does.

The star is a red-dwarf flare star.

 

[lpetrich]

A second possible planet, possibly with a huge ring system, orbiting Proxima Centauri. In another interesting development, scientists may have directly imaged a new planet orbiting the closest star to our sun, Proxima Centauri. Those results, from the SPHERE instrument on the Very Large Telescope (VLT), have just been published in the paper so far, without an official announcement, but it has stirred up conversations on Twitter. Astronomer Phil Plait wrote an excellent update about it for SYFY Wire on April 16.

Noting
Bad Astronomy | Is this an actual image of a planet orbiting the nearest star? Maaaaaaaybe.

[2004.06685] Searching for the near infrared counterpart of Proxima c using multi-epoch high contrast SPHERE data at VLT

A Habitable-zone Earth-sized Planet Rescued from False Positive Status - IOPscience -- that planet does indeed orbit a red-dwarf star with 0.2 solar masses.

Dedicated Team of Scientists Discover Habitable-Zone Earth-Size Planet in Kepler Data | SETI Institute

This young Canadian scientist has found 21 new planets—and counting - Macleans.ca - "Inspired by Star Trek to pursue astronomy, 26-year-old Michelle Kunimoto is one of the whip-smart young scientists who just might help us find the first evidence of extraterrestrial life"

It was ST:TOS

She fell hard for the now-campy 1960s show with its flimsy props, gauzy female costumes and flip-phone communicators. And she bonded irrevocably with the intrepid Captain Kirk and his implacable sidekick, Mr. Spock, as well as with their mission on the Starship Enterprise to “boldly go where no man has gone before.”

But it wasn’t only the search for the final frontier that grabbed her; it was also the show’s philosophical bent. “It really instilled a sense of curiosity in me. The entire world is represented on the bridge of the Enterprise working peacefully together,” she says.

 

[lpetrich]

For exoplanet research, the exploration of the Solar System has brought several data points into closer view, but there are additional data points that have long been available -- data points of the Earth's past. We also have some for the Moon and Mars, even if much less detailed ones.

Visible Paleo-Earth - Planetary Habitability Laboratory @ UPR Arecibo - maps of reconstructed continent positions. As to cloud cover, one should not blindly use present-day averages. One will have to do climate modeling, but here also, one has data from the past. Chris Scotese's PALEOMAP Home Page has Climate History maps. As a very rough approximation, the Earth has four kinds of climate zones between its equator and its poles:

• (Poles)
• Polar - dry
• Temperate - wet
• Subtropical - dry
• Tropical - wet
• (Equator)

This pattern holds more-or-less true over all the time that Chris Scotese has climate-reconstruction maps: the Early Cambrian (some 500 mya) to the present. Also interesting is that there have been tropical forests over nearly all the time that large macroscopic land vegetation has existed -- since the Middle Devonian, some 380 million years ago. There are scattered deposits from the Early Devonian, 400 million years ago, and none before then.

That means that the carbonate-silicate thermostat has successfully regulated the Earth's temperature over all that time, at least over its timescale of roughly a million years. Mass extinctions have been much faster events, too fast for this geochemical effect to compensate for them.

 

[lpetrich]

Simulations of Light Curves from Earth-like Exoplanets - Planetary Habitability Laboratory @ UPR Arecibo - contains simulations of Earth ones. Both present-day Earth and 500-million-year-old Earth.

For the present day, the Sahara Desert makes the Earth look neutral-colored when it is in full view in its local midday. Otherwise, the Earth looks bluish - a pale blue dot.

Earth Climate Models and the Search for Life on Other Planets - Astrobiology Magazine - applying climate-modeling software for exoplanets' possible atmospheres - going through a variety of possible atmospheres.

Alternative Earths - Astrobiology Magazine
NASA Astrobiology Institute - Alternative Earths: Explaining Persistent Inhabitation on a Dynamic Early Earth
Earth’s own evolution used as guide to hunt exoplanets - Astrobiology Magazine
noting journal paper
High-resolution Transmission Spectra of Earth Through Geological Time - IOPscience

Uses an underappreciated source of data: our planet's past states. I say underappreciated because our planet looked different in the past, and if one looks far enough, it had a very different atmosphere and biosphere. That journal paper discussed something that could be observable in transit observations: the Earth's apparent size as a function of light wavelength, its transmission spectrum. One would observe a planet's transit in different wavelength bands then work out the apparent size of the planet in each of them, giving its transmission spectrum.

