An international team of astronomers using the Subaru Telescope and
led by a graduate student member of SOKENDAI (The Graduate University of
Advanced Studies, Japan) has discovered companions circling
"intermediate-mass" stars. These are stars that are heavier than the Sun
and the companions were thought to be either planets or possibly small
stars. The excellent performance of the Subaru Telescope enabled the
detection of faint objects circling around three of six bright stars
surveyed.
The target objects γ Hya, HD 5608, and HD 109272 have companion stars (Figure 1)
called γ Hya B, HD 5608 B, and HD 109272 B. The other three stars
surveyed did not have them. Comparison of these results with the
frequency of companions predicted by planet formation theory is a
crucial step in better understanding how planets are formed.
Since the first discovery of an exoplanet in 1995, there have been
thousands of exoplanet candidates found. More than 3,000 have been
confirmed as planets. Compared to the planets of the solar system, these
distant worlds demonstrate a wide diversity of environments.
The discovery of new planets beyond our own solar neighborhood has
required frequent revisions of planetary formation theory. Among the
criteria that astronomers consider is the question of how many planets
of what mass range exist at what distances from their primary stars. In
particular, the frequency with which these distant planets form can be a
good test for theories about when and where they exist.
Planets are difficult to detect because they are smaller than their
stars and are often hidden by the star's glow, particularly if they
orbit close to their stars. The Radial Velocity (RV) detection method (Note 1) has been successfully used to detect many exoplanets at varying distances from their stars.
However, it is also a struggle to spot objects that lie too far away
from their primaries. These distant objects are difficult enough to even
find, let alone determine whether they are planets or another star. If
an object is far away from its star, its orbital period can be more than
10 years long. At such distances and periods, the RV method is very
difficult to use.
Short observation times make it difficult to "tease out" the
existence of a distant companion due to the longer time it takes to make
one orbit. The transit method, which is the other commonly used way to
find planets, (Note 2)
also faces similar limitations in the search for distant planets,
despite recent contributions to exoplanet discoveries using this method.
The presence of a planet more than 10 astronomical units (AU, 1 AU is
the distance between the Earth and the Sun) from its star, with an
orbital period of several decades remained unaccounted for in recent
surveys. To solve this problem, the research team added a direct imaging
approach to the RV method.
Direct Imaging Method is a Powerful Tool
Direct imaging is a very straightforward method. Astronomers take
pictures of the light from the faint planets. If the faint object is far
enough from the bright star, detection is easier. Or, in some cases, a
companion star brighter than planets will be detectable. Therefore, this
method enables an identification of the object that is causing the
long-term trend found via the RV method.
A group of researchers from Tokyo Institute of Technology has been
surveying the sky for exoplanets using the RV method for more than a
decade using a telescope at Okayama Astrophysical Observatory, National
Astronomical Observatory of Japan. Their targets are stars with mass 1.5
to 5 times heavier than the Sun. Already more than 30 objects in their
target list are known to have planets or brown dwarfs with relatively
short orbital periods. They have also found some unknown objects that
show a long-term orbital trend – that is, an orbit so long that it
indicates the object lies at a great distance from the primary (Figure 2).
- Radial Velocity (RV) detection method is used to detect exoplanets by measuring the periodic change of the radial velocity of a star. The orbital motion of the star-planet system around the center of gravity causes this variation (Figure 3, left panel). This is an indirect detection method that does not measure the planet itself. When a companion object is far away from the primary star and the period of observation is not long, the radial velocity shows the linear change (Figure 3, right panel).
- The transit method is another indirect method used in exoplanet detection. By monitoring the change in the brightness of the primary star as a planet "transits" between our point of view and the star, one can detect that planet. This method requires very precise measurements of the brightness. The shorter the orbital period of the planet, the easier it is to detect the presence of such a planet. Hence, this method is very useful in detecting the planets near their stars.
Source: Subaru Telescope
