The University of Texas at Arlington

The University of Texas at Arlington

UTA Planetarium

UTA Planetarium

Ask the Astronomer Q&A

Tag: "solar system"

  • 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)
  • Does the plane of revolution of planets have anything to do with the angle of which their parent star rotates?
  • The revolution of all planets are in a plane called the invariable plane, that is defined with the angular momentum of the solar system. But there is an uncertainty on the invariable plane as we don’t know every object in the solar system. For this reason, we call ecliptic, the plane Earth revolves around the Sun, the plane of the Solar System for convenience. Sun’s rotation axis is tilted by about 7 degrees to the perpendicular to the ecliptic. Stars’ rotation axis has a lot to do with the invariable plane during the formation of stars, but we can’t rule that the invariable planes will be in 90 degrees tilt with rotation axis of host stars.

    (Tags:  ecliptic  solar system  star  sun)
  • 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)
  • How far away are the stars?
  • The closest star system, the Alpha Centauri triple star system, is about 25 trillion (25,000,000,000,000) miles away. To put that in perspective, it could take us around 30 years to fly to Neptune, the furthest planet in our Solar System. But going at the same speed, it would take around 100,000 years to get to Alpha Centauri.

    (Tags:  solar system  star  universe)
  • 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 have an app on my phone that let's me explore the solar system and I noticed that Mercury and Venus don't appear to rotate. Is their rotation just really slow or is there none at all?
  • Mercury and Venus both rotate very slowly. It takes over 58 days for Mercury to rotate once, and 244 days for Venus to rotate once.

    (Tags:  mercury  solar system  venus)
  • I know that the Earth goes around the Sun, the Sun goes around the Milky Way, and the Milky Way is moving somewhere too. I was wondering, if there is a totally static point in space, how fast is the Earth moving from that static point of view?
  • Earth's average distance to the Sun is 150,000,000 km (93 million miles), therefore the distance it travels as it circles the Sun in one year is that radius x 2 x pi, or 942,000,000 million kilometers in a year of 24 hours/day x 365 1/4 = 8,766 hours so you divide to get 107,000 km/h or about 67,000 mph. You could also say the Earth moves around the Sun at 30 km/s. The Sun circles the center of our Galaxy at about 250 km/s. Our Galaxy is moving relative to the 'average velocity of the Universe' at 600 km/second (

    (Tags:  earth  physics  solar system  sun  universe)
  • 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 I were to fast-forward into the future, and I was setting up a base on Callisto, what might be the best place to do that? Would somewhere like in the Lofn crater be a good place to put it? If so, what would the area look like? Would I be able to see Jupiter from there? Would the ground feel and look like snow? Would the thin atmosphere carry any sort of sound?
  • Callisto is a tidally locked moon which means that the same side is always facing Jupiter (like our Moon). Unfortunately, the Lofn crater is located on the far side the moon (56˚ S and 23˚W) so your base would never see Jupiter. The surface of Callisto is covered by ice (water ice), carbon-dioxide and silicates; so it wouldn’t feel like snow, it would feel like hard ice. The thin atmosphere is so tiny; you can almost consider there is no atmosphere. It is surprising that this tiny moon has an atmosphere; however, it is not stable. The atmosphere is actually running away and is probably being replenished by the surface carbon-dioxide. Such an atmosphere wouldn’t carry any sound.

    (Tags:  jupiter  moon  physics  solar system)
  • If we suppose Jupiter has moved from the place that it was originally created to the spot it is now in our solar system, what affect would it's gravitational movement have on Earth?
  • The current model that astronomers use to explain how the solar system formed, does not include any changes in orbit after the planet forms. There may be slight adjustments to a planet’s orbit due to mass changes over time, but nothing that will explain back and forth movements of the planets, because that would require an external force. For Jupiter to move closer or farther away from the Sun, there would have to be an external force tugging it - allowing the orbital period to change. But there is no known source in the solar system that would have the power to change the orbits by that much. Even though the planets have a small gravitational effect on each other, a change in planet configuration wouldn’t cause a dramatic change to the orbits of the other planets. Most of the gravity in the solar system comes from the Sun, so as long as the Sun remains in the system, the orbits won’t change much (if at all). That’s why Earth and Mars don’t orbit around Jupiter when they get in conjunction with Jupiter.

    (Tags:  earth  gravity  jupiter  mars  solar system  sun)
  • 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 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)
  • 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)
  • 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'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)
  • 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)
  • Why is Pluto so cold?
  • Planets are warm because they trap heat from the Sun, but Pluto is so far away that it doesn't get much heat or light from the Sun. Light from the Sun takes 6 hours to reach Pluto. Compare that to the 4 minutes that it takes light from the Sun to reach Earth. In fact, Pluto is so far away that the Sun would begin to resemble all the other stars that we see at night.

    (Tags:  earth  light  pluto  solar system  star  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)