The University of Texas at Arlington

The University of Texas at Arlington

UTA Planetarium

UTA Planetarium

Ask the Astronomer Q&A

Tag: "planet"

  • Are planetary distances measured to the Sun’s surface or its center?
  • The answer is actually neither. Distances in the solar system are measured from an object's center of which can be deduced from Kepler’s laws. If an object is perfectly spherical, the center of mass would be the center of the object. However, the Sun and the planets are not perfectly spherical, so the center of mass is somewhere off center. For planets in the Solar System, either the semi-major axis is given as “distance”, or the average distance is given. Average distance is simply half of (perihelion + aphelion), with other words, half of (largest distance plus smallest distance).

    (Tags:  physics  planet  solar system  sun)
  • Can we make Jupiter, Saturn, Uranus or Neptune into mini-Suns using lasers to ignite the fusion process, like they do at the National Ignition Facility? If so, will their moons begin to rotate instead of being tidally locked?
  • Your question is an interesting one. However, I must say that there is no way to turn these planets into mini-Suns by sending laser beams. Here are several major problems with your suggestion. First, laser beams would be damped in atmospheres of these planets and the beams would never reach central parts of the planets. Second, lasers beams are highly collimated, which means that we could only trigger a fusion process very locally in the upper parts of the planetary atmospheres, where gas density and temperature are relatively low. Third, the fusion process (if ever triggered) would neither be sustained nor spread throughout the atmosphere. Fourth, neither of these planets would be able to sustain nuclear reactions in their interiors (even if we found a way to ignite them there) because the interior temperatures and densities in these planets are not high enough to initiate any fusion process. Finally, I do not think that our current lasers are strong enough to even trigger a very localized fusion process in the most upper parts of atmospheres of these planets. Now, the fact that the moons are tidally locked has to do with the planet’s gravity, which would not change whether nuclear reactions would occur or not.

    (Tags:  gravity  jupiter  laser  light  neptune  planet  saturn  sun  uranus)
  • Do rogue or lonely things in space such as stars, planets, etc exist... or is everything a part of a group of some sort?
  • Not everything in the universe is grouped together. Astronomers have started to find planets that are not attached to a parent star. And theory tells us black holes and stars can be flung out of galaxies when they collide.

    (Tags:  black hole  exoplanet  planet  star)
  • Gliese 710, an Orange dwarf star, will be headed our way in 1.5 million years. Gliese 710 is expected to come within 1.1 light years of our Solar System, perturbing the Oort Clouds and its comets. Can it grab some of the moons of the giant gas planets (Jupiter, etc) and make them satellites for itself, re-arranging our Solar System?
  • It is not likely that Gliese 710 will have any effect on the moons of the gas giant planets because 1.1 light years is still a very long way away. For reference, the Sun is about 8 light minutes away from Earth. Pluto is about 5.6 light hours away from the Sun. At 1.1 light years away, Gliese will be over 5 trillion miles from the solar system. This may be close enough for the star’s gravity to have an effect on the edge of the Oort Cloud, as you mentioned, but this will likely mean an increase in meteor showers and comets. It would not completely rearrange our solar system.

    (Tags:  brown dwarf  earth  gravity  jupiter  meteor  planet  pluto  saturn  solar system)
  • Has NASA spotted an exoplanet orbiting Alpha Centauri a and b and proxima centauri?
  • Yes! A planet, known as Alpha Centauri Bb, was discovered orbiting around Alpha Centauri B, which is part of a three-star system just 4.3 light-years away from us. Alpha Centauri Bb zips around its star every 3.2 days, orbiting at a distance of just 3.6 million miles (6 million kilometers) and is approximately the same size as Earth. For comparison, Earth orbits about 93 million miles, or 150 million km, from the sun.

    (Tags:  earth  exoplanet  nasa  observatory  planet  star  telescope)
  • How many planets have rings?
  • All the gas giant planets - Jupiter, Saturn, Uranus and Neptune - in our solar system have rings, but only Saturn's are bright enough to see from ground based telescopes. Although astronomers have not yet detected rings around planets orbiting other stars, it is likely that they do exist outside of our solar system as well.

