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I have heard many times that this planet is gaseous like jupiter this planet is like super earth like rocky planet etc. so is there any planet which is mix of gase and rock ?
Probably. Relatively little is known about exoplanets because they're very hard to get a good look at, but there's no reason why a rocky world couldn't accumulate enough ices and/or gas to also resemble a gas giant. Now half rocky half gas giant (hydrogen/helium) might be rare. Half rocky half "ices" is very possible and those have likely already been observed. It helps to understand planet formation and elemental abundance.
There's likely a size limit to rocky planets with thin atmospheres because mass tends to accumulate and retain atmosphere and with hydrogen and helium being the most abundant gases in the universe, once a planet gets massive enough, it should collect and retain hydrogen and helium and begin to resemble Jupiter or Saturn, even if it started out as a rocky world.
Rocky worlds, need elements like Magnesium, Silicon, and Iron, often bound with oxygen. close to 90% of the mass of rocky worlds in our solar system comes from those 4 elements and 90% isn't a bad estimate for rocky exoplanets.
Based on our solar-system:
Iron makes up about 0.117%, Silicon 0.065% and and Magnesium 0.051%, for about .234% of the mass of the solarsystem. Oxygen bound to those elements, using Earth's composition as an estimate adds about another 50%, up to about 0.35%. Hydrogen and Helium make up about 97.6%, which leaves about 2% of the mass of the solar system in the form of ices and heavier gases, not hydrogen and helium. Those are primarily water, ammonia, CO2 and Methane, with smaller amounts of other gases/ices.
These numbers are rough but good enough for an estimate. In our solarsystem there's about 6 times as much ice and heavier gases by mass than there is rocky material and there's close to 40 times as much hydrogen and helium as the other elements.
Planets need to be quite large to hold hydrogen and helium. Jupiter and Saturn are the only planets in our solar-system that are hydrogen and helium abundant. Even the smaller gas giants, Uranus and Neptune have comparatively little hydrogen and helium.
So there's probably no such thing as a small gas giant. Gas giants need to be large or they don't exist at all. (I should make a footnote that as a gas giant loses atmosphere, you can get a small gas giant, but that would be in a transition phase).
I don't like the term "Ice giants", though it's often used to define Neptune and Pluto because they are primarily ices (water, methane, CO2, NH3), not primarily hydrogen/helium. I don't like calling them "ice giants" when they can also be hot, so I prefer the term Neptune like planets or Neptunes.
Uranus (a Neptune type planet) is mostly made up of ices and heavier gases with an estimate of just 3%-10% hydrogen and helium. Estimates of about 70%-90% of ice/not hydrogen/helium gas and the remaining, about 7%-20% more dense material. That would be some other elements, perhaps some solidified carbon (diamonds), higher sulfur and lower Iron by percentage than Earth as well as some Earth like silicates. magnesium-oxides, Iron and Iron-oxides, though the high internal temperature might not leave much in the way of chemical bonds towards the center of the planet where temperatures reach 9,000 degrees C.
Neptune is similar to Uranus but more massive and more dense. It's thought to have more water and perhaps a larger internal mantle, though like Uranus, the majority of it's mass is ices and heavy gases (not hydrogen/helium).
Neptune and Uranus have hydrogen in the form of Methane, Ammonia and Water, but I'm counting that hydrogen as part of their "heavy gases/ices". They both have a comparatively low percentage of free hydrogen (3%-10%) making a clear distinction between Neptune like planets and gas giants like Jupiter and Saturn with which the majority of their mass is hydrogen.
It helps to think of how planets form. Smaller planets have too little gravity to retain hydrogen and helium gas and solar-systems are mostly too warm for hydrogen to freeze, so planets are made up of solid material for the most part, either rocky or ices that clump and stick together during formation. When they have enough gravity, then they can begin to hold onto an atmosphere and further from the star makes this easier.
The Moon Titan, as an example, is small to have an atmosphere, but it's quite far from the sun and largely made up of ices on it's surface, so as the ice on it's surface melts, it basically replenishes it's atmosphere. Titan is still out-gassing, and it losing atmosphere, but it loses atmosphere slowly enough that it retains an atmosphere, but in a sense, it's in a transition, where it's atmosphere is outgassed. When it runs low on surface ice that readily thaws, it should begin to resemble the icy moons of Jupiter.
So, back to your original question. Many combinations are possible, some are not. Baby Jupiters (the mass of Neptune) are probably unlikely, though a gas giant close to it's star that's lost a lot of it's atmosphere could resemble a baby Jupiter, but like Titan, that would be transitional.
Rocky Jupiters are unlikely because hydrogen outnumbers rocky material by over so much, over 200 to 1 in most of the milky-way. Now, some solarsystems are likely more "metalic" than others, so that ratio will have some variation, but it's once there's enough mass for a gas giant to form, hydrogen is likely to be the abundant element and Helium #2 and any rocky core would be dwarfed by the hydrogen and helium.
A rocky Neptune however, no reason why not. A planet with 8 earth masses of Earth like material and 8 Earth masses of ices (and 5%-10% hydrogen) would basically be a rocky Neptune. It would likely be a little smaller and certainly denser, but not all that different in outward appearance.
