McGill University
Astronomers have found that on the hot Jupiter CoRoT-2b, the wind blows in the “wrong” direction, which is why the hottest point on the planet is not where theories predict, according to an article in Nature.
Unlike Jupiter, which is 5 astronomical units from the Sun (that is, five times farther than the Earth), a hot Jupiter is a type of planet that is on the order of 0.05 astronomical units from the star. Such a planet makes one revolution around the main star in less than three days. Due to their proximity to the parent star, these gas giants are tidally captured and always turned to it on one side, as a result, the day side of the planet is noticeably hotter than the night side.
Theoretically, the hottest point of hot Jupiter should be closest to the star, but in reality this zone is usually shifted to the east: astronomers explain the observed feature by the movement of equatorial winds. Modern models say that they winds must blow in eastbound, causing the gas giant's hottest point to also move to the east. However, in the case of the planet CoRoT-2b, everything turned out differently. While studying the celestial body with the Spitzer Space Telescope, a team of researchers from McGill University noticed that the warmest point on the planet is shifted to the west.
The exoplanet CoRoT-2b was discovered about 10 years ago. It is located 930 light years from Earth in the constellation Serpens. Radius celestial body approximately 1.43 times the radius of Jupiter, and the mass is 3.3 times. As astronomers note, the CoRoT-2 system is interesting for several reasons at once: firstly, its main star, a yellow dwarf, is very active, secondly, it has a gravitationally bound companion, the star 2MASS J19270636+0122577, and thirdly , the exoplanet CoRoT-2b is very bloated and has an unusual emission spectrum.
Surface brightness CoRoT-2b
Lisa Dang et al / Nature, 2018
The effective surface temperature of CoRoT-2b is close to that of HD 209458b, a typical hot Jupiter from another system. Despite this, HD 209458b has its hottest region shifted to the east, while CoRoT-2b has its hottest region shifted to the west by 23 ± 4 degrees. According to the authors of the work, the anomaly can have three explanations. On the one hand, the exoplanet can rotate around its axis more slowly than around the star - simulations show that in this case, the equatorial winds will blow in the opposite direction to the west. On the other hand, the atmosphere of CoRoT-2b can interact with its magnetic field which affects the movement of the winds. Also, dense clouds covering the planet's east side may make it look "darker" than it really is (in infrared) - but such an explanation doesn't fully match current atmospheric circulation patterns on hot Jupiters.
More data is needed to build the most accurate CoRoT-2b model. They will help to reveal the features of the atmosphere of a hot Jupiter. In the future, astronomers plan to make observations with the space telescope, which is scheduled to launch in the spring of 2019.
Interestingly, clouds on hot Jupiters can also hide water in their atmosphere, and this obstacle is typical for this class of exoplanets.
Kristina Ulasovich
Initially, the note said that the launch of the James Webb telescope was scheduled for 2018, but this is outdated data. In September 2017, NASA announced the postponement of the launch to spring 2019. The editors apologize to the readers.
When astronomers discovered the first exoplanet around a sun-like star about two decades ago, their initial joy quickly turned to confusion. Planet 51 Pegasus b (Bellerophon) was one and a half times more massive than Jupiter, and its 4-day orbit was incredibly close to the star. Theorists studying planet formation could not explain how such a large body could have such a close orbit. Perhaps she was out of the general pattern? But no, now we know a lot.
Further searches for distant worlds presented scientists with several more surprises: planets with oblong and highly inclined orbits, and even planets moving in the direction opposite to the rotation of their parent star.
Artistic depiction of the exoplanet 51 Pegasus b. Credit: ESO/M. Kornmesser/Nick Risinger
The hunt for exoplanets picked up steam in 2009 with the launch of NASA's Kepler Space Telescope, which discovered more than 2,500 worlds. Kepler found that the most common type of planets are the so-called "super-Earths" (somewhere between the sizes of Earth and Neptune). There are none in our solar system.
