Stars | What is an Exoplanet? – Exoplanet Exploration: Planets Beyond our Solar System (2023)

In brief

Stars are the most basic building blocks of galaxies.

The age, distribution, and composition of stars trace the history, dynamics, and evolution of their galaxy. Stars are responsible for the production and distribution of heavy elements, such as carbon, nitrogen, and oxygen.

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Different star types have different habitable zones. This is the area around a star where conditions are just right, neither too hot nor too cold for liquid water to exist on a planet’s surface. (For this reason, a star's habitable zone is often referred to informally as its "Goldilocks zone.")

Statistically, there should be more than 100 billion planets in our Milky Way galaxy. They come in a wide range of sizes and characteristics. Complex organisms arose on Earth only 500 million years ago, and modern humans have been here for only 200,000 years – the blink of an eye on cosmological timescales. Earth will become uninhabitable for higher forms of life in a little over 1 billion years, as the Sun grows warmer and dries our planet. Therefore, stars slightly cooler than our Sun – called orange dwarfs – are considered better for advanced life. They can burn steadily for tens of billions of years. This opens up a vast timescape for biological evolution to pursue an infinity of experiments for yielding robust life forms. And, for every star like our Sun, there are three times as many orange dwarfs in the Milky Way.

The even more abundant star type called red dwarfs (also known as M dwarf stars) have even longer lifetimes. Planets in a red dwarf's comparatively narrow habitable zone, which is very close to the star, are exposed to extreme levels of X-ray and ultraviolet radiation, which can be hundreds of thousands of times more intense than what Earth receives from the Sun. Planets in the habitable zones of red dwarfs can be baked bone dry and have their atmospheres stripped away quite early in their lives. Red dwarfs typically calm down after a few billion years, but their early outbursts could prohibit their planets from evolving to be more hospitable.

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How are stars born?

How are stars born?

Stars are born from vast clouds of gas and dust, known as nebulae, that are scattered throughout most galaxies. Over thousands to millions of years, gravity can cause denser pockets within a nebula to collapse under their own weight. As a cloud – which is mostly hydrogen – collapses, the material at its center begins to heat up. Known as a “protostar,” this hot core of the collapsing cloud will is a star in the making. Some of these spinning clouds of collapsing gas and dust break up into two or three blobs that each form stars. This would explain why most of the stars in the Milky Way come in pairs or in multiples. Not all of this material ends up as part of the star, however – the remaining dust can become planets and moons, asteroids, and comets – or may simply remain as dust.

What is a main sequence star?

What is a main sequence star?

As millions of years pass, the core temperature of a protostar reaches a point at which nuclear fusion can begin. The star then begins the longest stage of its life, called the “main sequence.” Most stars in the galaxy, including our Sun, are categorized as main sequence. This is a state in which nuclear fusion in the star is stable and hydrogen is converted to helium. This process releases a lot of energy that keeps the star hot and bright, and it supplies an outward pressure against the incredible mass of material that would otherwise cause the star to collapse on itself. Ninety percent of a star’s life is spent in the main sequence phase.

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What does a star's color mean?

What does a star's color mean?

When you look at the night sky, you may notice that some stars shine more brightly than others. The brightness of a star is related to how much energy it puts out, as well as how close it is to Earth.

Stars also vary in color – because they vary in temperature. Hotter stars appear blue or white, while cooler stars look orange or red. Astronomers use these characteristics to classify main sequence stars into categories by color and temperature: O (blue), B (blue-white), A (white), F (yellow-white), G (yellow), K (orange), and M (red), from hottest and biggest to coolest and smallest. Stars at the ends of their lives are out of the main sequence. These include supergiants, red giants, and white dwarfs.

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What kind of star is our Sun?

What kind of star is our Sun?

Our Sun is categorized as a G-type yellow-dwarf main sequence star. It is predicted that our Sun will remain in the main sequence phase for a few billion more years.

Stars can live for billions of years, but their lives can be shorter or longer depending on their size (technically, their mass). The bigger (or more massive) the star, the shorter its life, as more massive stars burn their nuclear fuel faster.

(Video) Exoplanet Types: Worlds Beyond Our Solar System

How do planets form around stars?

How do planets form around stars?

The gas and dust that swirl around a star during its formation are critical to forming planets around it. The dust contains heavy elements such as carbon and iron that form the cores of planets.