Water worlds is a common term that might qualify as a "rocky-neptune". We don't know exactly what they are made of, but density estimates suggest a high percentage of water (and presumably CO2, CH4, NH3), which is the majority of Uranus and Neptune.
Here's a chart of exoplanets of less than 20 Earth masses. I suspect there's a considerable margin of error in these estimates, but it more or less agrees with the rocky-neptune argument. Planets with 2 or 3 grams per cc would be in that range.
See chart: www.hpcf.upr.edu/~abel/phl/hec_plots/exoplanet_df.png">Source.
Baby Neptunes of just 1 or 2 Earth masses might be possible too, but they'd probably need to be quite cold and far from their star or they'd be in danger of losing their atmosphere.
Iceworlds like Pluto can get quite small, but Pluto has very little atmosphere. Planets generally need to be fairly large to retain their atmosphere. If they are too cold, that atmosphere freezes. Titan, as I mentioned above, is in what could be called a slow transition where it's atmosphere comes from it's surface and it loses it slowly. Ceres, based on it's density, is an icy moon like object too, though it's lost nearly all it's surface ice, but it probably has a lot of water/"ices" below it's surface.
I think @userLTK answer is pretty complete, but I would like to add one from a different perspective.
Astronomers, as all humans, tend to categorize things we see using words. This is necessary for fast communication but sometimes there are objects that are at the edge of one category and another (think in a brown dwarf).
Reality is pretty analogic at big scales and we tend to cut it, "digitalize" it in order to understand what are we talking about, but almost always there will be examples in the limits from one categorize to the next, or even superposition, having a same object being in two categories.
As pointed, the amount of gas a planet has depends primarily on its mass. At the same time, there is one category of planets -by mass- missing in our solar system: super-earths (and probably mini-neptunes… again, this is about what granularity we want to use in definitions to a proper communication, and the bounds of each category. I won't get into that). So if Earth has 1 earth mass and Neptune-Uranus about 17-15 earth masses, you can easily imagine a planet having the right mass in the middle of those for being half rock and half gas. Maybe 3, 5 or 8 Earth masses, no idea about the exact mass for the exact 50%. It also depends on its formation and evolution.
Top 5 Strangest and Most Terrifying Planets in the Universe
HD 189733 b: winds up to 5,400 mph — that’s seven times the speed of sound. The blue comes from an atmosphere containing silicate particles, making it possible to rain glass sideways during its vicious winds. Credit: NASA
Exoplanets are defined as planets orbiting other stars outside of our own solar system. They come in a huge variety, from beautiful Earth-like environments to worlds nicknamed “hell planets” for their merciless surfaces. You’ll find everything from planets made of diamond to planets in a deep freeze. Planets so close to their stars that their mass is being ripped away every second and planets so far from their sun we still aren’t sure how it’s possible for them to exist. Some planets are rogues and wander in darkness through our universe, others have such long years that one year in their world is equal to 900,000 years on ours.
To date we have 3,730 confirmed exoplanets, 2,724 potential candidates to be exoplanets, and 2,783 planetary systems (stars with confirmed planets).
I’ve gathered here a list of some of the wildest and most terrifying of these planets.Depiction of a sunrise on CoRot-7 b’s surface. Credit: NASA
Size: A little more than 1.5 Earths
Orbital period: 20 hours or .9 Earth days
A world of violent extremes. Two hells in one.
The reason why the sunrise on this planet looks so intense is because it’s 60 times closer to its star than we are to our sun. That is, CoRot-7 b’s star is 360 times larger in the sky. On it’s day side, this planet can reach 4,700 degrees F, hot enough to melt and vaporize rock. Condensation during cooler fronts causes pebbles to form and then rocks to rain down. Theoretical models also give the planet boiling oceans and lava. This planet is locked into position with this fire-and-brimstone half always facing the sun. The other half of the planet is bitter cold and can reach -392 degrees F, hence why it is said to be two hells in one.
When discovered in 2009, CoRot-7 b was thought to be the first rocky, Earth-like exoplanet discovered. However, we now classify it as uninhabitable and place the probability of life on this world as none. It’s believed to have formed as a gas giant 100 times the size of Earth. As it migrated closer to its star, it shed gas until only a rocky core was left.
Size: about as big as Neptune
Orbital period: 2.6 Earth days
“One of the most exotic contradictions that we know of.”
30 Light years away from Earth lies the planet GJ 436 b, a planet a mere 2.5 million miles from its star (for comparison Mercury is 36 million miles from the sun). Its temperature is about 822 degrees F, certainly hot enough to evaporate water. And yet this planet is so fascinating precisely because its water remains in solid form. That is, GJ 436 b is a planet of burning ice. This isn’t the kind of ice you find here on Earth but rather a special one called Ice VII that results when it has a denser and more crystalline structure. Scientists believe this ice formed due to the strong gravitational force at the planet’s core.
The contradictions for GJ 436 b don’t end there. Its outer layer of hydrogen and helium gas should result in high levels of methane, yet scientists are seeing over 7,000 times less methane than what should exist on the planet. Instead there are huge quantities of carbon monoxide, which may come from the planet’s core where the temperatures are sweltering.