Currently, ground-based telescopes are collecting light directly from exo-worlds rather than detecting them indirectly, as Kepler did, and they, too, are baffling astronomers. Telescopes are discovering giant planets several times the mass of Jupiter, twice as far from their star as Neptune is from the Sun, where theorists thought they simply could not form. So far, not a single ordered one has been found like ours. star system, and theorists are constantly trying to come up with scenarios that will explain the appearance of previously "forbidden" planets in their "impossible" orbits.
“These are obvious things that don't fit into our models from day one. There has never been a theory that has caught up with observation,” said Bruce McIntosh, a physicist at Stanford University (USA).
The traditional model for the formation of both stars and their planets dates back to the 18th century, when scientists suggested that a slowly rotating cloud of gas and dust could collapse under its own gravity. Most of the material forms a ball that will ignite the star when its core becomes dense and hot enough. And the remaining material will be collected in a flat disk. The dust, made up of microscopic inclusions of iron and other hard particles, is the key to transforming this disk into a set of planets. Since it circulates in the disk that engulfs it, the particles sometimes collide and stick together due to electromagnetic forces. Over several million years, the dust will collect into grains, boulders, and eventually into kilometer-long planetesimals.
Artistic depiction of a protoplanetary disk. Credit: ESO/M. Kornmesser
At this point, gravity takes over, attracting dust and gas to the germs, which grow to the size of planets. By that time, most of the gas in the inner part of the disk has either been stripped away by the planets, eaten away by the star, or blown away by the stellar wind. The lack of gas means that the inner planets remain mostly rocky, with thin atmospheres.
This growth process, known as accretion, takes place faster in the outer part of the disk, where there is enough water ice. Ice beyond this allows protoplanets to consolidate faster. It manages to build solid cores (up to 10 times more massive than Earth) before the disk loses its gas. This allows the formation of planets with a dense atmosphere, like Jupiter (search for a solid core near the big planet The solar system will be one of the tasks of the spacecraft).
This scenario, of course, describes the evolution of planetary systems like ours: small, rocky planets with a thin atmosphere close to the star, and gas giants just beyond the boundary of eternal snows. In addition, the giants get smaller and smaller as they move away from the star, as their slow rotation in their orbits slows down the collection of material. All planets remain approximately where they formed, in circular orbits in the same plane. Nice and neat.
But the discovery of "hot Jupiters" showed that something was wrong with this theory. Forming so close to a star seemed incredible. The inevitable conclusion is that they formed further and then migrated.
Here, theorists have come up with two possible mechanisms for planetary shuffling. The first requires the presence of a huge amount of material in the disk after the giant planet has formed. Gravity will distort the disk, creating zones of increased density, which in turn will exert a gravitational effect on the planet, gradually dragging it towards the star.
Some observations support this idea. Neighboring planets often have a gravitational connection known as orbital resonance. It occurs when the lengths of their orbits are related as a small natural number. Pluto, for example, circles the Sun twice in every three revolutions of Neptune. It's unlikely that this is a fluke, and they may have once drifted locked in extra stability. Migration to early history our solar system could explain other oddities, including the small size of Mars and the asteroid belt. Based on this, theorists have suggested that Jupiter initially formed closer to the Sun, then went inward almost to the orbit of the Earth, and was blown back to its current location.
Do hot Jupiters migrate? Credit: NASA/JPL-Caltech
There are scholars who find the migration scenario too complex and unrealistic. “I believe in Occam's razor,” said Greg Laughlin, an astronomer at the University of California (USA). He is sure that the planets are more likely to be in their places and not twitch. “Perhaps protoplanetary disks that have major planets in close orbits, contained much more material than we used to think. Of course, some movement may occur, sufficient to explain the resonance, but these fine adjustments should not be put on stream, ”explained Greg Laughlin.
Others believe that there simply cannot be enough material to form planets like 51 Pegasus b. “They cannot form there. In addition, a large number of planets with elongated, inclined or even reverse orbits imply planetary shuffling,” said Joshua Wynn of the Massachusetts Institute of Technology (USA).
Some theorists resort to gravitational combat rather than power-law migration in an attempt to explain observations. Massive disks can spawn many planets close together, and the gravitational scramble between them will hurl some of them onto the star, others into strange orbits, and still others out of the system. Another potential troublemaker is a star's satellite in an elongated orbit. Most of the time it will be too far away to make an impact, but when it gets close it can make a splash. Or, if the parent star is a member of a friendly star cluster, a neighboring star may come too close during its walk and wreak havoc. “There are many ways to break the system,” said Joshua Wynn.