We chose atmosphere models representative of five geological epochs of Earth's history, corresponding to a prebiotic high CO2-world 3.9 billion years ago (Ga), an anoxic world around 3.5 Ga, and 3 epochs through the rise of oxygen from 0.2% to present atmospheric levels of 21%. Our transmission spectra show atmospheric spectral features, which would show a remote observer that Earth had a biosphere since about 2 billion years ago.

The atmosphere parameters that they used:

Time	SlFx	CO2	CH4	O2	O3	N2O
Now	1.00	3.65e-4	1.65e-6	2.1e-1	3.00e-8	3.00e-7
0.5 - 0.8	0.95	1.00e-2	4.15e-4	2.1e-2	2.02e-8	9.15e-8
1.0 - 2.0	0.87	1.00e-2	1.65e-3	2.1e-3	7.38e-9	8.37e-9
3.5	0.77	1.00e-2	1.65e-3	1.00e-13	2.55e-19	0
3.9	0.75	1.00e-1	1.65e-6	1.00e-13	2.55e-19	0

Time: billions of years before the present.
SlFx: solar flux ("solar constant")
Numbers: "chemical mixing ratios"
Total pressure for all the times: 1 bar

No discussion of atmospheric nitrogen or argon. Their present fractions are 0.78 N2 and 0.01 Ar - by volume.

 

[lpetrich]

Another bit of past-Earth data is amount of continental crust. It has grown over the Earth's history, growing from nothing to about 70% of present volume at 3 Ga, then more slowly growing to its present volume. Rates of generation and growth of the continental crust - ScienceDirect

Early continents were likely relatively small, about the size of present-day large islands like Madagascar.

Reddit has r/exoplanets and also r/astrobiology and r/exolife

The Weirdest Solar System We've Found So Far? You May Be In It – Exoplanet Exploration: Planets Beyond our Solar System

Before we found the first exoplanets — planets orbiting other stars — it seemed reasonable to suppose that other planetary systems looked like ours: small, rocky planets close to a Sun-like star, a big Jupiter and a few other gas giants farther out.

But after a quarter century of discovery revealing thousands of exoplanets in our galaxy, things look very different. In a word, we are “weird” — at least among the planetary systems found so far.

There are plenty of planets that we've observed that have no Solar-System counterparts: Hot Jupiters, Super-Earths and Mini-Neptunes.

"In all this variety, we’ve seen nothing yet that quite resembles our own setup: a Sun-like star with a retinue of rocky planets close in and more distant gas giants (including a domineering Jupiter)." - part of the problem is the difficulty of detecting similar planetary systems, a difficulty that the article did not mention.

 

[skepticalbip]

Another bit of past-Earth data is amount of continental crust. It has grown over the Earth's history, growing from nothing to about 70% of present volume at 3 Ga, then more slowly growing to its present volume. Rates of generation and growth of the continental crust - ScienceDirect

Early continents were likely relatively small, about the size of present-day large islands like Madagascar.

Reddit has r/exoplanets and also r/astrobiology and r/exolife

The Weirdest Solar System We've Found So Far? You May Be In It – Exoplanet Exploration: Planets Beyond our Solar System

Before we found the first exoplanets — planets orbiting other stars — it seemed reasonable to suppose that other planetary systems looked like ours: small, rocky planets close to a Sun-like star, a big Jupiter and a few other gas giants farther out.

But after a quarter century of discovery revealing thousands of exoplanets in our galaxy, things look very different. In a word, we are “weird” — at least among the planetary systems found so far.

There are plenty of planets that we've observed that have no Solar-System counterparts: Hot Jupiters, Super-Earths and Mini-Neptunes.

"In all this variety, we’ve seen nothing yet that quite resembles our own setup: a Sun-like star with a retinue of rocky planets close in and more distant gas giants (including a domineering Jupiter)." - part of the problem is the difficulty of detecting similar planetary systems, a difficulty that the article did not mention.

Right. Exoplanets comparable in size and mass to our system's rocky planets are extremely difficult to detect in distant systems so the planets detected so far are primarily the larger, more massives bodies. This doesn't necessarily mean their are few small rocky planets in those systems, just that our current technology makes them damn hard to detect if they are there.

 

[lpetrich]

I'll assess the observability of both Earth and Jupiter with exoplanet-detection methods.

