How do scientists spot planets? Aren't they dark?

Answer 1

There are now several new techniques whereby we can detect the presence of extra-solar planets orbiting other stars.

Current methods of detection
1 Astrometry
2 Radial velocity
3 Pulsar timing
4 Transit method
5 Gravitational microlensing
6 Circumstellar disks
7 Direct imaging

The reflected light from any planet is an extremely faint light source compared to its parent star. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. Therefore astronomers have generally had to resort to indirect methods to detect exoplanets.

At the present time, six different indirect methods have yielded success.

1. Astrometry

Astrometry is the oldest search method for extrasolar planets. It consists of precisely measuring a star's position in the sky and observing how that position changes over time. If the star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit. Effectively, star and planet each orbit around their mutual center of mass (barycenter),

Unfortunately, the changes in stellar position are so small that even the best ground-based telescopes cannot produce precise enough measurements. In 2002, however, the Hubble Space Telescope did succeed in using astrometry to characterize a previously discovered planet around the star Gliese 876.

Future space-based observatories such as NASA's Space Interferometry Mission may succeed in uncovering large numbers of new planets via astrometry, but for the time being it remains a minor method of planetary detection.

2, Radial velocity

Like the astrometric method, the radial-velocity method uses the fact that a star with a planet will move in its own small orbit in response to the planet's gravity. The goal now is to measure variations in the speed with which the star moves towards or away from Earth. In other words, the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star's spectral lines due to the Doppler effect.

Velocity variations down to 1 m/s can be detected with modern spectrometers, such as the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrometer at the ESO 3.6 meter telescope in La Silla Observatory, Chile.

This has been by far the most productive technique used by planet hunters. It is also known as the "Doppler method" or "wobble method". The method is distance independent, but requires high signal-to-noise ratios to achieve high precision, and so is generally only used for relatively nearby stars out to about 160 light-years from Earth. It easily finds massive planets that are close to stars, but detection of those orbiting at great distances requires many years of observation.

One of the main disadvantages of the radial-velocity method is that it can only estimate a planet's minimum mass. Usually the true mass will be within 20% of this minimum value, but if the planet's orbit is almost perpendicular to the line of sight, then the true mass will be much higher.

The radial-velocity method can be used to confirm findings made by using the transit method. When both methods are used in combination, then the planet's true mass can be estimated.

3. Pulsar timing

A pulsar is a neutron star: the small, ultradense remnant of a star that has exploded as a supernova. Pulsars emit radio waves extremely regularly as they rotate. Because the intrinsic rotation of a pulsar is so regular, slight anomalies in the timing of its observed radio pulses can be used to track the pulsar's motion.

Like an ordinary star, a pulsar will move in its own small orbit if it has a planet. Calculations based on pulse-timing observations can then reveal the parameters of that orbit.

This method was not originally designed for the detection of planets. But it is so sensitive that it is capable of detecting planets far smaller than any other method can, down to less than a tenth the mass of Earth. It is also capable of detecting mutual gravitational perturbations between the various members of a planetary system

The main drawback of the pulsar-timing method is that pulsars are relatively rare, so it is unlikely that a large number of planets will be found this way. Also, life as we know it could not survive on planets orbiting pulsars since high-energy radiation there is extremely intense.

4. Transit method

While the above methods provide information about a planet's mass, this method can determine the radius of a planet. If a planet crosses (transits) in front of its parent star's disk, then the observed visual brightness of the star drops a small amount. The amount the star dims depends on its size and on the size of the planet. For example, in the case of HD 209458, the star dims 1.7%.

This method has two major disadvantages. First of all, planetary transits are only observable for the small percentage of planets whose orbits happen to be perfectly aligned from astronomers' vantage point. Such alignment is especially unlikely for planets with large orbits.

Secondly, the method suffers from a high rate of false detections. At least at present, a transit detection requires confirmation from some other method.

The main advantage of the transit method is that when combined with the radial velocity method, one can determine the density of the planet, and hence learn something about the planet's physical structure. The nine planets that have been studied by both methods are by far the best-characterized of all known exoplanets.

The transit method also makes it possible to study the atmosphere of the transiting planet. When the planet transits the star, light from the star passes through the upper atmosphere of the planet. By studying the stellar spectrum carefully, one can detect elements present in the planet's atmosphere.

5. Gravitational microlensing

Gravitational microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. This effect occurs only when the two stars are almost exactly aligned. Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than a thousand such events have been observed over the past ten years.

If the foreground lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the lensing effect. Since that requires a highly improbable alignment, a very large number of distant stars must be continuously monitored in order to detect planetary microlensing contributions at a reasonable rate. This method is most fruitful for planets between earth and the center of the galaxy, as the galactic center provides a large number of background stars.

As of 2006 this is the only method capable of detecting planets of Earthlike mass around ordinary main-sequence stars

A notable disadvantage of the method is that the lensing cannot be repeated because the chance alignment never occurs again. Also, the detected planets will tend to be several kiloparsecs away, so follow-up observations with other methods are usually impossible. However, if enough background stars can be observed with enough accuracy then the method should eventually reveal how common earth-like planets are in the galaxy.

6, Circumstellar disks

Disks of space dust surround many stars. The dust is believed to be generated by collisions among comets and asteroids. Radiation pressure from the star will push the dust particles away into interstellar space over a relatively short timescale; therefore, the detection of such dust indicates that it is being continually replenished by new collisions.

The dust can be detected because it absorbs ordinary starlight and re-emits it as infrared radiation. Dust disks have now been detected around more than 15% of nearby sunlike stars.

