Where is the exoplanet Proxima b located? Astronomers have found the closest Earth-like planet. Looking in the mirror

After years of careful searching, observation and research, an international team of scientists has discovered irrefutable evidence of the existence of a planet orbiting the closest star to the sun, Proxima Centauri. This red dwarf is part of the neighboring star system Alpha Centauri. To date, this exoplanet is perhaps one of the most exciting space finds of recent times.

New exoplanet

The planet is called Proxima Centauri b (proxima b for short) and is in all likelihood only slightly larger than Earth. In addition, the planet is located at such a distance from the star to provide the ideal temperature for water to appear on the surface.

Proxima Centauri is a faint dwarf star located four light years from Earth. Despite its relatively short distance, Proxima cannot be seen with the naked eye from the surface of our planet; it is too small. The closest visible star is Alpha Centauri. Proxima is most likely connected to Alpha (which is a double star) by a gravitational field.

Search for new worlds

Once scientists realized the possibility of Earth-like exoplanets, their telescopes turned to the nearest star. The new discovery comes as a result of a thorough study of data collected by the Southern European Observatory over 14 years, from 2000 to 2014. Supporting data was collected in the first half of 2016.

The discovery of the exoplanet was made using the Doppler effect, which makes it possible to accurately observe the radial velocity of a celestial object and, by its change, determine the wavelength of light vibrations.

What do we know about the new planet?

At least one planet orbiting Proxima Centauri exists beyond any doubt. Its solar year consists of eleven days, because the distance between the planet and the star does not exceed seven million kilometers - much less than between the Sun and Mercury. From observations it follows that Proxima b is relatively small in size, its minimum mass is 1.27 Earth masses.

Compared to the glowing surface of the Sun, Proxima Centauri looks more like a small hot coal. For example, if the new planet were the same distance from Proxima as the Earth is from the Sun, it would be completely frozen. However, Proxima b's surface temperature may be just right for life. If the new exoplanet is in fact similar to Earth, then it is located in just the right place for water to exist on the surface.

Possibility of life

We know that Proxima b is the right size, in the right location, and likely maintains the right temperature, but are these characteristics sufficient to support life on a planet? Perhaps, but for this to happen a number of other characteristics must perfectly match.

Because Proxima b is so close to its star, it rotates almost exactly in lockstep while tidally locked. This means that the planet is turned to the star with only one side, and its position does not change. It turns out that on one half of the planet there is always light and warmth, and on the other there is darkness and cold.

For many years, scientists believed that such conditions could not possibly support life. The dark side would be so cold that any existing atmosphere would simply freeze to the surface, leaving the planet without air. However, later models suggest that even under tidal locking conditions an atmosphere can be preserved if it has the right composition and if the temperature of the planet's core, as well as the surface of its light part, is sufficient to provide minimal heat to the dark side. In such conditions, the dark side can also be quite habitable.

Scientists currently do not have enough information to determine whether Proxima b can support life on its surface.

Magnetosphere and stellar activity

Tidal locking is not the only possible problem for life on Proxima b. Proxima Centauri is an active star, and its surface is often subject to violent storms, megaflares, and coronal mass ejections that can destroy any atmosphere of a planet orbiting it.

This does not happen with Earth, even though the Sun is more active and larger than Proxima Centauri, because our planet has a strong magnetosphere that protects us from harmful solar activity. If the new planet has a similar magnetosphere, then it is equipped with a kind of shield that protects not only the atmosphere, but also the surface itself.

The estimated mass of Prxima b hints that, with a similar composition to Earth, it may well have its own protective magnetic field. Perhaps this planet is actually habitable.

Of course, scientists still know too little about the new planet, and the possibilities for studying it are extremely limited. However, with the constant development of new technologies, we can only hope that very soon we will learn more about the possible twin of our Earth.

The closest exoplanet to our solar system, orbiting the star Proxima Centauri just 4.2 light-years from the Sun, is found to be habitable, according to extensive computer modeling. Such conclusions were made possible by the latest model calculations made on the basis of data already known about the planet Proxima b.

Artist's representation of the planet Proxima b with its star Proxima Centauri and its companion stars Alpha Centauri A and B in the background. Copyright: ESO/M. Kornmesser

A team of researchers led by Anthony Del Genio of NASA's Goddard Institute for Space Studies wrote in a new issue of the journal Astrobiology, based on estimates from computer models used to simulate climate change on Earth, but with data known about Proxima Centauri b.