55 Cancri E
Orbital period: .7 Earth days
“Strange things transpire in the twilight zone, and stranger still is the planet Jenssen.”
Also known as Jenssen, this planet is 40 light years away and is most famously known for being 1/3 diamond. This could be the result of an atmosphere that’s mostly carbon, allowing the heat and pressure from both its sun and its interior to compress the planet’s mass into diamond. It is currently valued at 26.9 nonillion dollars (one nonillion is 10³⁰).
Similar to CoRot-7 b, Jenssen is tidal locked and has one side of the planet that always faces the sun and the other which is perpetually in darkness. It’s so close to its star that water cannot exist in a liquid state and instead exists in a supercritical state — a strange behavior somewhere between a liquid and a gas. Whether on the ridiculously hot side of the planet where temperatures reach 4,000 degrees F or in the dark “twilight zone”, there’s no chance you’ll survive on Jenssen. The added kick here is that the Hubble detected what could be hydrogen cyanide oozing from below the planet’s surface, giving us a superheated poisonous fluid.
Orbital period: 1.1 Earth days
It’s strange to see a planet stretched out into such an elongated shape and yet that’s exactly what the gravity of the parent star of Wasp 12 b is doing. The planet is estimated to have only 10 million years left to live as it gets ripped apart and consumed by its sun at a rate of about 189 quadrillion tonnes every year.
Its sun isn’t the only one doing the consuming, however. Wasp 12 b was a surprise to scientists who weren’t expecting to see such a pitch black exoplanet. The world has a unique property where it absorbs, instead of reflects, 94% of light. It has been called “black as asphalt”.
Size: about the size of Earth
Orbital period: 1.5 Earth days
“The light of six red, seemingly blood moons.”
In a system with seven planets and one red star at its center, views from the dark side could have six bright red “moons” in the sky. They’re not actually moons but are instead six large worlds reflecting light, three of them which are within the system’s habitable zone. The planet is hot (about 2,240 degrees F) and is estimated to have, along with the other planets in its system, a combined figure of 250 times more water than we do here on Earth. Planets in this system might be up to 5% water, whereas our planet is only .02% water. At least two of Trappist planets are thought to have liquid water whereas water on Trappist-1b would be in vapor form.
This planet is also thought to be a rocky world, like Earth, and not a gas giant like Jupiter.
Hubble Finds Earth-Sized Planet That May Be on Its Second Atmosphere
Orbiting a red dwarf star 41 light-years away is an Earth-sized, rocky exoplanet called GJ 1132 b. In some ways, GJ 1132 b has intriguing parallels to Earth, but in other ways it is very different. One of the differences is that its smoggy, hazy atmosphere contains a toxic mix of hydrogen, methane and hydrogen cyanide. Scientists using NASA’s Hubble Space Telescope have found evidence this is not the planet’s original atmosphere, and that the first one was blasted away by blistering radiation from GJ 1132 b’s nearby parent star. The so-called “secondary atmosphere” is thought to be formed as molten lava beneath the planet’s surface continually oozes up through volcanic fissures. Gases seeping through these cracks seem to be constantly replenishing the atmosphere, which would otherwise also be stripped away by the star. This is the first time a secondary atmosphere has been detected on a world outside our solar system.
This is an artist’s impression of the Earth-sized, rocky exoplanet GJ 1132 b, located 41 light-years away around a red dwarf star. Scientists using NASA’s Hubble Space Telescope have found evidence this planet may have lost its original atmosphere but gained a second one that contains a toxic mix of hydrogen, methane and hydrogen cyanide. Hubble detected the “fingerprints” of these gases as the parent star’s light filtered through the exoplanet’s atmosphere. The planet is too far away and too dim to be photographed by Hubble. This illustrates what astronomers believe is going on at this remote world. Beneath the planet’s smoggy, hazy atmosphere, there may be a thin crust only a few hundred feet thick. Molten lava beneath the surface continually oozes up through volcanic fissures. Gases seeping through these cracks seem to be constantly replenishing the atmosphere, which would otherwise be stripped away by blistering radiation from the planet’s close-by star. The gravitational pull from another planet in the system likely fractures GJ 1132 b’s surface to resemble a cracked eggshell. This is the first time a so-called “secondary atmosphere” has been detected on a planet outside of our solar system. Credit: NASA, ESA, and R. Hurt (IPAC/Caltech)
Transformed from a gaseous planet like Neptune to a hot, rocky world with a poisonous atmosphere, GJ 1132 b shows that planets can undergo drastic physical changes.
Scientists using NASA’s Hubble Space Telescope have found evidence that a planet orbiting a distant star may have lost its atmosphere but gained a second one through volcanic activity.
The planet, GJ 1132 b, is hypothesized to have begun as a gaseous world with a thick hydrogen blanket of atmosphere. Starting out at several times the diameter of Earth, this so-called “sub-Neptune” is believed to have quickly lost its primordial hydrogen and helium atmosphere due to the intense radiation of the hot, young star it orbits. In a short period of time, such a planet would be stripped down to a bare core about the size of Earth. That’s when things got interesting.
