Researchers from the University of Texas have discovered a multi-planet system around an unexpected star, which they say could alter planet formation theories.

Astronomers William Cochran and Michael Endl, using the 9.2-meter Hobby-Eberly Telescope (HET) at McDonald Observatory discovered a system of two Jupiter-like planets orbiting a star whose composition seemed at odd with planet formation theories.

Cochran and Endl, who have been monitoring the star, HD 155358, since 2001, using the High Resolution Spectrograph on HET found that the star's radial velocity, or motion toward and away from Earth, showed a wobble, which they believe is caused by unseen companions tugging onto the star.

Studies revealed HD 155358 is slightly hotter than the Sun, but a bit less massive.

It contains 20 percent as much of the chemical elements called metals - elements heavier than hydrogen or helium - as the Sun. Along with one other star (called HD 47536), it contains the fewest metals of any star found to harbour planets, the researchers said.

In-depth analysis of the star's spectrum however, revealed its metal-poor nature, while most of the planets found using the radial velocity technique are found around metal-rich stars.

Researchers deduced the star's age at roughly 10 billion years, and found that one planet orbiting the star had an orbital period of 195 days and, at a minimum, is 90 percent as massive as Jupiter.

It orbits HD 155358 at a distance of 0.6 AU (An astronomical unit, or AU, is the Earth-Sun distance of 150 million km, or 93 million miles).

The other planet was found orbiting HD 155358 in 530 days, with a minimum mass half that of Jupiter, at a distance of 1.2 AU.

Studies further revealed that the planets' orbits are not circular, and they orbit close to each other and thus interact gravitationally -- they push each other around.

"It's like a dance, Rob's calculations show us how the orbits change over time: first more eccentric, then more circular, and back again. The system is stable, and the pattern repeats about every 3,000 years," said Endl.

According to graduate student Robert Wittenmyer, the planets are trading eccentricity with each other.

"When one orbit is more circular, the other is more eccentric," he said.

Endl said: "the combination of massive planets orbiting a metal-poor star has consequences for theories of planet formation".

"There are two competing planet-formation models. Those models are known as the core accretion model and the disk instability model. Both models start with a rotating cloud with a star forming at its centre. As it rotates, the cloud flattens into a disk.

Over time, dust in the disk begins to clump together to form the seeds that will eventually become planets. Where the two models differ is in terms of timescale," he said.

"In the core accretion model, a Jupiter-like planet forms in a two-step process. Over about a million years, a proto-planetary core several times the mass of Earth forms through gravitational accumulation of solid materials.

When it reaches this mass, it has enough gravity to then pull huge amounts of gas onto itself. Over several million more years, it grows into a gas giant planet. This model relies on large amounts of heavy elements to be present in the disk - and, of course, in the star- to form the cores.

"The competing model of planet formation is called the disk instability model. It argues that the rotating disk of gas and dust around the forming star becomes unstable very soon after the disk forms, causes the disk to break into giant clumps.

Gravity within each clump can cause the gas to collapse under its own gravity, forming giant planets in only several hundred years. Gas giant planets formed this way might not have any solid core at all," he said.

Cochran and Endl believe that HD 155358 could have formed the two planets through either method of planet formation.