Modeling results show that Proxima b, even under various scenarios, may have large volumes of liquid water on its surface, which is why the likelihood of life on this planet increases significantly. “The key result of our comprehensive modeling is that the probability of the planet being suitable for life is indeed extremely high,” says Del Genio.

Proxima Centauri itself is a cool red dwarf star located 4.2 light-years from the Sun. And although it is located so close to our solar system, astronomers still know too little about the system surrounding this red dwarf. And even the first, and so far the only known planet of this star (Proxima Centauri b), was discovered only in 2016. Proxima b is 1.3 times the size of Earth, and it completes one revolution around its star in just 11 days.

“We assume that the planet has an atmosphere and oceans on the surface, while it orbits its star within its habitable zone,” the researchers say. “This allows the planet to receive enough light to keep surface temperatures above the freezing point of water.” But since the planet revolves around its star at a fairly close distance, it cannot be ruled out that between them there is the same connection as the Earth and the Moon, that is, the planet remains constantly turned to the star with only one side.

While earlier models suggested that only the star-facing side of the planet heats up, while the oceans freeze on the other side, Del Genio also did not rule out the possibility of such a thawed eyeball-shaped hemisphere facing the star, suitable for life - the so-called “eyeball planet”.

This is how the artist sees the “eyeball planet.” Copyright: NASA/JPL-Caltech

The complexity of the new simulations surpassed all previous model calculations, and they also included data on the dynamics of ocean and atmospheric circulation, through which global heat is distributed.

Given this state of affairs, it is quite possible to assume that although the side turned away from the star never sees the “sun’s rays,” a strip of liquid water stretches along the equator around the entire planet. Something similar is observed on our Earth, where, thanks to the Gulf Stream, the East Coast of the United States is always much warmer than it would have been without this warm current from the tropics.

In total, Del Genio's team carefully tested 18 potential planetary scenarios that included the effects of large continents, thin atmospheres, different atmospheric compositions, and even the effects of different salt concentrations in the proposed ocean.

And almost all of these options showed that at least parts of this planet must remain warm enough for liquid water and oceans to persist on the surface. “The larger the surface of potentially liquid water, the greater the likelihood that with the help of telescopes of future generations we will be able to discern life there,” Del Genio finally dreamed.

Since the beginning of the space age, people have used chemical rockets to get into space. While this method is certainly effective, it is quite expensive and resource intensive. Scientists became interested in the question: would hypothetical aliens be able to leave their planets using similar technologies?

Two studies

Harvard professor Abraham Loeb and astronomer Michael Gippke, an independent researcher associated with the Sonneberg Observatory, attempted to analyze this question in two recently published papers. Professor Loeb looked at the problems that extraterrestrial beings might encounter when launching rockets from Proxima b. Hippke's research addresses a similar question—whether aliens living on a super-Earth can get to .
In his study, Loeb argues that we humans are lucky to live on a planet that is well suited for space launches. In order for the rocket to leave the surface of the Earth and begin to orbit the Sun as its satellite, it needs to reach a speed of 11.186 km/s. The speed required to escape Earth's orbit and leave the solar system is about 42 km/s relative to the Sun.

Professor Loeb says:

“Accelerating a rocket to cosmic speeds requires a huge mass of fuel, which grows exponentially. By happy coincidence, the rate of escape from Earth's orbit around the Sun is at the limit of the speed achievable by chemical rockets. However, the habitable zone around fainter stars is closer to them, making it very difficult for chemical rockets to escape their gravitational clutches. your star."

As Loeb points out in his essay, escape velocity is calculated as the square root of the stellar mass divided by the distance from the star. This means that the rate of escape from the habitable zone is directly proportional to the stellar mass and inversely proportional to the distance from the star.

This infographic compares the planet's orbit around Proxima Centauri (Proxima b) with the same region of the Solar System.

Proximity to a star is not good for planets that orbit M-type stars (red dwarfs). These stars are the most common type of star in the Universe, making up about 75% of such objects in the Milky Way Galaxy. In addition, recent research has discovered many rocky planets orbiting red dwarf stars, and some scientists believe such planets are the most promising place to look for potentially habitable worlds.