To the surprise of astronomers, Hubble observed an atmosphere which, according to their theory, is a “secondary atmosphere” that is present now. Based on a combination of direct observational evidence and inference through computer modeling, the team reports that the atmosphere consists of molecular hydrogen, hydrogen cyanide, methane and also contains an aerosol haze. Modeling suggests the aerosol haze is based on photochemically produced hydrocarbons, similar to smog on Earth.
Scientists interpret the current atmospheric hydrogen in GJ 1132 b as hydrogen from the original atmosphere which was absorbed into the planet’s molten magma mantle and is now being slowly released through volcanic processes to form a new atmosphere. The atmosphere we see today is believed to be continually replenished to balance the hydrogen escaping into space.
“It’s super exciting because we believe the atmosphere that we see now was regenerated, so it could be a secondary atmosphere,” said study co-author Raissa Estrela of NASA’s Jet Propulsion Laboratory (JPL) in Southern California. “We first thought that these highly irradiated planets could be pretty boring because we believed that they lost their atmospheres. But we looked at existing observations of this planet with Hubble and said, ‘Oh no, there is an atmosphere there.'”
The rocky exoplanet GJ 1132 b, similar in size and density to Earth, possesses a hazy atmosphere made up of volcanic gases. Scientists say GJ 1132 b, orbiting a red-dwarf star about 41 light-years away, has some features in common with worlds in our own solar system as well as vast differences. Its hazy appearance might compare to Titan, Saturn’s largest moon, the only solar system moon with a substantial atmosphere – though Titan is much colder. Our own Earth might have had such a hazy appearance early in its history, although unlike Earth, the new planet is far too hot to be habitable. And GJ 1132 b likely has a “secondary atmosphere,” created by volcanic activity after its first hydrogen-helium atmosphere was stripped away by radiation from its star. Credit: NASA/JPL-Caltech/Lizbeth B. De La Torre
The findings could have implications for other exoplanets, planets beyond our solar system.
“How many terrestrial planets don’t begin as terrestrials? Some may start as sub-Neptunes, and they become terrestrials through a mechanism that photo-evaporates the primordial atmosphere. This process works early in a planet’s life, when the star is hotter,” said lead author Mark Swain of JPL. “Then the star cools down and the planet’s just sitting there. So you’ve got this mechanism where you can cook off the atmosphere in the first 100 million years, and then things settle down. And if you can regenerate the atmosphere, maybe you can keep it.”
This plot shows the spectrum of the atmosphere of an Earth-sized, rocky exoplanet, GJ 1132 b. The orange line represents the model spectrum. In comparison, the observed spectrum is shown as blue dots representing averaged data points, along with their error bars. This analysis is consistent with GJ 1132 b being predominantly a hydrogen atmosphere with a mix of methane and hydrogen cyanide. The planet also has aerosols which cause scattering of light. This is the first time a so-called “secondary atmosphere,” which was replenished after the planet lost its primordial atmosphere, has been detected on a world outside of our solar system. Credit: NASA, ESA, and P. Jeffries (STScI)
In some ways GJ 1132 b, located about 41 light-years from Earth, has tantalizing parallels to Earth, but in some ways it is very different. Both have similar densities, similar sizes, and similar ages, being about 4.5 billion years old. Both started with a hydrogen-dominated atmosphere, and both were hot before they cooled down. The team’s work even suggests that GJ 1132 b and Earth have similar atmospheric pressure at the surface.
But the planets have profoundly different formation histories. Earth is not believed to be the surviving core of a sub-Neptune. And Earth orbits at a comfortable distance from our Sun. GJ 1132 b is so close to its red dwarf star that it completes an orbit around its host star once every day and a half. This extremely close proximity keeps GJ 1132 b tidally locked, showing the same face to its star at all times—just as our Moon keeps one hemisphere permanently facing Earth.
“The question is, what is keeping the mantle hot enough to remain liquid and power volcanism?” asked Swain. “This system is special because it has the opportunity for quite a lot of tidal heating.”
Tidal heating is a phenomenon that occurs through friction, when energy from a planet’s orbit and rotation is dispersed as heat inside the planet. GJ 1132 b is in an elliptical orbit, and the tidal forces acting on it are strongest when it is closest to or farthest from its host star. At least one other planet in the host star’s system also gravitationally pulls on the planet.
The consequences are that the planet is squeezed or stretched through this gravitational “pumping.” That tidal heating keeps the mantle liquid for a long time. A nearby example in our own solar system is Jupiter’s moon Io, which has continuous volcanic activity due to a tidal tug-of-war from Jupiter and the neighboring Jovian moons.
Given GJ 1132 b’s hot interior, the team believes the planet’s cooler, overlying crust is extremely thin, perhaps only hundreds of feet thick. That’s much too feeble to support anything resembling volcanic mountains. Its flat terrain may also be cracked like an eggshell due to tidal flexing. Hydrogen and other gases could be released through such cracks.
NASA’s upcoming James Webb Space Telescope has the ability to observe this exoplanet. Webb’s infrared vision may allow scientists to see down to the planet’s surface. “If there are magma pools or volcanism going on, those areas will be hotter,” explained Swain. “That will generate more emission, and so they’ll be looking potentially at the actual geologic activity—which is exciting!”
The team’s findings will be published in an upcoming issue of The Astronomical Journal.
