孿生太陽周圍發現一顆孿生木星

據歐洲南天觀察台ESO的網站報道：一個由巴西天文學家領導的國際研究小組，用歐洲南天觀察台的HARPS（High Accuracy Radial velocity Planet Searcher），發現了一個孿生太陽系，並在其中發現了一個孿生的木星。

這裡的孿生指的是和我們的太陽大小和年齡都非常接近的一顆恆星。它的名稱是Hip 11915 。位於鯨魚座內。

HIP11915在天空的位置，用紅圈標出。它太暗淡了，肉眼無法觀察到，但用雙筒望遠鏡可以看到的。

這顆孿生木星的質量和我們太陽系的木星的質量幾乎一樣大小，並且它圍繞其宿主恆星的距離也和我們的木星非常接近的。

這一發現有里程碑的意義。按照目前的天文學理論，木星在太陽系行星系統的結構形成過程中及後來的演化過程有決定性作用。比如：木星的巨大引力傘保護了地球被其它天體撞擊的可能性大為降低了。同時，它的巨大引力使得地球的自轉軸幾乎不可能和其公轉軌道的平面垂直。這是地球上四季的由來。並且使得在它的內圈，不太可能存在氣體行星，只能是岩石行星存在——指的是，木星在離太陽合適的距離上，才會出現這樣的結果。

這一木星和其宿主恆星的諸多參數和我們的太陽系幾乎一樣，這意味着，在它的內圈，有像地球一樣的行星存在的概率是極大的。

在美國天文學家Donald Brownlee在他的《罕見的地球》一書中，所謂的宇宙生命公式中，木星是一個重要的因子。孿生木星的發現，使得他公式中的諸多項的概率等於1。

我們期待着在這顆恆星的周圍，也許會發現有生命的“地球”。

下面是ESO網站報道的英文全文。

Jupiter Twin Discovered Around Solar Twin

Brazilian-led team leading the search for a Solar System 2.0

An international group of astronomers has used the ESO 3.6-metre telescope to identify a planet just like Jupiter orbiting at the same distance from a Sun-like star, HIP 11915. According to current theories, the formation of Jupiter-mass planets plays an important role in shaping the architecture of planetary systems. The existence of a Jupiter-mass planet in a Jupiter-like orbit around a Sun-like star opens the possibility that the system of planets around this star may be similar to our own Solar System. HIP 11915 is about the same age as the Sun and, furthermore, its Sun-like composition suggests that there may also be rocky planets orbiting closer to the star.

So far, exoplanet surveys have been most sensitive to planetary systems that are populated in their inner regions by massive planets, down to a few times the mass of the Earth [1]. This contrasts with our Solar System, where there are small rocky planets in the inner regions and gas giants like Jupiter farther out.

According to the most recent theories, the arrangement of our Solar System, so conducive to life, was made possible by the presence of Jupiter and the gravitational influence this gas giant exerted on the Solar System during its formative years. It would seem, therefore, that finding a Jupiter twin is an important milestone on the road to finding a planetary system that mirrors our own.

A Brazilian-led team has been targeting Sun-like stars in a bid to find planetary systems similar to our Solar System. The team has now uncovered a planet with a very similar mass to Jupiter [2], orbiting a Sun-like star, HIP 11915, at almost exactly the same distance as Jupiter. The new discovery was made using HARPS, one of the world’s most precise planet-hunting instruments, mounted on the ESO 3.6-metre telescope at the La Silla Observatory in Chile.

Although many planets similar to Jupiter have been found [3] at a variety of distances from Sun-like stars, this newly discovered planet, in terms of both mass and distance from its host star, and in terms of the similarity between the host star and our Sun, is the most accurate analogue yet found for the Sun and Jupiter.

The planet’s host, the solar twin HIP 11915, is not only similar in mass to the Sun, but is also about the same age. To further strengthen the similarities, the composition of the star is similar to the Sun’s. The chemical signature of our Sun may be partly marked by the presence of rocky planets in the Solar System, hinting at the possibility of rocky planets also around HIP 11915.

According to Jorge Melendez, of the Universidade de São Paulo, Brazil, the leader of the team and co-author of the paper, “the quest for an Earth 2.0, and for a complete Solar System 2.0, is one of the most exciting endeavors in astronomy. We are thrilled to be part of this cutting-edge research, made possible by the observational facilities provided by ESO.” [4]

Megan Bedell, from the University of Chicago and lead author of the paper, concludes: “After two decades of hunting for exoplanets, we are finally beginning to see long-period gas giant planets similar to those in our own Solar System thanks to the long-term stability of planet hunting instruments like HARPS. This discovery is, in every respect, an exciting sign that other solar systems may be out there waiting to be discovered.”

Follow-up observations are needed to confirm and constrain the finding, but HIP 11915 is one of the most promising candidates so far to host a planetary system similar to our own.

Notes

[1] The current detection techniques are more sensitive to large or massive planets close to their host stars. Small and low-mass planets are mostly beyond our current capabilities. Giant planets that orbit far from their host star are also more difficult to detect. Consequently, many of the exoplanets we currently know are large and/or massive, and close to their stars.

[2] The planet was discovered by measuring the slight wobble it imposes on its host star while orbiting around it. As the inclination of the planet’s orbit is not known, only a lower limit to its mass can be estimated. Note that the activity of the star, which is linked to the variations of its magnetic field, could possibly mimic the signal that is interpreted as the signature of the planet. The astronomers have performed all the known tests to investigate this possibility, but it is currently impossible to completely rule it out.

[3] An example of another Jupiter Twin is the one around HD 154345, described here.

[4] Since the signature of the Brazilian accession agreement in December 2010, Brazilian astronomer have had full access to the ESO observing facilities.

More information

This research was presented in a paper entitled “The Solar Twin Planet Search II. A Jupiter twin around a solar twin”, by M. Bedell et al., to appear in the journal Astronomy and Astrophysics.

The team is composed of M. Bedell (Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois, USA; Visiting Researcher at the Departamento de Astronomia do IAG/USP, Universidade de São Paulo, São Paulo, Brazil), J. Meléndez (Universidade de São Paulo, São Paulo, Brazil), J. L. Bean (Department of Astronomy and Astrophysics, University of Chicago), I. Ramírez (McDonald Observatory and Department of Astronomy, University of Texas, Austin, Texas, USA), M. Asplund (Research School of Astronomy and Astrophysics, The Australian National University, Weston, Australia), A. Alves-Brito (Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazil), L. Casagrande (Research School of Astronomy and Astrophysics, Australia), S. Dreizler (Institut für Astrophysik, University of Göttingen, Germany), T. Monroe (Universidade de São Paulo, Brazil), L. Spina (Universidade de São Paulo, Brazil) and M. Tucci Maia (Universidade de São Paulo, Brazil).

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.