Scientists think planets start off as grains of dust smaller than the width of a human hair. They emerge from the giant, donut-shaped disk of gas and dust that circles young stars. Gravity and other forces cause material within the disk to collide. If the collision is gentle enough, the material fuses, growing like rolling snowballs. Over time, dust particles combine to form pebbles, which evolve into mile-sized rocks. As these planetesimals, or planets in the making, orbit their star, they clear material from their path, leaving tracks of space empty but for fine dust. At the same time, the star gobbles up nearby gas while pushing more distant material farther away. After a few million years, the disk will have totally transformed, much of it taking the form of new worlds.

The life cycles of stars

The life cycles of stars

Red giants and white dwarfs

When an average star like our Sun runs out of hydrogen to fuse, the star starts to collapse. But compacting a star causes it to heat up again and it is able to fuse what little hydrogen remains in a shell wrapped around its core. This burning shell of hydrogen greatly expands the outer layers of the star. When this happens, the star becomes a red giant. When our Sun enters the red giant phase of its life, in about 5 billion years, it will be so big that Mercury will be completely swallowed.

Our red giant Sun will still be consuming helium and cranking out carbon. When the helium is gone, the Sun will succumb to gravity again. When the core contracts, it will cause a release of energy and the Sun will become an even bigger giant with a radius beyond Earth's orbit.

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After about a billion years as a red giant, the Sun will have ejected its outer layers until, eventually, its stellar core is exposed. This dead (in terms of nuclear fusion) but still ferociously hot stellar cinder is called a white dwarf. White dwarfs are roughly the size of Earth, despite containing the mass of a star. Pressure from fast moving electrons keeps these stars from further collapse. The more massive the core, the denser the white dwarf that is formed. Thus, the smaller a white dwarf is in diameter, the larger it is in mass! White dwarfs fade into oblivion over many billions of years as they gradually cool down.

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This fate awaits only those stars with a mass up to about 1.4 times the mass of our Sun. Above that mass, electron pressure cannot support the core against further collapse. Such stars suffer a different fate.

White dwarfs may become novae

If a white dwarf forms in a binary or multiple star system, it may experience a more eventful demise as a nova. Nova is Latin for "new" – novae were once thought to be new stars in the act of being born. Today, we understand that they are very old stars – white dwarfs. If a white dwarf is close enough to a companion star, its gravity may drag matter (mostly hydrogen) from the outer layers of that star onto itself, building up on its surface. When enough hydrogen has accumulated on the surface, a burst of nuclear fusion erupts, causing the white dwarf to brighten substantially and eject its remaining material. Within a few days, the glow subsides and the cycle starts again. Sometimes, particularly massive white dwarfs (those near the 1.4 solar mass limit) may accrete so much mass in this manner that they collapse and explode completely, becoming what is known as a supernova.

Going supernova

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Stars more than eight times the mass of our Sun are destined to die in a titanic explosion called a supernova. A supernova is not merely a bigger nova. In a nova, only the star's surface explodes. In a supernova, the star's core collapses and then explodes. In massive stars, a complex series of nuclear reactions leads to the production of iron in the core. Having achieved iron, the star has wrung all the energy it can out of nuclear fusion. The star no longer has any way to support its own mass, and the iron core collapses. In just a matter of seconds the core shrinks from roughly 5,000 miles across to just a dozen, and the temperature spikes 100 billion degrees or more. The outer layers of the star initially begin to collapse along with the core, but rebound with the enormous release of energy and are thrown violently outward. Supernovae release an almost unimaginable amount of energy. For a period of days to weeks, a supernova may outshine an entire galaxy. Likewise, all the naturally occurring elements and a rich array of subatomic particles are produced in these explosions.

Neutron stars and pulsars

If the collapsing stellar core at the center of a supernova contains between about 1.4 and 3 solar masses, the collapse continues until electrons and protons combine to form neutrons, producing a neutron star. Neutron stars are incredibly dense. Because it contains so much mass packed into such a small volume, the gravity at the surface of a neutron star is immense. Like white dwarfs, if a neutron star forms in a multiple star system it can accrete gas by stripping it from nearby companions.

Neutron stars also have powerful magnetic fields that can accelerate atomic particles around its magnetic poles, producing powerful beams of radiation. Those beams sweep around like massive searchlights as the star rotates. If such a beam is oriented so that it periodically points toward Earth, we observe it as regular pulses of radiation that occur whenever the magnetic pole sweeps past our line of sight. In this case, the neutron star is known as a pulsar.

Black holes

If the collapsed stellar core is larger than three solar masses, it collapses completely to form a black hole: an infinitely dense object whose gravity is so strong that nothing can escape, not even light.