The surprising discovery of Kepler was that 60% of sun-like stars have in their orbits. This requires completely new theories. Most super-Earths are thought to be mostly solid rock and metal with some gas and orbit close to their stars. For example, the Kepler-80 system has four such exoplanets with orbits of 9 days or less. The conventional theory says that accretion in the interior of the disk is too slow to produce such big worlds. Plus, super-Earths are rarely found in resonant orbits, which does not support the migration theory.
Scientists have come up with a way to get out of the situation. One idea is to speed up accretion through a process known as stony accretion. The gas-rich disk offers great resistance to small stone objects, slowing them down. This causes them to drift towards the star. If they pass planetesimals on their way, the low speed will allow them to be captured. But fast accretion and gas-rich disks create a new problem: once they reach a certain size, super-Earths must pull a dense atmosphere towards them. "How do they keep themselves from becoming gas giants?" asks Roman Rafikov, an astrophysicist at the Princeton Institute for Advanced Study (USA).
Artistic representation of the formation of planets during pebble accretion. Credit: NASA/JPL-Caltech
“There is no need for accelerated accretion. If the inner region is 10 times denser than the disk from which it was born solar system, then one or more super-Earths can easily form in it. And they will not collect too much gas, since it will have already dissipated by the time they are finally formed,” retorted Eugene Chang, an astronomer from the University of California (USA).
Chang also has an explanation for another amazing discovery: "bloated" planets. Rare and equally problematic worlds that are lighter than super-Earths but have huge bloated atmospheres that make up 20% of their mass. Such exoplanets are believed by theorists to form in a gas-rich disk. However, in its inner part, warm gas will struggle with the planet's weak gravity, so the cold and dense gas of the outer disk is a more plausible candidate for their shells. In this case, Eugene Chang resorts to migration to explain their proximity to the star. In addition, it is confirmed by the fact that "bloated" ones are often trapped in orbital resonance.
The focus of exoplanet research so far has been on the inner regions of protoplanetary disks, roughly within a distance equivalent to the orbit of Jupiter. This is due to the fact that they can be seen by all existing methods. The worlds close to the stars are found in two main indirect ways: changes in brightness and fluctuations of the stars. But direct visualization of a nearby exoplanet is extremely difficult, as it is overshadowed by a host star that can be billions of times brighter than the target.
However, by pushing the limits of the world's largest telescopes, astronomers have been able to see several planets directly. And for a couple recent years two new tools designed specifically for imaging distant worlds have joined the hunt. The European "Spectro-Polarimetric High-contrast Exoplanet REsearch" (SPHERE) and the American "Gemini Planet Imager" (GMI) are installed on large telescopes in Chile and use sophisticated masks (coronagraphs) that block the light of the star.
Artist's rendering of the planetary system HR 8799. Credit: NASA, ESA
One of the earliest and most striking systems found by direct imaging is HR 8799. Four huge planets, more than five times the mass of Jupiter, orbit at "impossibly" distant distances from the star (from the orbit of Saturn to an orbit twice the orbit of Neptune). According to the theory, such distant exoworlds move very slowly, and they cannot accumulate mass more than Jupiter before the disk dissipates. However, the exoplanets' good circular orbits suggest they were not ejected there from nearby regions of the system.
Such distant giants provided support for the most radical theory, challenging the standard. According to her, some planets are formed not by accretion, but by a process called gravitational instability. This process requires a gas-rich protoplanetary disk that breaks up into clumps under its own gravity. Over time, these clumps turn directly into giant planets, which lack a solid core in the first place. The model assumes that the mechanism will only work under certain conditions: the gas must be cold, must not rotate very fast, and must lose heat efficiently. “Can this explain the planets HR 8799? Yes, but only two distant cold ones,” said Roman Rafikov.