Reference: “Detection of an Atmosphere on a Rocky Exoplanet” by Mark R. Swain, Raissa Estrela, Gael M. Roudier, Christophe Sotin, Paul Rimmer, Adriana Valio, Robert West, Kyle Pearson, Noah Huber-Feely and Robert T. Zellem, Accepted, The Astronomical Journal.
The long lost planet may still be out there. In May, astronomers in Japan announced that they had spotted lonely planets wandering through the dark space between stars. These lone wolfs may be even more common than star-bound worlds, the team reported. If the fifth gas giant is still out there, it will be one of the wandering exoplanets.
Today’s planets may have had other siblings as well. Previous researchers have suggested that a fifth rocky planet may have been ejected from an orbit between Mars and Jupiter, and that super-Earths may have been swallowed by Jupiter or Neptune.
“Our solar system looks calm and quiet now, but we pretty much know that it had this violent past,” Nesvorny says. “The question is, how violent was it?”
Nesvorny has had several suggestions from colleagues for a name for the new planet. These include Hades, the unseen god of the underworld from Greek mythology, and “Thing 1” from Dr Seuss’s book The Cat in the Hat. The latter leaves open the possibility for a “Thing 2” if future work shows the need for more than one planet. But he is not yet persuaded. “I’m not sure I like any of them,” he admits.
A new name for an old planet
What should the solar system’s long-lost planet be called?
Nesvorny has had several suggestions from colleagues. These include Hades, the unseen god of the underworld from Greek mythology, and “Thing 1” from Dr Seuss’s book. The latter leaves open the possibility for a “Thing 2” if future work shows the need for more than one departed planet.
But he is not yet persuaded. “I’m not sure I like any of them,” he says.
Readers put forward numerous names for the planet after asked for suggestions on Twitter. These included Mephitis, the Roman goddess of poisonous gases from @HLNdevon Liber, the “free one” and Roman god of wine from @nothingkitten and, Nesvorny’s favourite, “sar syem”, which is “lost” removed from “solar system” suggested by @Craig_McGlashan. More suggestions to @newscientist, please.
One of the Oldest Stars in the Galaxy has a Planet. Rocky Planets Were Forming at Nearly the Beginning of the Universe
Would it be surprising to find a rocky planet that dates back to the very early Universe? It should be. The early Universe lacked the heavier elements necessary to form rocky planets.
But astronomers have found one, right here in the Milky Way.
After the Big Bang, the Universe consisted of nothing but light elements like hydrogen and helium, with a little lithium. Rocky planets require heavier elements like carbon, oxygen, and iron, which astronomers call metals. Those heavier elements can only be formed in the hearts of stars. And the first stars didn’t form until about 200 million years after the Big Bang.
Any extremely ancient planets, formed not long after the Universe began, should be gaseous, not rocky. There wasn’t enough time for stars to seed the Universe with heavy elements for rocky planets. Or was there?
The planet in question orbits the star known as TOI 561. TOI stands for TESS Object of Interest, meaning it was spotted with NASA’s TESS (Transiting Exoplanet Survey Satellite). TOI 561 is one of the oldest stars in the Milky Way about 10 billion years old.
TESS found the planet, and a team of researchers used follow-up observations with the Keck Telescope to learn more about it. They presented their findings at the January 2021 meeting of the American Astronomical Society. They also published their findings in a paper titled “The TESS-Keck Survey. II. An Ultra-short-period Rocky Planet and Its Siblings Transiting the Galactic Thick-disk Star TOI-561.” It’s published in the Astronomical Journal and the lead author is Dr. Lauren Weiss, Beatrice Watson Parrent Postdoctoral Fellow at the Institute for Astronomy at the University of Hawaii.
TOI 561 is in rare company. It’s one of the stars in what’s called the galactic thick disk. The galactic thick disk is composed almost entirely of ancient stars, whose chemistry and motion are different from the thin disk. Thick disk stars, including TOI 561, have much lower metallicity than stars in the thin disk. So finding a rocky planet orbiting it is surprising.
Illustration showing the structural components of the Milky Way Galaxy. The star TOI-561 is located in the thick disk (marked in red-orange), which contains a rare, older population of stars. While nearly all known planets are found within the thin disk (marked in orange), the newly-discovered rock-and-lava exoplanet orbiting TOI-561 is one of the first confirmed rocky planets orbiting a galactic thick disk star.
Credit: Kaley Brauer, MIT
“The rocky planet orbiting TOI-561 is one of the oldest rocky planets yet discovered. Its existence shows that the universe has been forming rocky planets almost since its inception 14 billion years ago,” said lead author Weiss in a press release.
The planet, named TOI-561b, was discovered when it transited in front of its star. As its name says, TESS is designed to detect the dip in star light when a planet transits in front of a star in its field of view. Astronomers can gauge the planet’s size by measuring the drop in light, and in this case, it indicates that the planet is about 1.5 Earth radii.
Here’s what data on planetary transits looks like. It shows the measured dip in starlight when TOI 561b passes in front of its star from TESS’s perspective. Image Credit: Weiss et al, 2021
The team used the Keck Observatory for follow-up observations. The Keck has a special instrument called the High-Resolution Echelle Spectrometer (HIRES) to confirm the planet’s detection. HIRES allows astronomers to measure the wobble in the star caused by the planet’s gravitational tug. That measurement reveals the planet’s mass. In this case, the mass is large enough—three times greater than Earth’s—that TOI 561b has to be a dense rocky planet rather than a gaseous one. There are two other planets orbiting TOI-561, but they’re both gas planets.