Because photons are what our instruments are designed to see, black holes can only be detected indirectly. Indirect observations are possible because the gravitational field of a black hole is so powerful that any nearby material – often the outer layers of a companion star – is caught up and dragged in. As matter spirals into a black hole, it forms a disk, called an accretion disk, that is heated to enormous temperatures, emitting copious quantities of X-rays and gamma-rays that indicate the presence of the underlying hidden companion.

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Black holes that are quiet and not actively "feeding" on accretion disks can also be detected indirectly by observing the motions of nearby stars. For example, astronomers observe the supermassive black hole at the center of the Milky Way by watching as nearby stars whip around at astounding speeds only possible under the influence of an incredibly massive, but invisible object.

From the remains, new stars and planets arise

The dust and debris left behind by novae and supernovae, as well as by red giants puffing off their outer layers, eventually blend with the surrounding interstellar gas and dust, forming new nebulae. The products created in the ends of the lives of stars enrich galaxies with heavy elements and chemical compounds. Eventually, those materials are recycled, providing the building blocks for new generations of stars and planetary systems.

Next: What is the universe? (Hint: you're included)


Stars | What is an Exoplanet? – Exoplanet Exploration: Planets Beyond our Solar System? ›

An exoplanet is any planet beyond our solar system. Most orbit other stars, but free-floating exoplanets, called rogue planets, orbit the galactic center and are untethered to any star.

What is an exoplanet quizlet? ›

A planet that is considered an "extra," in that it was not needed for the formation of its solar system.

What are the planets outside our solar system? ›

The outer planets are gas giants Jupiter and Saturn and ice giants Uranus and Neptune. Beyond Neptune, a newer class of smaller worlds called dwarf planets reign, including longtime favorite Pluto.

What is a exoplanet NASA? ›

Planets that orbit around other stars are called exoplanets. All of the planets in our solar system orbit around the Sun. Planets that orbit around other stars are called exoplanets. Exoplanets are very hard to see directly with telescopes. They are hidden by the bright glare of the stars they orbit.

How many stars have planets NASA? ›

Our planetary system is the only one officially called “solar system,” but astronomers have discovered more than 3,200 other stars with planets orbiting them in our galaxy. That's just how many we've found so far.

Which statement best describes an exoplanet extrasolar planet )? ›

What best describes an extrasolar planet? A planet that orbits a star other than our Sun.

Where are most of the detected exoplanets located quizlet? ›

Most of the planets detected so far orbit closer to their star than in the Solar System, but the detection methods are biased. Few exoplanets have been images directly because of their small separation from and dimness relative to the star.

How many Earths are there? ›

There are a billion Earths in this galaxy, roughly speaking. Not a million. A billion. We're talking 1 billion rocky planets that are approximately the size of the Earth and are orbiting familiar-looking yellow-sunshine stars in the orbital “habitable zone” where water could be liquid at the surface.

What are the name of the 8 planets? ›

The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Mercury is closest to the Sun. Neptune is the farthest. Planets, asteroids, and comets orbit our Sun.

Is Earth an exoplanet? ›

Simply put, exoplanets are planets that lie beyond our solar system. So first, we have to understand the definition of a planet. Planets are worlds that orbit our Sun, like Mars, Jupiter, and of course, our own Earth.

Is there a new planet? ›

A still-forming exoplanet

(The sun, for context, is over 4.5 billion years old.). The new planet is giant. Scientists suspect its nine times bigger than Jupiter. And it orbits profoundly far from its star, at some 8.6 billion miles away.

Can you own a planet? ›

No, people cannot legally buy planets, at least for now. There isn't any way to legally enforce a claim to a planet, and courts have rejected similar claims in the past. International law forbids countries from claiming any celestial body, meaning a nation cannot grant space real estate to its citizens.

What is exoplanet made of? ›

Exoplanets are made up of elements similar to that of the planets in our solar system, but their mixes of those elements may differ. Some planets may be dominated by water or ice, while others are dominated by iron or carbon.

Is our sun a star? ›

Our Sun is an ordinary star, just one among hundreds of billions of stars in the Milky Way Galaxy. As the only star we can observe in detail, it provides a basis for our understanding of all stars. The Sun is composed almost entirely of hydrogen and helium gas.

What is a star made of? ›

Stars are huge celestial bodies made mostly of hydrogen and helium that produce light and heat from the churning nuclear forges inside their cores.

How old is the planet? ›

Why is exoplanet research important? ›

Observing exoplanets allows us to determine whether or not we actually understand those processes, even in our own solar system. In fact, what we've seen so far is that most stellar systems don't look like our solar system.

How was first exoplanet discovered? ›

On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12. This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets.