    (Tags:  exoplanet  jupiter  neptune  planet  saturn  solar system  uranus)
  • I received an email saying that we will see Mars as big as the moon on August 27, 2010. Is this true?
  • No, this is not true. The emails started circulating in 2003 when Mars made its closest approach to Earth. It was closer than it had been in 60,000 years! But, because the distances between Mars and Earth is so great (at and average of 46.5 million miles away) and their size so small (Mars is only half the size of Earth), Mars can never look as large as the Moon. That year, it appeared as a very bright red dot in the sky. Mars is visible this month in the Western part of the sky after sunset. It will appear as a red dot low in the sky. If you want to read more about this email hoax and others we see, check out these great articles on our website!

    (Tags:  astronomy  earth  light  mars  moon  planet  solar system)
  • I was wondering if you could recommend a good telescope to get a better look at space. I don't mean a very expensive one, but a reasonable one to get a good look at the stars, planets.
  • If you are just beginning to use a telescope for enhancing your stargazing experience, we would recommend the Celestron FirstScope, which we have available in our gift shop for $60. This is a table-top telescope with a 3-inch diameter, perfect for viewing the planets and bright deep space objects, such as the Orion Nebula and the Andromeda Galaxy. This telescope is easy enough for children to use, but will last a long time. If you would like to get an intermediate or advanced telescope, I would recommend contacting the Texas Astronomical Society at They have a great online resource for finding the perfect telescope for your needs, and experts able to answer all your questions.

    (Tags:  astronomy  jupiter  mars  moon  planet  saturn  solar system  star  telescope)
  • If the Sun's gravity holds the planets in orbit, why is it that we can not duplicate this?
  • Gravity is something we can replicate. It is a force that is proportional to the mass of two (or more) objects and the distance between those objects. Gravity is what holds you to the Earth. It’s what keeps the Moon in orbit around us, and us in orbit around the Sun. If we had sufficient enough mass we would be able to create a “second sun” that would change the orbits of all the planets. The only problem is creating that much mass – the sun makes up over 90% of all the mass in our solar system. It weighs 1,980,000,000,000,000,000,000,000,000,000 kg! That’s 333,000 times more than the Earth weighs.

    (Tags:  gravity  planet  sun)
  • Is anything is space completely stationary? Or is everything moving in some way?
  • Everything in the universe is moving. Planets and stars rotate on their axes. Planets, asteroids, and comets orbit around stars. Stars orbit around the center of the galaxy. Galaxies and the whole fabric of the universe are expanding and moving away from the center of the universe.

    (Tags:  asteroid  comet  planet  star  universe)
  • Is everything we see in the universe on fire or able to be seen because it is reflecting light from something on fire?
  • The answer kind of depends on what you are referring to. Take the planets of our solar system for example. They do not create their own light, they are just reflecting the Sun’s light back to us, the same is true for our Moon. On the other hand, the Sun, stars, galaxies and nebulae are all creating their own light. But I wouldn't say that they are on fire. It is true that astronomers talk about stars “burning” but we aren't referring to a fire like you may create in your fireplace or grill. What’s actually happening is Hydrogen atoms in the center of the star (like our Sun) are fusing to form Helium atoms. This creates an incredible amount of heat and light. The Sun is more than 10,000 degrees Fahrenheit!

    (Tags:  hydrogen  light  milkyway galaxy  moon  planet  star  sun)
  • Is it possible that the gas giant planets are just planets that haven't fully formed yet? Like gravity hasn't fully brought them together and compressed them to a solid yet?
  • The gas giant planets are fully formed planets. They just don’t have solid ground. But they definitely have gravity. Everything that has mass, has gravity. The more massive an object, the more gravity it has. And the gas giants have A LOT of mass. Jupiter, for example, is 318 times more massive than Earth and has 2.5 times more gravity than Earth. This incredible amount of gravity holds the planet together and compresses the gas into a liquid closer to the center.

    (Tags:  gravity  jupiter  neptune  planet  saturn  uranus)
  • Is it safe to look at Jupiter through a big telescope this month, September 2010?
  • Absolutely! Jupiter is currently near opposition. It is the best time to see Jupiter and the Galilean moons Io, Europa, Ganymede, and Callisto. Unlike the Sun, it is always safe to look at planets with telescopes. To look at the Sun, specially designed filters that take 99.9% of light away must be used. Otherwise permanent damage to the eye will occur. It is even harmful to look at the Sun directly (with unaided eye).