This figure from the study shows planet bulk density vs. planet radius for small planets with measured radii and masses, based on results from the NASA Exoplanet Archive. TOI 561 b, c, and d are shown. Planets in our Solar System are shown for comparison. Image Credit: Weiss et al, 2021.
The origin of the old stars in the galactic thick disk is unclear. They could be the remnants of an ancient galaxy that was swallowed up by the Milky Way. Or they could be the first stars to form in the Milky Way. Or it could be something else. Nobody’s certain.
As a planet orbiting an ancient, 10 billion-year-old star, it’s been through a lot. The wandering motion of stars in the disk sometimes takes them above the galactic plane. An observer on TOI 561b would have been gifted some stunning views of the Milky Way’s beautiful spiral structure. “I wonder what view of the night sky would have been accessible from the rocky planet during its history,” said Weiss.
Artist’s rendition of TOI-561, one of the oldest, most metal-poor planetary systems discovered yet in the Milky Way galaxy. This 10 billion-year-old system has a hot, rocky exoplanet (center) that’s one and a half times the size of Earth as well as two gas planets (to the left of the rocky planet) that are about twice as large as Earth. Credit: W. M. Keck Observatory/Adam Makarenko
But there were likely no observers. Not now, anyway. Though the long history of the planet is unknown, it’s too hot for life in the present-day. TOI 561b in an ultra-short period (USP) planet. It orbits TOI 561 twice each Earth day at a distance that keeps the surface at about 2000 degrees Kelvin (1721 C 3140 F.) So while it is a rocky planet, that rock is likely magma on the surface.
What does this discovery mean? Well, it would be surprising if it was the only one. It’s likely an indication that there’s a whole population of ancient, rocky planets orbiting ancient stars.
In fact, TOI 561b isn’t the first planet detected around a galactic thick disk star. The Kepler mission found five sub-Earth size planets orbiting the triple star system Kepler 444. Kepler 444 is estimated to be just over 11 billion years old, and its five planets are all rocky planets smaller than Venus. They’re also very close to their star and are blisteringly hot.
There’s also LHS 1815b. It’s orbiting an M dwarf star in the thick disk and was discovered in early 2020. It’s a rocky super-Earth and has a blistering surface temperature due to its proximity to its star.
An illustration of LHS 1815b, an ancient rocky exoplanet orbiting a star in the galactic thick disk. Image Credit: NASA
There’s still some mystery in this discovery. Not so much over the planet itself, but the thick disk. There’s disagreement and uncertainty on the nature of the galactic thick disk itself. Some astronomers think it doesn’t even exist as a distinct structure. In 2012 a group of astronomers published a paper in the Astrophysical Journal titled “The Milky Way Has No Distinct Thick Disk.” They argued that there is no distinct thick disk and say their data showed that “…the Milky Way has a continuous and monotonic distribution of disk thicknesses: there is no “thick disk” sensibly characterized as a distinct component.”
Nobody’s denying the existence of the ancient stars themselves or the ancient rocky planets orbiting them. But what the existence of those planets means for our understanding of the Universe is not clear yet.
Rocky Versus Gaseous Planets
In our Solar System, we have two kinds of planets: small, rocky , dense planets that are similar to Earth and large, gaseous planets like Jupiter. From what we astrophysicists have detected so far, all planets fall into these two categories.
In fact, when we look at the data from planet-hunting missions such as the Kepler mission or from the Transiting Exoplanet System Satellite , there is a gap in the planet sizes. Namely, there aren’t many planets that fulfill the definition of a “super-Earth,” with a radius of one and a half to twice Earth’s radius and a mass that is five to 10 times greater.
So the question is, why aren’t there any super-Earths? Why do astronomers only see small rocky planets and enormous gaseous planets?
The differences between the two kinds of planets, and the reason for this super-Earth gap, has everything to do with a planet’s atmosphere – especially when the planet is forming.
When a star is born , a huge ball of gas comes together, starts to spin, collapses in on itself and ignites a fusion reaction within the star’s core. This process isn’t perfect there is a lot of extra gas and dust left over after the star is formed. The extra material continues to rotate around the star until it eventually forms into a stellar disk: a flat, ring-shaped collection of gas, dust, and rocks.
During all of this motion and commotion, the dust grains slam into each other, forming pebbles which then grow into larger and larger boulders until they form planets. As the planet grows in size, its mass and therefore gravity increases, allowing it to capture not only the accumulated dust and rocks – but also the gas, which forms an atmosphere.
There is lots of gas within the stellar disk – after all, hydrogen and helium are the most common elements in stars and in the universe. However, there is considerably less rocky material because only a limited amount was made during star formation.
Kepler looked for exoplanets by measuring changes in the amount of light received from a particular star. An exoplanet passing in front of a star blocks some of the light, albeit a tiny percentage. So Kepler looks for any small changes.
