Iota Horologii

ESO






Larger image.






Constellation Horologium is
located near constellations
Orion and Taurus and the
bright stars Canopus and
Sirius. Iota Horologii appears
to have drifted from the
Hyades Cluster in Taurus
after its birth more than
600 million years.

In 1999, astronomers announced the discovery of a Jupiter-like planet around this Sun-like star (Kürster et al, 2000; and ESO press release -- details below), with indications of at least one other planet in an outer orbit. A subsequent announcement of the discovery of a dust disk around the star was later determined to be an instrumental artifact (ADONIS press release in 2001). (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.) As Iota Horologii has become one of the top 100 target stars for NASA's planned Terrestrial Planet Finder (TPF), images of this star and its position relative to the Milky Way in Earth's night sky are now available from the TPF-C team.

JPL, CalTech, NASA


Larger illustration


Astronomers have identified Iota
Horologii as a prime target for the
Terrestrial Planet Finder (TPF),
now planned for launch between
2014 and 2020.

The Star

Iota Horologii is a yellow-orange main sequence dwarf star of spectral and luminosity type G0 Vp, but it has also been previously classified as G3 and a subgiant (IV). It has about 1.25 times the mass of Sol (based on asteroseismology, see ESO press releases of April 15, 2008; and July 29, 1999; and Vauclair et al, 2008), 1.5 times its diameter, and 1.8 times its luminosity. It appears to more enriched than Sol (around 50 percent more) in elements heavier than hydrogen based on its abundance of iron (ESO press release and Vauclair et al, 2008). The star exhibits higher chromospheric activity than Sol and so is probably around only 625 million years old, much younger than Sol's 4.6 billion years (ESO press release and Vauclair et al, 2008). Useful catalogue numbers and designations for the star include: Iot Hor, HR 810, Gl 108, Hip 12653, HD 17051, CD-51 641, CP(D)-51 330, SAO 232864, and LTT 1322.

ESO









Larger image.









Iota Horologii has
at least one large
Jupiter-class
planetary companion.

Planet "b"

In 1999, a team of astronomers (Martin Kürster, Michael Endl, S. Els, Artie P. Hatzes, William D. Cochran, S. Döbereiner, and Konrad Dennerl) announced the discovery of a Jupiter-class planet around Iota Horologii using radial-velocity analysis (Kürster et al, 2000; and ESO press release). Planet b has at least 2.1 times Jupiter's mass. It moves around Iota Horologii at an average distance of only 0.92 AUs in a elliptical orbit (e=0.18, see HD 17051 at exoplanets.org). This orbit would cross the orbits of Earth and Venus in the Solar System and take almost a year (314 days) to complete. In addition, a long term trend in the residuals suggest that either there is a second planet in in the system, or activity on the surface of the star causes slight changes in its spectrum, influencing radial velocity measurements.

© Christoph Kulmann -- larger image


Artwork from Exoplaneten.de
(used with permission).


An interstellar "starship" maneuvers
into orbit around a large moon of
planet candidate "b," as imagined
by Kulmann (more).

Recent astrometric analysis, however, suggests that planet b may have as much as 24 times the mass of Jupiter with an inclination of 5.5° from Earth's line of sight (Han et al, 2000). Thus, the "planet" could be an extremely dim brown dwarf, substellar companion of Iota Horologii. The authors consider their analysis to be preliminary, needing confirmation with additional astrometric as well as other observations.

The orbit of an Earth-like planet (with liquid water) around Iota Horologii may be centered around 1.3 AU -- between the orbital distances of Earth and Mars in the Solar System -- with an orbital period of about 1.5 years. However, the presence of planet b at its orbital distance of around 0.9 AU may have disrupted the development of an Earth-type planet in the water zone. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.) Astronomers are hoping to use NASA's Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around Iota Horologii. As currently planned, the TPF will include two complementary observatory groups: a visible-light coronagraph to launch around 2014; and a "formation-flying" infrared interferometer to launch before 2020, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub beginning in 2015.

Brown Dwarfs or Planets?

When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.

© American Scientist



Artwork by Linda Huff
(for Martin et al, 1997)
used with permission.

University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover a nearby brown dwarf, Gliese 229 b, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 80 (or so) Jupiters. While these objects cannot fuse "ordinary" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter masses would be defined as planets.

Other prominent astronomers, such as San Francisco State University astronomer Geoffrey W. Marcy who also has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. Similarly, there may also be many different processes that lead to the creation of brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.