    (Tags:  jupiter  light  planet  solar system  sun  telescope)
  • Is Jupiter a "brown dwarf" star?
  • No. A brown dwarf is a kind of star. For Jupiter to exhibit any resemblance to a star it would have to fuse lighter elements into heavier ones in the core. An object needs to be about 80 times more massive than Jupiter for this to occur.

    (Tags:  brown dwarf  jupiter  planet  star)
  • Is Pluto a planet, or what?
  • It's more of an "or what." When Pluto was discovered, it was thought that it was bigger than the Earth. But it was figured out that Pluto was even smaller than the Earth's Moon. But that's not what did it in. Starting in 1992 icy objects similar to Pluto were found past Neptune, many in similar orbits to Pluto. It was discovered that there was a second asteroid belt, but made of icy objects, instead of rock. Pluto was still called the smallest planet, and the biggest Kuiper Belt object (Kuiper Belt is named for Dutch-American astronomer : Gerard Kuiper), or KBO. In 2005 a KBO slightly bigger than Pluto was discovered (now called Eris). In light of the new evidence, the voting members of the naming committee to the International Astronomical Union decided to reclassify Pluto.

    (Tags:  planet  pluto  solar system)
  • Is there a planet Nibiru which is reported to come close to Earth on 12 December 2012?
  • No, there is no such planet Nibiru. Nibiru is also often called Planet X. According to hoaxists, Nibiru is a planet that was discovered by the Mayans that will collide with the Earth on December 21st, 2012. This is just one of the "theories" about the world ending in 2012. The truth is, the Mayans knew of no such planet. Although they were very skilled at tracking the motions of the planets, Sun and Moon, they did not know what the planets were. They would have only been able to see the 5 naked eye planets; Mercury, Venus, Mars, Jupiter and Saturn. Telescopes had not yet been discovered. If there is a planet called Nibiru it would have to be very large or very close to Earth in order for the Mayans to see it. And if it is very large or very close, then you and I would be able to see it today. If it were on a collision course with Earth, it would get brighter and brighter until we could even see it in the daylight. But of course, we can't, because it doesn't exist. Rest easy tonight, the world will continue well past the year 2012.

    (Tags:  2012  apocalypse  earth  mayan culture  nibiru  planet)
  • Isn't it possible that other planets rotate in and out of our solar system on longer cycles in elliptical orbits. E.G. Nibiru at 3,600 year cycles that our current civilization has not observed?
  • Although it is theoretically possible that more planets exist in our solar system that we have not detected, it is very unlikely. Everything in the universe has gravity, and this gravity effects everything in the universe. The closer 2 objects are to each other, the more their gravity effects surrounding objects. The same is true for large objects. The larger the object, the more gravity will effect surround objects. These effects are measurable by astronomers, even outside our own solar system. Astronomers have used this technique to find hundreds of planets in orbit around other stars. Since we have never seen any gravitational interaction on our Sun or the planets in our solar system, astronomers believe there are not any very large planets beyond the orbit of Pluto.

    (Tags:  2012  gravity  nibiru  physics  planet  pluto  solar system  sun)
  • Isn't there strong evidence to indicate that planets are formed as a collection of 'cold stars?' Planets are so variable in their composition, isn't it unlikely that they were formed by the 'snowball' effect that most scientists seem to prefer as an explanation? We see stars forming and cooling. We don't observe dust particles conglomerating in space into massive planets by cumulative means. Can scientists explain the diversity of composition of planets if they are indeed formed from rings of orbiting dust from a single central star?
  • Astronomers are still working on models that can accurately predict what type of planet will form around various stars. It is currently unknown why some planets can form with solid surfaces and some as pure gas (like Jupiter). Astronomers have created models that explain how outer planets (like those of our solar system) could form as gas giants. But the models don’t work if the gas giants are close to their host star. But, we have detected many examples of gas giant planets that orbit their host star in just a few days. We do know planets form from gas & dust (and NOT from cold stars), which accumulates - after some dynamic processes – in the form of a disc around a star (or stars in case of close binary systems). Additional processes occur within the disc such as gravitational bonding and cooling, which eventually results in the build-up of planets. The idea of "cool stars" is probably a misunderstanding in the sense that when stars age (and typically, attain a cooler surface temperature), they lose mass to free space, which is the decisive step for the existence of gas & dust necessary for planets to form. This gas and dust is thought to be “recycled” when it becomes a large enough cloud for gravity to collapse it again into a new star. This process takes many thousands (even millions) of years.