But this approach assumes that stars produce a relatively constant amount of light, just like our Sun. But one of Kepler’s early discoveries was that this assumption is wrong. Sun-like stars turn out to be much more variable than astronomers had thought and this variability hides the presence of exoplanets. “Identifying habitable zone rocky planets proved to be a greater challenge than anticipated,” according to Bryson and his team.
In response, NASA extended Kepler’s mission from four to eight years, in the hope that the extra data would help to reveal more exoplanet candidates. But then disaster struck. To gather light, the space telescope must point with great accuracy at a small piece of sky and the spacecraft is equipped with four reaction wheels that control its orientation.
But in 2012 and 2013, two of the wheels failed leaving the spacecraft crippled. Lower precision observations continued until 2018 when the spacecraft finally died but the new data never quite made up for the problems associated with star variability.
As a result, the final data release from Kepler contains just 9 potentially habitable candidates. These are unambiguously rocky planets within the parent star’s habitable zone that are a similar size to Earth (between 0.5 and 1.5 times Earth’s radius).
Why planets have size limits
Artist’s concept of a planet-forming disk made from rock and gas surrounding a young star. Image via NASA/JPL-Caltech/ SwRI/ MSSS/ Gerald Eichstädt/ Seán Doran.
Scientists have discovered over 4,000 exoplanets outside of our solar system, according to NASA’s Exoplanet Archive.
Some of these planets orbit multiple stars at the same time. Certain planets are so close to their star that it takes only a handful of days to make one revolution, compared to the Earth which takes 365.25 days. Others slingshot around their star with extremely oblong orbits, unlike the Earth’s circular one. When it comes to how exoplanets behave and where they exist, there are many possibilities.
And yet, when it comes to sizes of planets, specifically their mass and radius, there are some limitations. And for that, we have physics to blame.
I am a planetary astrophysicist and I try to understand what makes a planet able to support life. I look at the chemical connection between stars and their exoplanets and how the interior structure and mineralogy of different sized planets compare to each other.
This sketch illustrates a family tree of exoplanets starting from the protoplanetary disk, which is a swirling disk of gas and dust surrounding a planet (much like a stellar disk but smaller). Gas and dust is pulled onto the planet, depending on the planet’s mass and gravity. Image via NASA/ Ames Research Center/ JPL-Caltech/ Tim Pyle.
Rocky versus gaseous planets
In our solar system, we have two kinds of planets: small, rocky, dense planets that are similar to Earth and large, gaseous planets like Jupiter. From what we astrophysicists have detected so far, all planets fall into these two categories.
In fact, when we look at the data from planet-hunting missions such as the Kepler mission or from the Transiting Exoplanet System Satellite (TESS), there is a gap in the planet sizes. Namely, there aren’t many planets that fulfill the definition of a super-Earth, with a radius of 1 1/2 to twice Earth’s radius and a mass that is five to 10 times greater.
So the question is, why aren’t there any super-Earths? Why do astronomers only see small rocky planets and enormous gaseous planets?
The differences between the two kinds of planets, and the reason for this super-Earth gap, has everything to do with a planet’s atmosphere – especially when the planet is forming.
When a star is born, a huge ball of gas comes together, starts to spin, collapses in on itself and ignites a fusion reaction within the star’s core. This process isn’t perfect there is a lot of extra gas and dust left over after the star is formed. The extra material continues to rotate around the star until it eventually forms into a stellar disk: a flat, ring-shaped collection of gas, dust, and rocks.
During all of this motion and commotion, the dust grains slam into each other, forming pebbles which then grow into larger and larger boulders until they form planets. As the planet grows in size, its mass and therefore gravity increases, allowing it to capture not only the accumulated dust and rocks, but also the gas, which forms an atmosphere.
There is lots of gas within the stellar disk. After all, hydrogen and helium are the most common elements in stars and in the universe. However, there is considerably less rocky material because only a limited amount was made during star formation.
Comparison of confirmed super-Earth planets compared to the size of the Earth. Image via NASA/ Ames/ JPL-Caltech.
The trouble with super-Earths
If a planet remains relatively small, with a radius less than 1.5 times Earth’s radius, then its gravity is not strong enough to hold onto a huge amount of atmosphere, like what’s on Neptune or Jupiter. If, however, it continues to grow larger, then it captures more and more gas which forms an atmosphere that causes it to swell to the size of Neptune (four times Earth’s radius) or Jupiter (11 times Earth’s radius).
Therefore, a planet either stays small and rocky, or it becomes a large, gaseous planet. The middle ground, where a super-Earth might be formed, is very difficult because, once it has enough mass and gravitational pull, it needs the exact right circumstances to stop the avalanche of gas from piling onto the planet and puffing it up. This is sometimes referred to as unstable equilibrium, such that when a body (or a planet) is slightly displaced (a little bit more gas is added) it departs further from the original position (and becomes a giant planet).
Another factor to consider is that once a planet is formed, it doesn’t always stay in the same orbit. Sometimes planets move or migrate towards their host star. As the planet gets closer to the star, its atmosphere heats up, causing the atoms and molecules to move very fast and escape the planet’s gravitational pull. So some of the small rocky planets are actually the cores of bigger planets that have been stripped of their atmosphere.
