3 Super-Earths found around a Star

Super-Earth is a planet more massive than our Earth but less massive than Uranus or Neptune (about 15 Earth’s masses).

And astronomers at the ESO La Silla Observatory recently announced that they have found not only one, but three super-Earths orbiting a star 42 light-years away.

This star is called (boringly) HD40307. It is a K-type star, a rather normal one slightly less massive and cooler than our Sun, located in the southern constellation Pictor.

The three super-Earths are 4.2, 6.7 and 9.4 times the mass of our Earth. Their orbital periods are 4.3, 9.6 and 20.4 days respectively, very very short compared to the ~365-day period of our planet, which suggest that they orbit very closely to their parent star. It must be very very hot there…

A trio of super-earth. ESO.

How do the astronomers found the planets?

By using a technique called Doppler shift. Doppler shift is the change in wavelength and frequency of a source due to the movement of the source itself or the observer or both.

The most frequent example of Doppler effect in our daily life is an ambulance’s siren. As the ambulance approaches us, the wavelengths of the sound are “squeezed” and become smaller, the frequency increases and we hear a higher pitch. Since blue wavelength is smaller than red wavelength, we say that the radiation is “blueshifted”.

The opposite of blueshift is redshift. As the ambulance passes and moving away from us, the wavelengths now are stretched out and become longer, the frequency decreases, so we hear a lower pitch.

Now we look at a star-planets system and see how Doppler shift can be used to detect the planets.

In a star-planet system (or any two-body system), the planet does not orbit the centre of the star but instead both the planet and star orbit their common centre of mass, which is not exactly at the centre of the star. Since the mass of a planet compared to its parent star is so small, the centre of mass actually lies inside the star. So, that why for simplicity we just say the planet orbits the star.

As the star and planet orbit about their common centre of mass, the planet tugs on the star gravitationally and causes the star to wobble. If the wobble is towards us, we see the light from the star is blueshifted; and when the wobble is away from us, the light is redshifted. By analysing the Doppler shift in the spectrum of starlight, these slight changes in the star’s velocity can be detected on Earth. This information can then be used to deduce the planet’s mass and orbit.

This is called the radial velocity method. In this method, the motion of the wobble can only be detected in the direction toward and away from us or the radial direction, hence the name.

The amount of wobble depends on the distance from the star and the mass of the planets. More massive means more wobble and more wobble means easier to detect. Also the nearer the planet to the star, the faster it moves, thus causing larger shift in the star’s spectrum. That’s why most of the extrasolar-planets (planets outside the Solar System, exoplanets for short) found are giant, massive planets, with orbits very close to their parent stars – the hot jupiters.

However, things are changing now.

The wobbles caused by the three newly found super-Earths are really tiny, they induce a motion of their parent star only a few metres per second, and only the high sensitivity of HARPS spectrograph on ESO’s 3.6-m telescope at La Silla made it possible to detect them.

“With the advent of much more precise instruments such as the HARPS, we can now discover smaller planets, with masses between 2 to 10 times the Earth’s mass.” The analysis of all the stars studied with HARPS shows that one solar-like star out of three has either super-Earth or Neptune-like planets with orbital periods shorter than 50 days.

And that is only the super-Earths planets. We have not yet talk about Earth-like planets out there waiting to be discovered. Adding all that to the Jupiter-like planets already known, we may found that planets are very common after all.

Source: ESO Science Release


~ by thChieh on June 19, 2008.

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