Planet Found Orbiting Red Giant

In the distant future, when the Sun begins to expand and evolves into a "giant" star, the surface temperature on the Earth will rise dramatically and our home planet will eventually be incinerated. Fortunately for us, this dramatic event is several billion years away. However, that sad fate will befall another planet within a few tens of millions of years.

This planet, just discovered by a team at the European Southern Observatory (ESO), orbits around the giant star HD 47536.

The doppler effect
The 6th-magnitude giant star HD 47536 where the new exoplanet has been found (reproduced from the Digital Sky Survey).
Credit: STScI Digitized Sky Survey, (C) 1993, 1994, AURA, Inc. all rights reserved

The distance of the planet from the star is about 300 million kilometers (or twice the distance of the Earth from the Sun). The planet’s orbital period is 712 days – somewhat less than two Earth years – and the planet’s mass is 5 to 10 times that of Jupiter.

The structure and evolution of stars like our Sun are well understood. They are born by contraction in immense clouds of dust and gas, and when the central density and temperature become high enough, nuclear fusion ignites in their interiors. A long period of relative calm follows (the Sun is now in this phase).

That calm period ends when the star’s nuclear fuel runs out. The star then begins to expand and soon becomes a "giant". During this phase, the surface temperature drops somewhat (but is still several thousand degrees) and this lower temperature causes the color of the star to change from yellow to red.

In the case of the Sun, this will happen about 5 billion years from now. Our star then will become larger and the surface of our home planet will become exceedingly hot, incinerating whatever life forms remain.

The giant planet in orbit around HD 47536 probably is starting to witness some dramatic events. As its central star slowly but steadily expands and occupies a progressively larger fraction of the sky above the planet, atmospheric changes like rising temperatures and violent winds probably will occur. Some tens of millions of years from now, the unlucky planet is doomed to lose its gaseous layers entirely and the surface will become burning hot.

At a distance of nearly 400 light-years from us, this is the second-most remote planetary system discovered to date. HD 47536 is in the southern constellation of Canis Major (The Great Dog). This is only the fourth case known of a giant star hosting a giant planet. With a diameter of about 33 million kilometers (or 23.5 times that of our Sun), HD 47536 is by far the largest of those giant stars.

"We are very excited about this discovery," says Luca Pasquini of ESO, "because it now widens the search for exoplanets towards more massive stars."

The problem with looking for planets around massive stars is that the stars rotate very rapidly during the first phase of the lives. This rapid rotation makes accurate measurements of minute velocity variations caused by the gravitational pull of accompanying planets virtually impossible. However, in the later phase of their lives, when the stars become giants, they slow down considerably and scientists then have a much better chance of detecting possible exoplanets in orbit around them.

Gas giant planet
Artist’s conception of a gas giant planet orbiting a nearby star.
Credit: NASA and G. Bacon (STScI)

The discovery has other interesting implications. For years, the present team of astronomers has been studying certain giant stars that are found to contain lithium. However, this light element normally is consumed very rapidly in such stars.

"One obvious possibility is that those stars have obtained their lithium by recently swallowing a nearby planet," says team member Licio da Silva from the Observatorio Nacional in Rio de Janeiro, Brazil. But until recently, this hypothesis was considered rather exotic, because of the lack of evidence of planets in danger." But with this discovery of a giant planet orbiting near a giant star, that explanation now is looking more plausible.

What’s Next

With over 70 other giant stars still under close scrutiny, the ESO team is working hard to sift through the observational data. They say it is possible that they will find other giant stars with planet-induced velocity variations. Soon, the HARPS very high-precision spectrometer will be installed at the ESO 3.6-m telescope on La Silla. It has been built by the Geneva Observatory in collaboration with ESO, and will be dedicated to the search for exoplanets.

The Kepler Mission, scheduled for launch in 2006, will use a spaceborne telescope to search for Earth-like planets around stars beyond our solar system. A key criterion for such suitable planets would be whether they reside in habitable zones, or regions sometimes protected by gas giants but with temperate climates and liquid water.

Future missions, such as ESA’s Herschel mission will search for many more and take detailed pictures of stars that might harbor dust rings. As these images become available, astronomers will be able to predict the sizes and orbits of giant planets within the alien solar system.

Research article ("Evidence of a Sub-Stellar Companion around HD 47536" by Johny Setiawan et al.) will appear as a Letter to the Editor in the European research journal "Astronomy & Astrophysics" (Vol. 398 No. 2, p. L19 – February 2003).