When was the first exoplanet discovered? ›

Previously, scientists have used this method to discover thousands of exoplanets, or planets outside of our solar system (but still in our galaxy). The first exoplanet discovered was in 1992 and, since then, most exoplanets found have been less than 3,000 light-years from Earth.

What are some methods used to detect exoplanets? ›

The main techniques used to detect exoplanets are:
  • Direct imaging: The exoplanet is imaged directly using large telescopes fitted with adaptive optics and coronagraphs. ...
  • Radial velocity: ...
  • Transits: ...
  • Microlensing: ...
  • Transit timing variations:

Which of the following has been the most effective method for finding exoplanets quizlet? ›

The Doppler-shift method for detecting the presence of exoplanets is best able to detect: massive planets near the star.

What is an important difficulty with the transit method of detecting exoplanets quizlet? ›

Why is it so difficult to directly observe exoplanets? They are too close to the (much brighter) stars which they orbit.

How many Multiverses are there? ›

In a new study, Stanford physicists Andrei Linde and Vitaly Vanchurin have calculated the number of all possible universes, coming up with an answer of 10^10^16.

Is Earth in a galaxy? ›

We live on a planet called Earth that is part of our solar system. But where is our solar system? It's a small part of the Milky Way Galaxy. A galaxy is a huge collection of gas, dust, and billions of stars and their solar systems.

Is there another Earth with humans? ›

The Kepler space telescope identified the exoplanet, and its discovery was announced by NASA on 23 July 2015.
Physical characteristics
Mean radius1.5 +0.32 0.22 R Earth
Mass5 ± 2 M Earth
14 more rows

What are 9 planets called? ›

The order of the planets in the solar system, starting nearest the sun and working outward is the following: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and then the possible Planet Nine.

Is Pluto still a planet? ›

According to the International Astronomical Union, the organization charged with naming all celestial bodies and deciding on their statuses, Pluto is still not an official planet in our solar system.

Why Pluto is not a planet? ›

Answer. The International Astronomical Union (IAU) downgraded the status of Pluto to that of a dwarf planet because it did not meet the three criteria the IAU uses to define a full-sized planet. Essentially Pluto meets all the criteria except one—it “has not cleared its neighboring region of other objects.”

Who found Earth? ›

By around 500 B.C., most ancient Greeks believed that Earth was round, not flat. But they had no idea how big the planet is until about 240 B.C., when Eratosthenes devised a clever method of estimating its circumference.

Whats is a star? ›

A star is any massive self-luminous celestial body of gas that shines by radiation derived from its internal energy sources. Of the tens of billions of trillions of stars in the observable universe, only a very small percentage are visible to the naked eye.

Why is it called an exoplanet? ›

An exoplanet is also called an “extrasolar planet” - both terms simply mean a planet which is in orbit around a star which is not ours. The 'exo' part comes from the same root as an “exoskeleton”, “exothermic” or “exotic”.

Is Planet 9 a black hole? ›

Is Planet 9 a black hole? A group of astronomers, including Avi Loeb at Harvard University, suggested Planet Nine may be a tiny black hole somewhere out in the Oort Cloud. If Planet Nine turns out to be Black Hole Nine instead, it's probably about the size of a grapefruit but about 5 to 10 times the mass of Earth.

Is there a Planet 9? ›

Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the peculiar clustering of orbits for a group of extreme trans-Neptunian objects (ETNOs), bodies beyond Neptune that orbit the Sun at distances averaging more than 250 times that of the Earth.

What is the 10th planet? ›

Because Eris appeared to be larger than Pluto, NASA initially described it as the Solar System's tenth planet.
Eris (dwarf planet)
Eris (center) and Dysnomia (left of center); image taken by the Hubble Space Telescope
Discovered byM. E. Brown C. A. Trujillo D. L. Rabinowitz
40 more rows

Who is owner of Moon? ›

The Outer Space Treaty means therefore that - no matter whose national flags are planted on the lunar surface - no nation can 'own' the Moon. As of 2019, 109 nations are bound by the Treaty, and another 23 have signed the agreement but have yet to be officially recognised.

Can I buy land on moon? ›

Buying land on the moon is illegal as per the Outer Space Treaty, which was designed by the Soviet Union and the United States at the height of the cold war in 1967 to prevent an imminent space colonization race and it has since been signed by 109 nations, including India.

Can I sell a planet? ›

That's because, in 1967, the United Nations ratified its Outer Space Treaty, which states that “outer space shall be free for exploration and use by all States.” According to the treaty, any past, present or future sales of stars, moons, planets or asteroids are null and void.

