# How is the size of a distant planet determined?

How is an object's size determined from other solar systems than our own? How are new large bodies being found in our solar system currently?

One method of finding planets orbiting stars is to analyze the transit effect. This is when a planet crosses the face of a star between the star and observer. This will block some of the light reaching the observer. By measuring this decrease in light from the star we can determine how much of the surface area of star is occluded.

The speed of a planet across the face of a star will let us calculate the orbital radius considering a circular orbit. The temperature of the star can be determined from its brightness and location on HR diagram and spectroscopic data. Combining this information lets us determine the size and of the planet.

Using space-based or ground-based telescopes and automated data collection and analysis methods large regions of the sky can be viewed for these transits. This method is highly accurate and will probably surpass the radial velocity method as most common method to discover new planets.

Another method for finding planets is known as the radial velocity method. When a planet orbits a star it is really the planet and star rotating about the center-of-mass of the system. Hence the star wobbles due to the bodies orbiting the star. It wobbles because usually the center-of-mass is inside the star and this results in the star rotating about an axis running thru the star but offset from the center. By doing a doppler analysis it can be determined whether the star is revolving or not and the period of revolution. The radial velocity method is the most effective method. The transit method is very limited by the viewing angle, the planet must pass between observer and star.

A good source to better learn about these methods is here.

• According to Wikipedia, as of 1st June this year there were 4071 known exoplanets, of which 2662 were discovered with the Kepler space telescope. Kepler only used the transit method, so your statement "[the] radial velocity method is responsible ... for most of the planets located to date" is not correct. – Martin Bonner supports Monica Jun 6 at 9:09
• The statement was taken directly from my source but source may be dated to the early years of the Kepler mission. I will edit my answer. It was pointed out in the reference that Kepler had found hundreds of candidates so there may be a difference in the reference between actual planets and candidates. But I'm just guessing really. – jmh Jun 6 at 21:22

The size of a planet is determined by

1. The amount of gas & dust available to build it.
2. The type of star it orbits.
3. Its distance from the star.
4. Whether or not the sort of resonances we see in Bode's Law come into play. We don't know for sure whether Bode's Law operates for all stars, some stars, or only our star.
• Bode's law does not operate for any star. – Sir Cumference Jun 29 at 1:23
• How do you know? – Michael Walsby Jul 15 at 12:09
• Because it completely failed to predict Neptune's orbit? The "law" was simply a hypothesis with no theory behind it, and now that it has failed there's no reason to expect it to hold for exoplanets, let alone Solar System planets. – Sir Cumference Jul 15 at 12:50