    (Tags:  exoplanet  gravity  planet  star)
  • The sidereal period of an imaginary planet in our solar system is 8 yrs. What is the semi major axis of its orbit?
  • To calculate the semi-major axis of a planet just use the following equation: (p^2)/(a^3) Where a is the semi-major axis and p is the orbital period.

    (Tags:  planet  solar system)
  • There is a bright light every evening to the West. I thought it was a satellite but I have been looking at it since last year. Do you know what it is?
  • What you have been seeing is not a satellite but the planet Venus. Venus appears as the brightest object in the night sky right after sunset, or right before sunrise. Venus is so bright, because of the thick carbon dioxide clouds that reflect a lot of sunlight back to Earth. We only see it near the Sun, because it is closer to the Sun than Earth. Did you know, that many people mistake Venus for a UFO? This is because it can appear to flicker, move and change color in the sky when it is low in the horizon. First time skywatchers also underestimate how bright celestial objects can be in the sky.

    (Tags:  astronomy  light  planet  sun  venus)
  • There's so much talk about the alignment of the earth and sun on 12-21-2012...But what about the other planets in the same alignment? Do solar flares affect other planets when they align with the sun and the blackhole?
  • We’ve answered the first half of the question before. There are no planetary alignments in the next few decades, Earth will not cross the galactic plane in 2012, and even if these alignments were to occur, their effects on the Earth would be negligible. Each December the Earth and Sun align with the approximate center of the Milky Way Galaxy but that is an annual event of no consequence. The black hole at the center of the galaxy is over 26,000 light years away (meaning it would take 26,000 years to reach it if you were travelling at 186,000 miles per second!) This is incredibly far away, so it’s effects are not noticeable to us. Solar flares are not at all related to the black hole at the center of the galaxy. Solar flares are a natural occurrence on all stars. The number of flares and sunspots seen on the Sun changes on an 11 year cycle. Currently we are nearing a solar maximum, meaning there is an increase in solar activity including flares. The effects of these flares are seen on the other planets too. NASA has many pictures of auroral displays on Jupiter and Saturn.

    (Tags:  2012  apocalypse  black hole  milkyway galaxy  planet  solar system  star  sun)
  • What causes planets to rotate?
  • Planets are formed inside giant clouds of gas and dust that begin to rotate and collapse in on itself until it forms a star and planets. The rotation of the planets, is the left over rotation from the gas cloud that formed the planet.

    (Tags:  gravity  physics  planet  solar system  star  sun)
  • What holds the planets in place?
  • The planets are held in place by the Sun's gravity. Because the Sun is by far the largest object in the solar system, it extends its gravitational force far out into the solar system. Anything that comes inside our solar system will be sent into orbit around our Sun.

    (Tags:  gravity  physics  planet  solar system  sun)
  • What is a "dwarf planet" exactly?
  • An unfortunate name. Many people have an emotional tie to Pluto being a planet. They even have "When I was your age Pluto was still a planet" t-shirts. The pro- "Pluto is a planet" group was strong, so a compromise was made and "dwarf planet" was born. A planet now has to meet three criteria. First, it is big enough to make itself round (Pluto is), goes around the Sun, and not a planet (Pluto does), but also has to clear its orbit (Pluto does not). A dwarf planet meets the first two conditions but not the third. A better term would have been "big asteroid."

    (Tags:  planet  pluto  solar system)
  • What is the average number of planets that orbit a star?
  • In most cases, only one planet is discovered around stars that are known to host planets. There are a few stars where second, third and fourth planets have been discovered. With numbers, there are 334 planetary systems discovered with a total of 394 planets as of today. Out of 334, only 41 systems are multi-planet systems. Discovery of a planet system like our solar system is has not been made yet; however, this does not indicate the Solar System is unique. Discovery of smaller planets is an extremely difficult task. For example, if we were looking to the Solar System from a nearby star, we probably would not discover smaller planets in the first place, and think the Sun has 2 planets; Jupiter and Saturn.

    (Tags:  jupiter  planet  saturn  solar system  star  sun)
  • What is the largest planet of the solar system?
  • The largest planet is Jupiter. It is so large, that we could fit 1000 Earth's inside of it. However, compared to the Sun, Jupiter is very small. We could fit 1000 Jupiter's inside the Sun.