• Luminosity (absolute visual magnitude): S +4.83, E +29.15, +J 23.59
• Luminosity (visual, Sun fraction): E: 1.9*10^(-10), J: 3.1*10^(-8)
• Luminosity (10-mcm IR, Sun fraction): E: 1.5*10^(-6), J: 2.5*10^(-4)
• Deviation (at 10 pc): E: 0.30 mcas, J: 0.50 mas
• Radial Velocity: E: 0.089 m/s, J: 12.5 m/s
• Transit Depth: E: 8.4*10^(-5), J: 9.9*10^(-3)
• Transit Probability: E: 4.7*10^(-3), J: 8.9*10^(-4)
• Period: E: 1 yr, J: 11.86 yr

Sources: some Wikipedia articles, like
 Absolute magnitude - calculated for Earth and Jupiter at 90d phase angle
The IR luminosity I estimated by multiplying the visual luminosity by (10 mcm (IR peak) / 0.5 mcm (visual peak) )^3
mcm = micrometer, mas = milliarcsecond, mcas = microarcsecond

So it's very difficult to observe the Solar System from across interstellar space, even with present-day exoplanet-observation systems.

 

[lpetrich]

Rescuing an Overlooked Planet

In every batch of detections from the Kepler spacecraft, some transit signals get relegated to “false positive” status by an automated vetting pipeline. How do we ensure that real exoplanet detections don’t accidentally get discarded by the pipeline?

The Kepler False Positive Working Group is on the case — and they just rescued quite a find from being relegated to a false-positive fate.

...
Kepler-1649c is a planet the same size as Earth that orbits around its M-dwarf host star once every ~20 days, placing it firmly in its host star’s habitable zone. Its star also hosts a previously known inner planet that appears to be equivalent to Venus in its size and the amount of flux it receives.

A Habitable-zone Earth-sized Planet Rescued from False Positive Status - IOPscience

Searching Pulsars for Planets
noting
The NANOGrav 11 yr Data Set: Constraints on Planetary Masses Around 45 Millisecond Pulsars - IOPscience

No success in observing planets for any of them, with upper limits as small as the mass of the Moon.

 

[lpetrich]

There’s Metal in the Air

With atmospheric temperatures ranging from roughly 3,000 to 6,500 degrees Fahrenheit, ultra-hot Jupiters are ready-made laboratories for extreme planetary science. For instance, any molecules in the atmosphere of an ultra-hot Jupiter will be broken down into their component atoms and ions. So what can be found in the atmosphere of the ultra-hot Jupiter WASP-121 b?

Neutral Cr and V in the Atmosphere of Ultra-hot Jupiter WASP-121 b - IOPscience

Are We Watching a Planet Disintegrate?
noting
A Possible Transit of a Disintegrating Exoplanet in the Nearby Multiplanet System DMPP-1 - IOPscience

Cloudy Challenges to Exploring Exoplanet Atmospheres

Komacek and collaborators examine the results of three-dimensional general circulation models of tidally locked planets orbiting M-dwarf stars. The authors generate simulated transit spectra for planets with different rotation rates, incoming starlight, surface pressure, radius, and more. They then explore whether the presence of clouds in the atmospheres of these planets will impede JWST’s ability to detect the water vapor features that arise from lower in the atmosphere.

The result? Bad news. The authors find that the presence of clouds significantly mutes spectral features; when clouds are present, JWST would typically need to observe 10–100 times more transits of the planet to be able to detect the water vapor features in its atmosphere.

This impact is especially strong for slower-rotating planets. The climate models show that planets with periods longer than about 12 days form significantly more cloud cover on their daysides, due to more water vapor being carried to high altitudes. This leads to even stronger muting of these planets’ spectral features.

Clouds will Likely Prevent the Detection of Water Vapor in JWST Transmission Spectra of Terrestrial Exoplanets - IOPscience

One finds the transmission spectrum of an exoplanet's atmosphere by finding the planet's apparent size in different wavelengths as it transits. More extinction (absorption, scattering) makes a planet look larger.

 

[lpetrich]

Scientists reveal first-ever photo of a solar system like ours - CBS News

Scientists have captured the first direct image of a solar system that closely resembles our own. The new image is a family portrait of sorts, showing two giant exoplanets orbiting a young, sun-like star, roughly 300 light years away.

The picture was taken using the European Southern Observatory's Very Large Telescope, located in Chile's Atacama Desert. ...

The star, known as TYC 8998-760-1 and located in the Southern constellation of Musca, is only 17 million years old, which researchers called a "very young version of our own sun." Comparatively, the sun is roughly 4.6 billion years old.

