Water ice in the young planetary system - future ocean of the planets?
February 17, 2009
What is the origin of the ocean on the Earth where the life began? Researchers think that the Earth was formed by the aggregation of a huge amount of dust particles in the circumstellar disk around the Sun during its birth. There is a hypothesis that water ice in the dust at that time is the material of the sea on the Earth. The observation from Subaru Telescope shows that there is water ice in the gas and dust disk around a young star HD142527 toward the constellation Lupus. The ice discovered by Subaru may become the sea on a planet revolving around HD142527 in the future.
About 4 billion years ago, the first life was formed in the ocean that covered the surface of the Earth. Ever since, the water is essential for life on the Earth. For example, the primary composition of the human body is water. While the Earth is called "water planet", where does this abundant water come from?
The formation of the planets in the Solar system including the Earth is described by scientists as following; about 4.6 billion years ago, there was a disk consisting of gas and dust around the Sun which had been just formed. In the disk, dust particles collided and stuck with each other, then, these particles aggregated and finally became the planets. At that time, some particles of dust making the Earth had ice, the solid water. In addition, many comets (bodies of dust and ice) bombarded the early Earth.
A hypothesis of the origin of the sea on the Earth goes as the following; ice from the dust particles and the comets heated up and evaporated due to the heat from their collisions. Some portion of the water vapor was trapped in the atmosphere of the early Earth. Then, the vapor cooled down and became the rain, and then, fell down to create the sea. Thus the water ice in the dust particles in the protosolar system could be the source of the sea on the Earth.(Note 1)
State-of-the-art telescopes in the world such as the Hubble Space Telescope and the Subaru Telescope have discovered protoplanetary disks around young stars. Those gas and dust disks are considered to be the formation site of the planets. More than 300 extrasolar planets have been detected since 1995. However, there has been no planets discovered to have oceans like the Earth does.
Protoplanetary disks where planets are now forming should have plenty of dust with ice as the primordial disk in the solar system once did. Indeed, previous observations found some indication of the ice inside and around such disks. However, the location is not resolved where the abundant ice is.(Note 2) This research team had an idea of depicting the light scattered at the surface of the disk after being emitted from the central star. What is the signal of ice imprinted in this light? Molecules of water ice absorb the light at the wavelength of 3.1 micron, in the infrared wavelength longer than that of the visible light. If the light is scattered by the ice on the disk surface, the scattered light at the wavelength of 3.1 micron becomes fainter than that at other wavelengths because of the absorption. On the other hand, there is no such absorption in the light scattered by dust without ice. With this method, they were able to examine whether ice exists in the disk surface or not.(Note 3)
Observations of this research team were performed on 30 June 2005 and 26 July 2007 with the Coronagraphic Imager with Adaptive Optics (CIAO) on the Subaru Telescope. The target is a young star 650 light-year away from the Earth, HD142527, in the Constellation Lupus. The mass and the age of the star are estimated to be about 2 times larger than the Sun and about 2 million years, respectively. Previous observations at Subaru Telescope showed that this star has a protoplanetary disk in which planets may be forming (Press Release in 2006, "Diversity the Norm in Protoplanetary Disks: Astronomers Find Donuts, Spirals and Now Banana Splits"). The research team took images of the disk around the star at the wavelengths of 3.1 micron and 3.8 micron. These images show the distribution of the light from the central star scattered at the surface of the disk. When the observed intensities of the scattered light at 3.1 micron and 3.8 micron were compared with the previous measurements at around 2 micron, the research team found that the scattered light at 3.1 micron was really fainter than that at other wavelengths. Ice does exist on the disk surface!
Water ice was found in the disk surface more than 100 AU (1 AU is the distance between the Sun and the Earth) away from the central star. The planet formation site is probably closer to the central star.The water ice reported in this article may be incorporated into comets in the future rather than to the planets. However, the ice may supply the water on the planets. The ocean on a planet is a big step forward to harbor life forms there.
The observations from Subaru Telescope reported here show that water ice exists in the disk surface relatively far from the central star. How about the region closer to the star where planets are forming? Unfortunately, the inner part of the disk is difficult to study because the light of the central star is too bright to see anything small or faint. Ice in a part near the central star would evaporate because of the heat from the central star. Thus, water stays in solid form at a certain distance away from the central star. The boundary between the solid water and the water vapor is called "snow line". Due to the significance of the snow line in the process of the planet formation, future observations will be targeted to define the location of the snow line.
This research was made by a team of scientists from the Kanagawa University, Osaka Sangyo University, Osaka University, Tokyo Institute of Technology, National Astronomical Observatory of Japan, Kyoto Sangyo University, Ibaraki University, and Kobe University, and was published in The Astrophysical Journal Letters, vol.690, pp.110-113 (issue published on 10 January 2009).
Mitsuhiko Honda (Kanagawa University)
Akio Inoue (Osaka Sangyo University)
Motohide Tamura (NAOJ)
Note 1: There are three hypotheses of the origin of water on the Earth: (1) ice (solid water) in dust which made the early Earth, (2) ice in comets which fell on the early Earth just formed, and (3) chemical reaction between hydrogen from the gas of the protopnanetary disk and oxygen in the magma ocean on the early Earth.
Note 2: Malfait et al. reported the detection of emission lines of ice at the wavelengths of 44 micron and 62 micron from the star HD142527, with the Infrared Space Observatory, ISO. However, it was unclear whether the ice exists in the protoplanetary disk around HD142527 or in the gas cloud surrounding the star because the spatial resolution of the ISO was not adequate (Malfait et al. 1999, A&A, 345, 181).
Terada et al. found an absorption of ice at the wavelength of 3.1 micron with spectroscopic observations (i.e. observations to measure intensity at each wavelength) of two protoplanetary disks with the Subaru Telescope. However, it was uncertain where in the disks the ice exists at that time (Terada et al. 2007, ApJ, 667, 303).
Eisner found water vapor in another gaseous disk within 1 AU from the central star with the near-infrared interferometer of the Very Large Telescope (Eisner 2007, Nature, 447, 562). This water vapor is interpreted as a result of the evaporation of ice in the disk heated by the central star.
Note 4: Let's compare the scale with the Solar system. The distance from the Sun to the Earth is 1 AU, Jupiter is at 5 AU, Saturn is at 10 AU, and Neptune is at 30 AU. Further than the Neptune, there are dwarf planets like the Pluto and many small icy bodies (called trans-Neptunian objects). Some of them probably travel closer to the Sun and are seen as comets.
Recent observations showed that planets can be formed at a distance of 100AU (for example, a planet around Fohmalhaut, Kalas et al. 2008, Science, 322, 1345). So, it is possible that ices detected around HD142527 can become the ingredients of planets.