Is it possible to fly away from Proxima b?

Using the closest star to our own world (Proxima Centauri) as an example, Loeb explains that a rocket using chemical fuel would have a much harder time reaching escape velocity from a planet located in its habitable zone.
“The closest star to the Sun, Proxima Centauri, is an example of a faint star with only 12% of the mass of the Sun,” he said. “A couple of years ago, this star was discovered to have a planet the size of , called Proxima b. It is located in the habitable zone, which is 20 times closer to the star than the Earth is from the Sun. At this location, the ejection speed is 50% greater than in the Earth's orbit around the Sun. It will be difficult for the civilization on Proxima b to leave their world using chemical rockets."

What did Hippke research?

Hippke's research begins with the assertion that Earth may not, in fact, be the most common type of planet in our universe. For example, planets that are more massive than Earth will have higher surface gravity, meaning they will be able to retain a denser atmosphere, which will provide protection from harmful cosmic rays and solar radiation.

An artist's impression of a Super-Earth, a class of planets that has many Earth masses but is smaller than the planet Uranus or Neptune. Source: NASA/Ames/JPL-Caltech.

Additionally, a planet with higher gravity would have a flatter topography, resulting in archipelagos instead of continents and smaller oceans—an ideal situation when it comes to promoting biodiversity. However, when it comes to launching rockets, increased surface gravity will mean the need to reach higher flight speeds. As Hippke pointed out in his study:

"Rocket propulsion obeys the Tsiolkovsky equation (1903): if the rocket is carrying propellant, the ratio of the total rocket mass to the final velocity is an exponential function, making high speeds (or heavy loads) more expensive."

For his calculations, Hippke uses Kepler-20 b, a “Super-Earth” located 950 light-years away. This planet is 1.6 times the size of Earth and has 9.7 times the mass of our planet. While the speed at which a body leaves orbit around Earth is approximately 11 km/s, a rocket attempting to leave a super-Earth like Kepler-20 b would have to achieve an exit speed of ~27.1 km/s. As a result, a single-stage rocket on Kepler-20 b would have to burn 104 times more fuel than a rocket on Earth in order to reach orbit.

To put this all into perspective, Hippke looks at specific payloads launched from Earth. “To launch a payload of 6.2 tons, as is required in the case of the space telescope. James Webb from the planet Kepler-20 b, the fuel mass will increase to 55,000 tons, which is equal to the mass of the largest ocean battleships,” he writes. "For a classic Apollo to the Moon (45 t) the rocket would have to be significantly larger, ~400,000 t."

Project Starshot, designed to be humanity's first interstellar journey.

Hippke's analysis allows us to conclude that chemical rockets will still provide the speeds necessary for departure from the planet on super-Earths with up to 10 Earth masses. However, the amount of fuel required makes this method impractical. As Hippke noted, this could seriously affect the development of an alien civilization.

"I'm amazed at how lucky we humans are to find ourselves on a planet that's good enough for spaceflight," he said. “Other civilizations, if they exist, may not be so lucky. On more massive planets, spaceflight will be more expensive, and its opportunity will decrease exponentially depending on the mass of the planet. Such civilizations will not have satellite television, a mission to the moon or the Hubble Space Telescope.”

Both of these articles provide some clear conclusions that relate to the search for extraterrestrial intelligence (). First, it means that civilizations on planets that orbit red dwarfs or super-Earths are less likely to explore space, making them harder to detect. The results of the study also indicate that humanity may be one of the few civilizations that have been given the opportunity to explore space by.

In August 2016, staff at the European Southern Observatory announced an amazing discovery. It turned out that an unusual exoplanet, Proxima Centauri b, revolves around Proxima Centauri, the nearest star just 4.25 light years away, with a period of 11.2 Earth days. Its main feature is that the probability of life on it is extremely high, although the conditions in which Proxima Centauri b is located are not at all the same as in the Solar system. And if so, the story about this distant, near planet is directly related to our beloved biology.

In general, we are serious people. The granite of science crunches on our teeth. We cover such harsh, complex corners of biological knowledge that the lights of other popular science sites have not reached. But sometimes we just want to fool around. And talk about science in a fun way, show it from a different angle. Draw funny pictures, write light and funny text. That's why we opened a new section - “12 biological news in pictures.”