Two Directly Imaged, Wide-orbit Giant Planets around the Young, Solar Analog TYC 8998-760-1 - IOPscience
Their projected distances are 160 AU and 320 AU - and they have plenty of leftover heat from their formation, enough for them to be seen. Their masses are at 14 +- 3 MJup and 6 +-1 MJup, likely from cooling models.


Scientists identify rain of molten iron on distant exoplanet | Science | The Guardian - "Conditions on Wasp-76b in Pisces include temperatures of 2,400C and 10,000mph winds"

Wasp-76b, which is 640 light years away in the constellation of Pisces, is an ultra-hot gas giant. It orbits its star at about 3% of the distance between the Earth and the Sun, resulting in scorching surface temperatures and the weird phenomenon of molten iron falling from the sky.

...
On the day side, which is 1,000 degrees hotter, molecules separate into atoms, and iron evaporates into the atmosphere to form metallic clouds. The extreme temperature difference between the day and night sides produces ferocious winds that carry the iron vapour to the cooler night side, where temperatures decrease to about 1,500C and the iron condenses and falls as rain that constantly peppers the planet’s gas surface and vanishes beneath it.

The article in Nature magazine

 

[lpetrich]

Convergence Time in Simulating Extrasolar Atmospheres | News | Astrobiology - that's how long one runs the simulation, in simulated days

Researchers supported in part by the NASA Astrobiology program have reported gray gas general circulation model (GCM) simulations of a tidally locked sub-Neptune planet. Gliese 1214b (GJ 1214b) was discovered in 2009 orbiting the star Gliese 1214, and is located roughly 48 light-years from the Sun. Because GJ 1214b is relatively nearby and transits its star, the planet has been a target for atmospheric studies.

Results of the new study agree with previous work that was performed on timescales of 1000-10,000 Earth days. However, when the timescales of the simulation were increased (50,000 – 250,000 Earth days), significant differences arose in atmospheric circulation and atmospheric features. The authors suggest that previous studies of tidally-locked exoplanets may need to be revisited, taking convergence time into consideration.

Extremely Long Convergence Times in a 3D GCM Simulation of the Sub-Neptune Gliese 1214b - IOPscience - "We demonstrate that this long convergence time is related to the long radiative timescale of the deep atmosphere and can be understood through a series of simple arguments."

The Exoplanets Channel - YouTube

Close Encounters in the Milky Way — and What They Mean for Planets - Sky & Telescope - Sky & Telescope

New research suggests that more than half of the bulge stars undergo a relatively close flyby — close enough to breeze the outskirts of what would be their solar system — as much as 35 times in a billion years, a frequency that could have a significant effect on how young worlds grow.

... But in a new study graduate student Moiya McTier (Columbia University) has reported that the bulge might not be the best environment for growing worlds.

After simulating how bulge stars interact with each other over time, she found that roughly 80% of them have close encounters within the lifetime of the universe. In fact, half the bulge stars will pass within 1,000 astronomical units (a.u.) of another star, brushing closer than the inner edge of the solar system's Oort cloud, not just once but dozens of times over the course of a billion years.

Nearly a third of bulge stars have a visitor within 100 a.u., just outside the Kuiper Belt where Pluto and other dwarf planets live. A handful, less than 1 in 5,000, have company swing by within 10 a.u., just past the distance of Saturn.

8 in 10 Stars in the Milky Way Bulge experience stellar encounters within 1000 AU in a gigayear | Monthly Notices of the Royal Astronomical Society | Oxford Academic

 

[lpetrich]

An Exposed Planetary Core at TOI-849

Exceptions tweak our thinking, and do have NGTS-4b, a world 20 percent smaller than Neptune and 20 times as massive as Earth in a 1.3-day orbit around a K-dwarf (see Into the Neptunian Desert for more on this one, which is now joined by an even more puzzling object).

For today we learn of the discovery of a world of roughly Neptune’s mass with an orbital period of a scant 18 hours, and researchers reporting the discovery in Nature suggest that we are actually looking at a ‘failed’ gas giant, an exposed planetary core. We can thank TESS (Transiting Exoplanet Survey Satellite) for the original data on this one, which is labeled TOI 849b. The object orbits a star much like our own about 730 light years from the Sun.

Follow-up observations with the HARPS spectrograph at ESO’s La Silla Observatory in Chile complement the TESS transit data with the radial velocity readings used to determine that the object is two to three times more massive than Neptune but also incredibly dense. It must, then, consist largely of iron, rock and water, with little hydrogen and helium.

