November 26, 2010

An Intriguing Planet From Outside the Galaxy

An artist's impression of HIP 13044 and the planet HIP 13044 b.
Image: ESO/L. Calçada

Recently, exoplanets1 have been discovered at an extremely rapid pace. In just about two decades, astronomers have found and confirmed over 500 planets, with many more waiting to be confirmed. Despite the large number, we're still finding a great amount of new things to be excited about in many of these discoveries.

The telescope used to make the discovery.
Image: ESO/H. H. Heyer
A newly announced exoplanet can claim to be the first one discovered to have originated outside our galaxy. Discovered by the Max Planck Institute in Germany using a 2.2m telescope at the European Southern Observatory in Chile, the planet, HIP 13044 b, orbits around a star, HIP 13044, that is located in the Helmi stream about 2,000 light years from the Sun. This particular group of stars originated from a small satellite galaxy of the Milky Way. The galaxy was later absorbed by the Milky Way about six to nine billion years ago, and gravitational tidal forces subsequently tore it apart and stretched it into a stream of stars. Both the star and planet orbiting around it were likely swept along for the ride.

This particular planet was discovered by the “wobble” method, just as many other exoplanets have been discovered in the past. The host star is studied for a long period of time, and a wobble, found for this star by a doppler shift, indicates a planet gravitationally tugging back on the star as it orbits around. The wobble in this particular case suggests a giant massive planet (similar to Jupiter) orbiting very closely to the star. This is very surprising, since the host star has also already passed through its red giant phase. When a Sun-like star enters the red giant phase, it grows extremely large, increasing its radius by ten to even hundreds of times larger that its original radius. Our Sun is expected to have a radius extending beyond the Earth's orbit when it becomes a red giant in around five billion years. However HIP 13044 b lies very close to its star, inside the area that was likely taken up by the star's red giant size before. This is likely due to the planet migrating inwards from a larger original orbit after the star shrunk, which is very intriguing. Later, the host star is expected to become an asymptotic giant branch2 star, and the discoverers of the planet believe it will be devoured by the star at that time.

Furthermore, this exoplanet's host star is very metal-poor, meaning that it does not have many elements heavier than helium. Other exoplanets discovered so far, on the other hand, have had host stars that are at least as metallic as the Sun. HIP 13044, like the other stars in the Helmi stream, has metal content of about 1% of that of the Sun, in a mass that is about equal to the Sun's. This is just simply not a curiosity, but a potential reconsideration of how planets are formed. In the widely accepted core-accretion planet formation model, the matter around a star gradually coalesces to form planets. However, this model requires heavier elements to begin the process of planet formation, by forming the rocky core first. Without a rocky core, a gas giant, like this particular exoplanet, could not be formed since there is not enough mass to retain the gas. There is an alternative model for the formation of giant planets, called the disk instability , where a giant disk of gas around a star breaks off into planet-sized self-gravitating pieces. These pieces eventually each result in a giant planet. The model may be relevant in this case and this exoplanet discovery may provide substantial evidence for the disk instability model, or perhaps may lead to another future model.

HIP 13044 b's discovery continues to show that although exoplanet discoveries may no longer by novel in and of themselves, they still bring forth fresh considerations and interesting ideas.


1: Short for extrasolar planets, which are planets located outside our solar system.
2: In order to explain the asymptotic giant branch, I should explain the Hertzsprung-Russell (H-R) diagram first. The H-R diagram is essentially a scatter graph of stars plotted by their temperatures and luminosities (some use other related classifications like absolute magnitude, spectral types, etc.). Most stars in the H-R diagram lie on an area that looks like a curved line called the main sequence. There are also some branches that come out of the main sequence. The asymptotic giant branch is one of these branches, and consists of low to intermediate mass stars (about 0.6 to 10 times the mass of the Sun) in the late part of their stellar evolution.

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