The intellectual partner of these illustrated stories is RVC JSC.

Information about the existence of Proxima Centauri b (simply Proxima b) was leaked online on August 12, 2016. Literally two weeks after this, on August 24, employees of the European Southern Observatory confirmed rumors about the discovery of a new planet. And several preprints of scientific articles appeared on the Internet, the authors of which discussed its habitability. Subsequently, a number of these articles were published in the journal Astronomy & Astrophysics .

First, we must say how the planet was discovered. Not all biologists are strong in physics, so it is all the more worth noting the method that made it possible to “see” the potential nearest habitable planet to us. It's called radial velocity method or Doppler method. The fact is that not only does the star influence the planets it belongs to, but also the planets change the behavior of their star. The planet's gravity slightly shifts the radial speed of the star associated with it, as if rocking it. Changes of this kind are recorded by spectrographs, because the spectrum of the star changes.

In order for a planet to significantly influence the movement of its star, it must be quite large - have a mass no less than that of Earth, and preferably at least several times more. So we can say for sure that Proxima b is “more plump” than our space home. It is even known how much - 1.3 times.

What else do we know about this planet? It is known that it is 300 million years older than Earth, which means that if life once arose there, it had more time to develop. Who knows, maybe the Proxima people are already developing devices capable of flying not much slower than the speed of light? Be that as it may, we cannot yet fly to check this: with our current space engines it will take tens of thousands of years, and only if trillions of dollars are spent on the flight.

It is also known that the “host” of Proxima b, the star Proxima Centauri, is a red dwarf. The emission spectrum of stars of this type differs sharply from the solar one. Red dwarfs emit much more ultraviolet light than yellow dwarfs, such as the Sun. In addition, they “pamper” their planets with X-ray radiation. But in the case of Proxima b, this may not be so bad. The atmosphere there, apparently, is very dense, well blocking various harmful rays. Nevertheless, at the dawn of its times, the planet was unlikely to be inhabited: then Proxima Centauri emitted a lot of ultraviolet and X-ray radiation, and only then “settled down.”

How to describe livability?

The suitability of a planet for life is determined by several parameters:

• relatively small size and mass (about the same as the Earth);
• the temperature is at least in some places above zero Celsius, but not by much;
• presence of liquid water;
• absence of hard types of radiation that can damage DNA and other biological molecules.

This is not the entire list, but perhaps the main criteria are listed in it. In other words, potentially habitable planets should be similar to Earth in many ways. And no wonder: this is the only planet known to us that is absolutely inhabited.

More detailed discussions about the suitability of different planets for life can be seen in the article “ Wild wild space“, well, as for our Earth, it’s best to familiarize yourself with the book by Mikhail Nikitin “ Origin of life. From nebula to cell» .

Hard ultraviolet radiation plays a cruel joke on vitality because it splits water molecules into hydrogen and oxygen. Hydrogen also easily escapes from planets with moderate gravity. If so, scientists should have calculated how much water Proxima b could have lost during the time its red dwarf “behaved badly.” Let the density of the atmosphere of this exoplanet be approximately the same as that of the Earth. Then Proxima b could lose from 0.4 to 0.9 of the volume of Earth's oceans. This seems like a lot, but in reality it is quite small, considering that our Earth has lost a quarter, or even more, of the water of its oceans over its history. And besides, a significant part of the losses can be restored due to the water contained in the planet’s mantle. If so, most likely there is enough water on Proxima b now. Moreover, this water is liquid: the planet is 20 times closer to its star than the Earth is to the Sun, and therefore it is quite warm there.

In October 2016, there was even a hypothesis that the ocean on Proxima b could be up to 200 kilometers deep. It is based on calculations of the diameter of the planet depending on its composition at a known mass (remember, this is 1.3 Earth). According to the authors of the scientific article, Proxima b most likely has a compact core of silicates, covered with water on top. The mass of water is almost half of the mass of the entire planet.

The most interesting thing is that such an abundance of water does not help life, but rather reduces the likelihood of its presence on Proxima b. The fact is that such a thickness of H 2 O creates too much pressure at the bottom. Under such conditions, even hot water can turn into an exotic version of ice - so unusual that it does not exist on Earth. It will absorb up to 95% of the total mass of water on the planet. But ice is ice: its crust will not allow the ocean to communicate with the silicate core, which means there will be nowhere for salt to come from. Well, distilled water is far from the best place for the emergence and maintenance of life. Let us recall the textbook blood cells that swell and even burst under the influence of water with a minimal salt content.