    (Tags:  earth  jupiter  planet  solar system  sun)
  • What is the size and brightness of the Sun as viewed from the different planets in the Solar System?
  • We have a few simple calculations that you can use to find the size (in degrees) and the brightness of the Sun on each planet. In fact, these calculations will work for the brightness and size of the Sun from anywhere in the universe, as long as you know the distance of the object from the Sun in AU. To find the size of the Sun in degrees: a = 0.5/distance (in AU) where a is the size in degrees and 0.5 is the size of the Sun as seen from Earth. To find the brightness of the Sun as compared to its brightness as seen on Earth: M = 2.5 * log(distance)^2 – 26.74 Where M is the apparent magnitude of the Sun and 26.74 is the magnitude of the Sun as seen on Earth.

    (Tags:  earth  jupiter  mars  moon  planet  pluto  saturn  solar system  star  sun  venus)
  • What's going on on Uranus? Why is it's tilt so different than every other planet?
  • Uranus was probably hit by a planet sized chunk of rock, early in the formation of the solar system, causing it to be tipped over on its side.

    (Tags:  planet  solar system  uranus)
  • When will we get actual pictures of the class m planets recently discovered by Kepler? Artist conceptions are cool and optimistic, however the truth is always more amazing.
  • Unfortunately, astronomers do not have the technology to actually see, or take pictures of, exoplanets. Most exoplanets are discovered by indirect methods, like looking for a dimming in a star’s light, or for the wobble in a star’s orbit – which is caused by a planet pulling on the star. In order to directly see the planet, astronomers would need much larger telescopes than have currently been built, in order to see the incredibly small bodies at great distances. One of the closest exoplanets discovered is about 22 light years away – that’s approximately 135 trillion miles away – and is only 4 times the size of Earth (at approximately 32,000 miles in diameter)! This is way too small to be seen by our telescopes. So for now, the best we can do is imagine what the planets look like, with artist conceptions.

    (Tags:  exoplanet  gravity  light  observatory  planet  star  telescope)
  • Why are planets round?
  • Simply put, planets are round because of gravity. Gravity pulls all the matter of a planet equally towards the center, creating a ball. Objects that are smaller, like asteroids, are not round, because there isn’t enough gravity to compress the object into a sphere. Instead they look like giant potatoes floating in space. In fact, planets themselves are not perfectly round either. We call them oblate spheres. Because they rotate on their axis as they go around the sun, the motion flattens the top and bottom of the planet a little bit, making the diameter at the equator greater than the diameter measured from the poles. This same effect is seen on the Sun and stars.

    (Tags:  asteroid  earth  gravity  planet  star  sun)
  • Why do planets rotate around the sun?
  • Planets and stars are formed in immense clouds of gas and dust. This gas and dust collapses in on itself and begins to spin as it collapses. The densest parts of the cloud become stars and planets. The planets get their motion around the Sun because of the rotation of the cloud that formed them. And Newton tells us that they will continue to orbit the Sun forever (an object in motion will stay in motion unless acted upon by another force).

    (Tags:  gravity  physics  planet  solar system  sun)
  • Will our magnetic field ever get destroyed by magnetic storms from the sun?
  • Although the Sun may give off a strong shock wave that may disturb our magnetic field, it cannot be destroyed by them. Geomagnetic storms are a temporary disturbance of Earth’s magnetic field by a solar wind shock wave. Earth’s magnetic field is providing adequate protection for life on Earth. However, astronauts are under a greater risk. Satellites can also be affected by a strong solar storm. Even high flying jets have a small risk of being exposed. There is a nice article in Wikipedia about the largest geomagnetic storm in history: From August 28 until September 2, 1859, numerous sunspots and solar flares were observed on the sun, the larges flare occurring on September 1. A massive CME headed directly at Earth due to the solar flare and made it within eighteen hours – a trip that normally takes three to four days. On September 1-2, the largest recorded geomagnetic storm occurred. The horizontal intensity of the geomagnetic field was reduced by 1600 nT as recorded by the Colaba observatory near Bombay, India. Telegraph wires in both the US and Europe experienced induced emf, in some cases even shocking telegraph operators and causing fires. Auroras were seen as far south as Hawaii, Mexico, Cuba, and Italy – phenomena that are usually only seen near the poles. If you would like to know more about this superstorm, you can read more on the Wikipedia site by searching for the 1859 solar superstorm.

    (Tags:  earth  electromagnetic spectrum  planet  solar system  star  sun)