How big is a exoplanet? ›

Exoplanets come in a wide variety of sizes, from gas giants larger than Jupiter to small, rocky planets about as big around as Earth or Mars. They can be hot enough to boil metal or locked in deep freeze. They can orbit their stars so tightly that a “year” lasts only a few days; they can orbit two suns at once.

Is a exoplanet a star? ›

And it's because – unlike stars – exoplanets don't shine with their own light. Like our own Earth, they shine only with light reflected from their local stars. In contrast to their stars, exoplanets are exceedingly dim; even the largest are drowned in the light of their vastly brighter stars.

Can we live in exoplanets? ›

In recent years, scientists have discovered a large amount of exoplanets, but close to none of them could actually support human life. For example, Kepler 10b, an exoplanet in another solar system, is close to the size of Earth, but it's too close to its star for human life.

Is the Earth a star? ›

The Earth is an example of a planet and orbits the sun, which is a star. A star is usually defined as a body of gas which is large enough and dense enough that the heat and crushing pressure at its center produces nuclear fusion.

How old is our galaxy? ›

Is moon a star? ›

In reality, the moon is not considered a star. While it shines just like many of the stars in the sky, its light comes from the sun, not itself. To be a star, a celestial body must be capable of igniting itself because of its mass. The moon's core has never ignited, so it does not fall under the definition of a star.

Are stars on fire? ›

with stars, they are not actually on fire. The heat and light are released by the chemical process of atoms joining together. This middle stage in the life cycle of a star is called the main sequence. As the hydrogen is used up, the star begins to fuse helium and heavier elements.

How are stars born? ›

A star is born when atoms of light elements are squeezed under enough pressure for their nuclei to undergo fusion. All stars are the result of a balance of forces: the force of gravity compresses atoms in interstellar gas until the fusion reactions begin.

Is a star a rock? ›

Instead of solids (like rock and soil) and liquids (like water), the Sun is made up mostly of gases and plasma. In fact, the Sun is comprised almost entirely of two extremely hot gases: hydrogen and helium. Stars also usually have trace amounts of heavier elements, such as oxygen, nitrogen, carbon, and iron.

What age do we live in? ›

We are all in the midst of a new geological age, experts say. This age, dubbed the Meghalayan, began 4,250 years ago when what was probably a planetwide drought struck Earth, according to the International Union of Geological Sciences (IUGS).

How old is the world in Bible? ›

Concerning the age of the Earth, the Bible's genealogical records combined with the Genesis 1 account of creation are used to estimate an age for the Earth and universe of about 6000 years, with a bit of uncertainty on the completeness of the genealogical records, allowing for a few thousand years more.

Do you age in space? ›

In space, people usually experience environmental stressors like microgravity, cosmic radiation, and social isolation, which can all impact aging. Studies on long-term space travel often measure aging biomarkers such as telomere length and heartbeat rates, not epigenetic aging.

What kind of planet is easier for us to detect quizlet? ›

What kinds of planets are easiest to detect with each method? -Both techniques are most sensitive to large planets close to their stars. -The transit method is most sensitive to large planets close to their stars. Doppler technique is most sensitive to large planets far from their stars.

What types of exoplanets are astronomers finding in our galaxy? ›

So far scientists have categorized exoplanets into the following types: Gas giant, Neptunian, super-Earth and terrestrial.

Why is it difficult to observe an exoplanet directly through a telescope? ›

Why is it difficult to observe an exoplanet directly through a telescope? Exoplanets are small compared to their parent star, so they reflect only a small portion of the star's light.

What kinds of exoplanets seem to be the most common so far? ›

What kinds of exoplanets seem to be the most common so far? Super-Earths and sub-Neptunes. Astronomers have discovered massive gas giant planets like Jupiter orbiting companion stars at closer than 0.7 AU (about the distance of Venus' orbit).

Which planetary properties we can measure using transit method quizlet? ›

In the transit method, the fraction of light absorbed is the ratio of the planet's area to the star's area, so we can find the physical size of the planet. If we also measure its mass via radial velocity, we can calculate its density.

Which of the following is a major reason why it's difficult to obtain direct images of extrasolar planets group answer choices? ›

Which of the following is a major reason why it so difficult to obtain direct images of extrasolar planets? The light of the planets is overwhelmed by the light from their star.

What is the relationship between the size mass average density composition and location of the planets? ›

A planet's density is how much material it has in the space the planet occupies: density = mass/volume. Planets can have a wide range of sizes and masses but planets made of the same material will have the same density regardless of their size and mass.


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