On the other hand, such a harsh model of Proxima b is an extreme. The ocean there may not be so deep, it all depends on the physical parameters “driven” into the program. Their values ​​have yet to be confirmed experimentally.

As for climate, the issue is complex. The planet is close to its star, which means their mutual attraction may prevent Proxima b from rotating on its axis. As a result, the planet is probably always facing one side of its star, which is very warm, while its other hemisphere is always cold. True, the difference in temperatures between the “day” and “night” sides can be significantly smoothed out by a dense atmosphere. It is most likely on Proxima b.

By the way, calculations published already in 2017 reduced the likelihood that the planet does not really rotate around its axis. They showed that the orbit of Proxima b is quite elongated: its eccentricity is 0.25. This means that, at least at the extreme points of its orbit, the planet is not so strongly attracted by the star. Probably, on it, like on our Mercury, a day is equal to 2/3 of its year and in this case lasts about one earthly week. Taken together, this means that the difference in climate at different points on the exoplanet is not so great, so the chances of finding life there increase.

One of the most outstanding astronomers of 2016 according to the magazine Nature Guillem Anglade-Escudé even fantasized about what such a life would look like. He said in an interview with a Spanish popular science publication that “Proximian” plants, if any, look unusual because the radiation from Proxima Centauri contains most of the infrared part of the spectrum that is not visible to our eyes. But this red dwarf produces almost no visible light of green shades, like the Sun. Thus, in order to survive under its rays as efficiently as possible, it would be better for plants on Proxima b to have a reddish tint, and not green.

It’s not just the local flora that can look unusual. The fauna of red dwarfs will also be slightly unusual. If we assume that the biosphere there has reached large multicellular organisms, then among them there will be no analogues of polar bears, arctic foxes and other hares in winter coloring. It just doesn't make sense there. 95 percent of Proxima Centauri's radiation is infrared. The snow and ice in it are “black”, that is, they absorb light well and, unlike the Earth, melt quickly even with short days. Stable snow cover in winter in such conditions is unlikely - just like animals in “winter” camouflage colors.

In general, whether there is life on Proxima b or not, it is not yet possible to say with absolute certainty. But the information available at the moment indicates that the patient is more likely alive than dead. And if he is alive, we can theoretically see a lot of interesting things on him.

Literature

1. Guillem Anglada-Escudé, Pedro J. Amado, John Barnes, Zaira M. Berdiñas, R. Paul Butler, et. al.. (2016). A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature. 536 , 437-440;
2. Ortega I. (2016). European scientists praised the viability of Proxima b. "Life";
3. Martin Turbet, Jérémy Leconte, Franck Selsis, Emeline Bolmont, François Forget, et. al.. (2016). The habitability of Proxima Centauri b. "Life";
4. Ortega I. (2016). Astronomer of the Year: The planet near the nearest star is covered with strange vegetation. "Life"..

Perhaps we have found a second Earth?

A planet, possibly suitable for life, has been discovered near the star closest to the Sun; the imagination already pictures a dense atmosphere and oceans on it.

The discovered planet, named Proxima b, has an almost circular orbit, separated from the star by approximately 7.6 million kilometers (0.05 astronomical units, that is, the average distance of the Earth from the Sun). A year in this world lasts only 11 days, the mass of the planet is 1.3 times greater than that of Earth, and the average surface temperature is close to zero degrees Celsius - this is only ten degrees lower than that of Earth and several tens of degrees higher than that of Mars .

By cosmic standards, Proxima Centauri is very close - only 4.24 light years away.

The parent star Proxima Centauri itself, which is characterized by strong ultraviolet and X-ray flares, can prevent the emergence of this paradise. This is described in a study published in the journal Nature.

Computer modeling has long told astronomers that our neighbor has at least one planet, and exoplanets are mostly found around red dwarfs.

Opening Proxima b was accomplished by observing the Doppler shift of the star's spectrum due to the gravitational influence of the planet. The work was performed on two scientific instruments of the European Southern Observatory - HARPS (High Accuracy Radial velocity Planet Searcher) and UVES (Ultraviolet and Visual Echelle Spectrograph).

