Reasons to Believe

Search for the Sun’s Twin

The conclusion that Earth displays many unique, apparently designed, features that enable it to support life, and, in particular advanced life (known as the rare planet doctrine) is now well established within the scientific community.

The discovery of over 270 extrasolar planets, combined with spacecraft missions to explore the outer solar system, likewise is beginning to establish the rare planetary system doctrine. In July 2007, I wrote a TNRTB about the emerging rare Milky Way Galaxy (MWG) doctrine. Now, yet another new principle is arising to complement the others: the rare Sun doctrine.

Many features of a galaxy, planetary system, star, and planet must be fine-tuned in order to support life. I list and describe several such requirements in The Creator and the Cosmos1 and in Creation As Science.2 The observed rarity of galaxies, planetary systems, and stars exhibiting the same life-essential properties as the MWG, the solar system, and the Sun confirms the fine-tuning.

For over fifty years astronomers have scoured our Milky Way Galaxy in search of a star that would qualify as the Sun’s twin, possessing all the characteristics necessary to possibly harbor a planet that could sustain advanced life. The thinking in the astronomical community was that given the MWG’s two hundred billion stars and the great variety of differing properties for all those stars, it should not be too difficult to find many exact analogs to the Sun. However, in spite of diligent searches over several decades, astronomers have found only four “twins” of the Sun: the stars 18 Scorpii, HD 98618, HR 6060, and HIP 100963.3 The table below lists the observed properties for three of them relative to the Sun’s.

  18 Scorpii HD 98618 HR 6060
Mass 1.02 1.02 1.01
Luminosity 1.05 1.06 1.05
Metallicity 1.05 1.12 1.12
Age 0.85 0.91 1.05
Rotation 0.90 0.96 ?
Temperature 1.01 1.01 1.003

All three of the stars listed in the table are only marginally solar twins. According to astronomers Jorge Meléndez and Iván Ramírez, they should all be called “quasi-solar twins.”4 Moreover, all four stars have a lithium abundance factor of 3-6 times higher than what astronomers measure in the Sun. Thus, the Sun has no true twin. There is no known star similar enough to the Sun to qualify as a candidate for possessing a habitable planet. In the words of three astronomers who have spent most of their careers searching for a true solar twin, “Despite the observational effort carried out in the last few decades, no perfect solar twin has been found.”5

The lack of similarity between the three stars and the Sun raises the question, does any star in the MWG possess a true twin? The answer is an unequivocal “yes.” A great many stars have twins. In fact, two stars in the table, namely 18 Scorpii and HD 98618, are twins in that all of their observed properties are identical to within astronomers’ measurement uncertainties. Thus, the Sun’s lack of a twin seems to be indicative of supernatural design for the specific benefit of human beings.

In a paper in the November 10, 2007 issue of Astrophysical Journal Letters, Meléndez and Ramírez claim to have found a true solar twin.6 They present measurements on the star HIP 56948 showing that its characteristics are closer to solar values than any of the stars listed in the table. What they find especially exciting is that HIP 56948 apparently has a lithium abundance nearly the same as the Sun’s.

Meléndez and Ramírez identified HIP 56948 as the best solar twin by searching through a sample of more than 100,000 stars in the Hipparcos Catalog of stars with accurate distance determinations. (Accurate distance determinations are crucial for establishing the properties of the stars.) For comparison, the measured characteristics of HIP 56948 have been added to the table below.

  18 Scorpii HD 98618 HR 6060 HIP 56948
Mass 1.02 1.02 1.01 1.00
Luminosity 1.05 1.06 1.05 1.15
Metallicity 1.05 1.12 1.12 1.02
Age 0.85 0.91 1.05 1.26
Rotation 0.90 0.96 ? ?
Temperature 1.01 1.01 1.003 1.00

Unlike the quasi-solar twins (18 Scorpii, HD 98618, and HR 6060) HIP 56948 exhibits a lithium abundance indistinguishable from that of the Sun. Its mass and temperature measure to be the same as the Sun. Its metallicity is only slightly higher than the Sun’s. However, it is 15 percent more luminous than the Sun and 1.2 billion years older.

As long as a star has hydrogen to burn in its core, it will get brighter as it ages. This brightening happens because, as hydrogen is fused into helium as a result of nuclear fusion burning, the added helium increases the core density of the star. This increased core density causes nuclear fusion to proceed with greater efficiency. The 15 percent higher luminosity of HIP 56948 relative to the Sun is in keeping with its older age.

The greater age and increased luminosity of HIP 56948 makes it a noncandidate for harboring a planet that could possibly support life. If the Sun were to get brighter by just 1 percent, it would make Earth sterile.

The Creator carefully introduced just the right amounts and kinds of life-forms on Earth to lower greenhouse gas levels in the planet’s atmosphere so as to perfectly compensate for the increasing luminosity of the Sun. However, that decrease cannot continue much longer. Greenhouse gases in Earth’s atmosphere already have been drawn down to a level barely above the minimum amount necessary to permit the existence of photosynthetic life. Astronomers calculate that within 30 million years from now the Sun will either heat Earth’s surface beyond what life can tolerate or the quantity of carbon dioxide and water vapor in Earth’s atmosphere will be at levels too low to support life.

HIP 56948 is a better twin for the Sun than any of the other candidates. However, it is still a quasi-solar twin in that its properties rule out the possibility that it could possess a planet like Earth. To quote Meléndez and Ramirez, “The question of whether the Sun is unique or not [is] a question that has important philosophical consequences. An anomalous Sun favors some forms of the anthropic principle.”7 I would agree and add that it favors the anthropic principle that says the universe and its constituent components were exquisitely designed by a supernatural, super-intelligent Creator to provide a bountiful home for human beings.

Subjects: Habitable Planets, SETI, Solar System Design, TCM - Cosmic Design

Dr. Hugh Ross

Reasons to Believe emerged from my passion to research, develop, and proclaim the most powerful new reasons to believe in Christ as Creator, Lord, and Savior and to use those new reasons to reach people for Christ. Read more about Dr. Hugh Ross.

References:

  1. Hugh Ross, The Creator and the Cosmos 3rd ed. (Colorado Springs: NavPress, 2001), 176-99.
  2. Hugh Ross, Creation As Science (Colorado Springs: NavPress, 2006), 102-6.
  3. G. F. Porto de Mello and L. da Silva, “HR 6060: The Closest Ever Solar Twin?” Astrophysical Journal Letters 482 (June 1997): L89-L92; Jorge Meléndez, Katie Dodds-Eden, and José A. Robles, “HD 98618: A Star Closely Resembling Our Sun,” Astrophysical Journal Letters 641 (April 2006): L133-L136; Y. Takeda et al., “Behavior of Li Abundances in Solar-Analog Stars: Evidence for Line-Width Dependence,” Astronomy and Astrophysics 468 (June 2007): 663-77.
  4. Jorge Meléndez and Iván Ramírez, “HIP 56948: A Solar Twin with a Low Lithium Abundance,” Astrophysical Journal Letters 669 (November 2007): L89-92.
  5. Meléndez, Dodds-Eden, and Robles, “HD 98618,” L133.
  6. Meléndez and Ramírez, “HIP 56948,” L89-L92.
  7. Ibid., L92.