These dust disks provide strong indirect evidence for the existence of small bodies like comets and asteroids that orbit the parent star. For example, the dust disk around the star tau Ceti indicates that that star has a population of objects analogous to our own Solar System's Kuiper Belt, but at least ten times thicker

Features in dust disks sometimes suggest the presence of full-sized planets. Some disks have a central cavity, meaning that they are really ring-shaped. The central cavity may be caused by a planet "clearing out" the dust inside its orbit. Other disks contain clumps that may be caused by the gravitational influence of a planet. Both these kinds of features are present in the dust disk around epsilon Eridani, (the nearest star to us known to have a planet) hinting at the presence of a planet with an orbital radius of around 40 AU (in addition to the inner planet detected through the radial-velocity method).

The Hubble Space Telescope is capable of observing dust disks with its NICMOS (Near Infrared Camera and Multi-Object Spectrometer) instrument. Even better images have now been taken by its sister instrument, the Spitzer Space Telescope. The Spitzer Telescope was designed specifically for use in the infrared range and probes far deeper into the spectrum than the Hubble Space Telescope can.

And finally

7. Direct Imaging

In a few unusual cases current telescopes may be capable of directly imaging planets. Specifically, this may be possible when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and young (so that it is hot and emits intense infrared radiation).

In early 2005 two groups, both using the European Southern Observatory's Very Large Telescope array in Chile, announced direct infrared images of extrasolar planets: 2M1207b and GQ Lupi b. Both planets are believed to be several times more massive than Jupiter and to orbit at distances greater than 40 AU from their parent star.

In December 2005, the planetary status of 2M1207b was confirmed. As of November 2005, there has been no such confirmation for GQ Lupi b, which might instead be a small brown dwarf.

Future methods of detection

Several space missions are planned that will employ already proven planet-detection methods. Astronomical measurements done from space can be more sensitive than measurements done from the ground, since the distorting effect of the Earth's atmosphere is removed, and the instruments can view in infrared wavelengths that do not penetrate the atmosphere. Some of these space probes should be capable of detecting planets similar to our own Earth.

The Terrestrial Planet Finder

The European Space Agency's COROT satellite and NASA's Kepler Space Observatory will both use the transit method. COROT will be sensitive enough to detect planets slightly larger than Earth, while Kepler should be capable of detecting planets even smaller than Earth.

Answer 2

Yes, they ARE dark, in that they don't radiate light (unless they're inhabited).

There are, however, three main ways to detect a planet.

While a planet does not radiate its own light, it may REFLECT the light of its parent star. If it has water, ice or cloud cover, it is more reflective (has a higher albedo).

Planets and other bodies also move against the stellar background. If the planet passes in front of something else that is being observed, it can be spotted that way.

Finally, and this is the way Pluto and all the other planets in distant solar systems have been found, planetoid-sized bodies have measureable gravity. This interacts with the mass of other nearby bodies, including the parent star, causing anomalies in their movements. By viewing the effect of this mass on a known body, the presence of the planet can be deduced and its mass and trajectory calculated (so astonomers know where to look).

Answer 3

Well first of all usually depending it's distance to a star , a planet will have light on it. Like the way our sun shines on Jupiter & saturn and even our moon !

As an Astronomer I get questions like this at our observatory and I know a planet hunter so he wrote this to answer your question ... his name is Andrew Gould and here is a page about him ...

http://researchnews.osu.edu/archive/nuplanet.htm

~~~~~~~~~~~~~~
Hi ! I'm Andrew Gould

What us planet hunters do is take lots and lots of pictures every night of the sky and if we see one seems to be in a different spot than another picture we look tosee if it's been recorded from other observatory's all over Earth .
Then if it hasn't been a planet not found yet we ask then to photograph it's movements. Then if it hasn't been discovered it becomes a new planet !

Geoff Marcy .. One of the top planet hunters in the world sees if it's traveling by a star , he checks to see if that star has any known planets orbiting it . If there is no record of it ..it becomes a new planet and gets named by whoever discovered it !

That's how new planets get spotted!

Answer 4

As regards objects in our own Solar System, Neptune was discovered in 1846 because it caused perturbations in the orbit of Uranus so astronomers knew that it existed and where to look.

Galileo observed Neptune in 1610 but did not know the significance of his observations and thought it was a star. When Herschel found Uranus in 1781 he thought at first it was a comet.

Modern astronomers have more powerful telescopes, some of them in space, and they can photograph images to notice movement of objects over a period of days and weeks against a background of stars.

So that is why we have found 1000+ Trans Neptunian Objects in 13 years and we are now discovering asteroids at an average rate of 5000 a month.

Faint though their reflected light may be, it is still trackable.

Answer 5

That's the exact reason that they CAN spot planets. When the planet goes in front of the star it's orbiting, it darkens the light from the star... so they know that SOMETHING is orbiting it.

The more it darkens and for a longer time, means the bigger the planet is.
tada

Answer 6

They spot planets by spotting the "shadows" they make in the light of stars as they orbit around them . . . as long as the planets are close enough to the sun to make those shadows and large enough for our telescopes to spot. So far we don't have any telescopes sensitive enough to spot anything smaller than a gas giant.

Answer 7

Light from the sun reflects off all the planets. The planets themselves don't give off any light.

Just like looking at our moon - it's reflected light!

Answer 8

They watch for bubbles in stars. As a planet revolves around its sun it will show a slight pull in the gravitational area around the sun. The star will appear to go oblong for a second. You can see it stretch briefly and slightly from side to side.

Answer 9

They detect the wobble of the star the planet is orbiting.

Answer 10

The moon, by itself, is dark, too, but we can see it because it reflects sunlight.

Sometimes they deduce that planets are there by the way their gravity affects the movement of the stars around them.

Answer 11

1)a noticeable wobble in the parent stars rotation
2)Ocultation,or, a planet passing in front of a parent star.
3)Direct observation using I.R.,U.V., or X-Ray instruments.