Despite the seemingly catastrophic proximity to the star, this world can be very good from the point of view of supporting life, because they are cold stars.

Proxima Centauri's surface temperature is more than twice (almost three thousand kelvins), its mass is ten times, and its luminosity is four orders of magnitude less than that of the Sun.

And in order for the water on the surface of the planet not to freeze, it must be much closer to its star than the Earth is to the Sun.

In the Solar System, Venus, Earth and Mars are located in a similar zone, and the distance range for the Proxima Centauri system is from 0.04 to 0.08 astronomical units. It would seem that everything speaks in favor of the emergence of life, but there is one unpleasant moment that can cancel out all the advantages.

A distinctive feature of red dwarfs is their high activity. X-ray flares that periodically occur on Proxima Centauri are about 400 times stronger than the most intense flare on the Sun. How such radiation will affect the emergence and maintenance of life is unknown. Perhaps such a superflare can generate a chain of chemical reactions with the formation of molecules of organic substances, but, on the other hand, it can “rip” the atmosphere from the planet. Possess planet Proxima b, like the Earth, by its own magnetic field, the harmful effects of radiation would be reduced, but its presence could not be detected remotely.

As a result of the most powerful solar flares, up to a trillion megatons of TNT are released into the surrounding space in a few minutes. This is about a fifth of the energy emitted by the Sun in one second, and all the energy that a person would produce in a million years (assuming it was produced at modern rates). Superflares occur, as a rule, on larger stars of spectral classes F8-G8 - massive analogues of the Sun (belonging to the G2 class). These luminaries usually do not rotate quickly around their axis and may be part of a close binary system. The power of superflares exceeds typical solar flares by tens of thousands of times, however, scientists do not exclude the possibility of such a cataclysm on the Sun.

Besides, planet Proxima b due to its proximity to the star, one side is always turned towards it, that is, it is in a state of tidal locking, like the Moon in relation to the Earth. This means that one half of the planet is constantly warm, and the other is always cold. Modeling has shown that this will not be an insurmountable obstacle to the existence of life, provided there is a dense atmosphere. Constant convective currents will ensure heat exchange between the halves of the planet and a comfortable temperature can be established in the “border zone”.

Most likely, such a large planet formed in remote areas of the system and, over time, moved to its current position. Looking at the solar system, we can say that this celestial body contains a large amount of water.

Proxima Centauri is likely part of a triple star system, which also includes the double star Alpha Centauri, the stars in it are separated by only 23 astronomical units. The red dwarf's orbital period around two Sun-like stars is more than 500 thousand years.

Flight to Alpha Centauri

Astrophysicist Philip Lubin (University of California at Santa Barbara) proposed sending a group of small automatic stations with . A system of lasers in Earth orbit will accelerate them to near light speed. A similar idea was proposed by Russian businessman Yuri Milner and British theoretical physicist Stephen Hawking.

The plans for both missions include only flying through the system, because it will be impossible to slow down.

Difficulties in implementing the project are related to its technical component and price. To implement Lyubin’s project, it will be necessary to deploy a constellation in Earth orbit that is a hundred times larger in mass than the ISS. It would take the miniature probe 15 years to reach Alpha Centauri and send back a few photos, but the cost would be tens of trillions of dollars.

A modern spacecraft could do this much cheaper, but it would take 70 thousand years.

Lubin's idea was supported by Congressman John Culberson, who called on NASA to begin work on an automatic mission to Alpha Centauri as early as 2017. According to the Republican's plans, the station should launch in 2069 - the centenary of the astronaut landing on the Moon. The Milner-Hawking team also did not stand aside. At an event dedicated to the discovery of Proxima b, it was stated that the Russian businessman planned to send probes to the mother star and planet as early as 2030. The devices should reach the goal in 20 years. The first images of the closest exoplanetary system on Earth will be seen in 2055.

The ideas of scientists and politicians were met with skepticism by most of their colleagues, and the remote study of Proxima b remains in the foreground. Problems with observation from Earth and from near space may arise due to the low luminosity and modest size of Proxima Centauri.

The open world's proximity to the Sun makes it a prime target for future exploration. In addition, there is likely a super-Earth in orbit around Proxima Centauri, located outside the zone suitable for life. The period of its revolution around the star ranges from 100 to 400 days.