So, while there are no super huge rocky planets or small fluffy planets, there is still a huge amount of diversity in planet sizes, geometries and compositions.
Natalie Hinkel, Planetary Astrophysicist, Senior Research Scientist at the Southwest Research Institute and Co-Investigator for the Nexus for Exoplanet System Science (NExSS), Arizona State University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Rogue Planets That Float in Space Without Orbiting a Sun Could Outnumber the Stars
Artist’s conception of SIMP J01365663+0933473, a planetary-mass object beyond our Solar System. The object, about a dozen times more massive than Jupiter is traveling through space unaccompanied by any parent star.. Credit: Chuck Carter, NRAO/AUI/NSF
Upcoming NASA mission will search for planets in the Milky Way without their own sun.
An upcoming NASA mission could find that there are more rogue planets — planets that float in space without orbiting a sun — than there are stars in the Milky Way, a new study theorizes.
“This gives us a window into these worlds that we would otherwise not have,” said Samson Johnson, an astronomy graduate student at The Ohio State University and lead author of the study. “Imagine our little rocky planet just floating freely in space — that’s what this mission will help us find.”
The study was published today in the Astronomical Journal.
The study calculated that NASA’s upcoming Nancy Grace Roman Space Telescope could find hundreds of rogue planets in the Milky Way. Identifying those planets, Johnson said, will help scientists infer the total number of rogue planets in our galaxy. Rogue, or free-floating, planets are isolated objects that have masses similar to that of planets. The origin of such objects is unknown, but one possibility is they were previously bound to a host star.
“The universe could be teeming with rogue planets and we wouldn’t even know it,” said Scott Gaudi, a professor of astronomy and distinguished university scholar at Ohio State and a co-author of the paper. “We would never find out without undertaking a thorough, space-based microlensing survey like Roman is going to do.”
The Roman telescope, named for NASA’s first chief astronomer who was also known as the “mother” of the Hubble telescope, will attempt to build the first census of rogue planets, which could, Johnson said, help scientists understand how those planets form. Roman will also have other objectives, including searching for planets that do orbit stars in our galaxy.
That process is not well-understood, though astronomers know that it is messy. Rogue planets could form in the gaseous disks around young stars, similar to those planets still bound to their host stars. After formation, they could later be ejected through interactions with other planets in the system, or even fly-by events by other stars.
Or they could form when dust and gas swirl together, similar to the way stars form.
The Roman telescope, Johnson said, is designed not only to locate free-floating planets in the Milky Way, but to test the theories and models that predict how these planets formed.
Johnson’s study found that this mission is likely to be 10 times more sensitive to these objects than existing efforts, which for now are based on telescopes tethered to the Earth’s surface. It will focus on planets in the Milky Way, between our sun and the center of our galaxy, covering some 24,000 light years.
“There have been several rogue planets discovered, but to actually get a complete picture, our best bet is something like Roman,” he said. “This is a totally new frontier.”
Rogue planets have historically been difficult to detect. Astronomers discovered planets outside Earth’s solar system in the 1990s. Those planets, called exoplanets, range from extremely hot balls of gas to rocky, dusty worlds. Many of them circle their own stars, the way Earth circles the sun.
But it is likely that a number of them do not. And though astronomers have theories about how rogue planets form, no mission has studied those worlds in the detail that Roman will.
The mission, which is scheduled to launch in the next five years, will search for rogue planets using a technique called gravitational microlensing. That technique relies on the gravity of stars and planets to bend and magnify the light coming from stars that pass behind them from the telescope’s viewpoint.
This microlensing effect is connected to Albert Einstein’s Theory of General Relativity and allows a telescope to find planets thousands of light-years away from Earth–much farther than other planet-detecting techniques.
But because microlensing works only when the gravity of a planet or star bends and magnifies the light from another star, the effect from any given planet or star is only visible for a short time once every few million years. And because rogue planets are situated in space on their own, without a nearby star, the telescope must be highly sensitive in order to detect that magnification.
The study published today estimates that this mission will be able to identify rogue planets that are the mass of Mars or larger. Mars is the second-smallest planet in our solar system and is just a little bigger than half the size of Earth.
Johnson said these planets are not likely to support life. “They would probably be extremely cold, because they have no star,” he said. (Other research missions involving Ohio State astronomers will search for exoplanets that could host life.)
But studying them will help scientists understand more about how all planets form, he said.
“If we find a lot of low-mass rogue planets, we’ll know that as stars form planets, they’re probably ejecting a bunch of other stuff out into the galaxy,” he said. “This helps us get a handle on the formation pathway of planets in general.”
Reference: “Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. II. Free-floating Planet Detection Rates” by Samson A. Johnson, Matthew Penny, B. Scott Gaudi, Eamonn Kerins, Nicholas J. Rattenbury, Annie C. Robin, Sebastiano Calchi Novati and Calen B. Henderson, 21 August 2020, Astronomical Journal.
Planet hop from TRAPPIST-1e
The first discovery of seven Earth-sized planets around a single star offers a hopeful target in the search for life.
TRAPPIST-1e Coloring Page
What hues would appear under a red sun? Grab crayons, markers, paint or colored pencils and color in the hues of our TRAPPIST-1e coloring page based on our popular Exoplanet Travel Bureau poster.