Either a failed gas giant or one that has been stripped of its volatile outer layers by being near its star.

Many Worlds: Multiple Super-Earths Discovered Around a Nearby and Unusually Quiet Red Dwarf | astrobites

Gliese 887 is a nearby red dwarf (or M dwarf) star about half the size of our own Sun. Red dwarfs are the smallest, coolest, and by far the most common type of main sequence star in the Galaxy and are found in abundance near the Earth. At a distance of only about 11 light years, Gliese 887 is one of the 12 closest stars to the Sun and also the brightest red dwarf visible from the Earth.

...
The researchers detected the unmistakeable periodic signals of two exoplanets – Gliese 887b and Gliese 887c – located close to the host star with orbital periods of 9.3 and 21.8 days, and masses of 4.2 ± 0.6 and 7.6 ± 1.2 times that of the Earth respectively. These figures place both planets within the super-Earth category, defined as planets which have a mass exceeding that of the Earth but substantially lower than our own ice giants, Uranus and Neptune. These two newly discovered planets may have a rocky composition and are located close to the inner edge of the stars habitable zone. The team also detected hints of a third planet located further out and potentially within the habitable zone, though they acknowledge that this could also be a false signal related to small stellar variations with a similar period.

Red Dots – A search for terrestrial planets around the nearest red dwarfs - including Proxima Centauri, Barnard's Star, and Ross 154 / Gliese 729
 Lacaille 9352 - Gliese 887
A multiplanet system of super-Earths orbiting the brightest red dwarf star GJ 887 | Science
[2006.16372] A multiple planet system of super-Earths orbiting the brightest red dwarf star GJ887
Why boring could be good for this star’s two intriguing planets - "A nearby red dwarf doesn’t emit flares or harmful radiation — so its planets might have atmospheres."

 

[lpetrich]

Habitable Planet Reality Check: Kepler 1649c & The Implications for Earth-Size Exoplanets in Red Dwarf Habitable Zones | Drew Ex Machina - another article on that earlier overlooked planet.

Three Nearby Exoplanets to Explore - Sky & Telescope - Sky & Telescope - another article on GJ 887's 2 or 3 recently-discovered planets

News | NASA's TESS, Spitzer Missions Discover a World Orbiting a Unique Young Star

For more than a decade, astronomers have searched for planets orbiting AU Microscopii, a nearby star still surrounded by a disk of debris left over from its formation. Now scientists using data from NASA's Transiting Exoplanet Survey Satellite (TESS) and retired Spitzer Space Telescope report the discovery of a planet about as large as Neptune that circles the young star in just over a week.

The system, known as AU Mic for short, provides a one-of-kind laboratory for studying how planets and their atmospheres form, evolve and interact with their stars.

"AU Mic is a young, nearby M dwarf star. It's surrounded by a vast debris disk in which moving clumps of dust have been tracked, and now, thanks to TESS and Spitzer, it has a planet with a direct size measurement," said Bryson Cale, a doctoral student at George Mason University in Fairfax, Virginia. "There is no other known system that checks all of these important boxes."

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AU Mic is a cool red dwarf star with an age estimated at 20 million to 30 million years, making it a stellar infant compared to our Sun, which is at least 150 times older. The star is so young that it primarily shines from the heat generated as its own gravity pulls it inward and compresses it. Less than 10% of the star's energy comes from the fusion of hydrogen into helium in its core, the process that powers stars like our Sun.

The system is located 31.9 light-years away in the southern constellation Microscopium. It's part of a nearby collection of stars called the Beta Pictoris Moving Group, which takes its name from a bigger, hotter A-type star that harbors two planets and is likewise surrounded by a debris disk.

Although the systems have the same age, their planets are markedly different. The planet AU Mic b almost hugs its star, completing an orbit every 8.5 days. It weighs less than 58 times Earth's mass, placing it in the category of Neptune-like worlds. Beta Pictoris b and c, however, are both at least 50 times more massive than AU Mic b and take 21 and 3.3 years, respectively, to orbit their star.

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In July and August 2018, when TESS was observing AU Mic, the star produced numerous flares, some of which were more powerful than the strongest flares ever recorded on the Sun. The team performed a detailed analysis to remove these effects from the TESS data.

A planet within the debris disk around the pre-main-sequence star AU Microscopii | Nature