In the past, radio telescope observations of protoplanetary disks have provided some support for gravitational instability. Having sensitivity to cold gases, radio telescopes saw tangled, asymmetric clumps in the disks. But recent images from the Atacama Large Millimeter Array (ALMA) radio telescope paint a different picture. ALMA is sensitive to shorter wavelengths that come from dust particles in the plane of the disk. His images of the stars HL Taurus in 2014 and TW Hydra in 2015 showed smooth, symmetrical disks with dark, circular "gaps" that extend far beyond Neptune's orbit. “It came as a huge surprise. There was no mess in the disks, they have a nice, regular, beautiful structure. This is a blow to the proponents of gravitational instability. Nature is smarter than our theories,” Roman Rafikov explained.
ALMA image of the disk around the young star TW Hydra. Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)
It's too early to tell what other surprises SPHERE and GMI will bring from the outer reaches of the planetary systems. But the regions between these outlying regions and the close orbits of "hot Jupiters" and super-Earths remain stubbornly out of reach: too close to the star for direct imaging and too far for indirect methods. As a result, it is still difficult for theorists to get the full picture. “We rely on fragments and incomplete observations. Right now, it’s probably all wrong,” said Greg Laughlin.
However, astronomers won't have to wait long. Next year, NASA will launch the Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency (ESA) the Characterizing Exoplanets Satellite (CHEOPS). Unlike the Kepler mission, which surveyed a large number of stars while conducting a population census, TESS and CHEOPS will focus on bright, nearby, sun-like stars, allowing researchers to study "average" orbits. And since the targets will be close to Earth, ground-based telescopes will need to be able to estimate their mass, with which scientists can calculate density and indicate whether they are rocky or gaseous.
NASA's James Webb Space Telescope, set to launch in 2018, will go even further. It will analyze the star's light passing through the exoplanet's atmosphere to determine its composition. “This is an important key to the formation of the planet. For example, having more heavy elements in the super-Earth's atmosphere would suggest that the disk is rich in these elements. necessary for the rapid formation of the planetary core,” explained Bruce McIntosh. In the next decade, spacecraft such as NASA's Wide Field Infrared Survey Telescope (WFIRST) and ESA's Planetary Transits and Oscillations (PLATO) will join the search, as well as a new generation of huge ground-based telescopes with 30-meter (or more ) mirrors.
This illustration shows exoplanet WASP-121b, an ultra-hot Jupiter that is so close to its star that even iron boils on its dayside. Credit & Copyright: Engine House VFX, At-Bristol Science Centre, University of Exeter.
Ultra-hot Jupiters are a new class of exoplanets that astronomers are increasingly finding in various parts of the universe. These incredibly hot gas giants are much closer to their stars than Mercury is to the Sun, which invariably results in tidal lock, meaning the planet always faces the same side of the star. This causes daytime temperatures there to exceed 1,900 degrees Celsius, while temperatures on the night side are around 1,000 degrees Celsius. In addition, ultra-hot Jupiters exhibit unique atmospheric characteristics that other planets do not, such as the absence of molecules.
Despite the intriguing nature of these strange, hellish worlds, scientists still know little about them. However, a new study accepted for publication in the journal Astronomy and Astrophysics may change this state of affairs.
In this study, an international team of scientists simulated the atmospheres of four known ultra-hot Jupiters that were previously explored using the Hubble and Spitzer space telescopes. And based on the data, the team concluded that ultra-hot Jupiters are even more unusual than originally thought.
In particular, the team found that these exoplanets are so hot during the day that the heat can break most types of molecules into their component parts. And because these molecules are destroyed, they are not visible even to our most advanced observatories. This led the researchers to a surprising conclusion: the atmosphere on the dayside of ultra-hot Jupiter resembles a star more than a planet.
In addition to being interesting in itself, this result may also explain why astronomers find only water molecules at the edge of the day and night sides of ultra-hot Jupiters. The team found that as hydrogen and oxygen atoms make their way to the cooler night side of the planet, they recombine, which in turn leads to the formation of water. However, because the planet's night side is too dark to see directly, astronomers can only detect these water molecules at the edge of day and night.
This new study not only sheds light on an understudied class of exoplanets, but also provides valuable data that will help astronomers better understand the physical processes that take place on them.
