[Paper Review] Some Comments on Possible Preferred Directions for the SETI Search

S. Nussinov|ArXiv.org|Mar 9, 2009

Space Science and Extraterrestrial Life3 references4 citations

TL;DR

The paper proposes focusing SETI searches on specific celestial directions where Earth's transit across the Sun would be detectable by extraterrestrial civilizations, based on the likelihood that advanced societies would use transit observations to discover habitable planets. This approach narrows the search to 1.5–2.5% of the sky (or up to 7% with broader stripes), increasing the probability of detecting targeted signals from civilizations that could have observed Earth's transits.

ABSTRACT

The search for extraterrestrial intelligence by looking for signals from advanced technological civilizations has been ongoing for some decades. We suggest that it could possibly be made more efficient by focusing on stars from which the solar system can be observed via mini-eclipsings of the Sun by transiting planets.

Motivation & Objective

• To identify preferred directions in the sky for SETI searches based on the visibility of Earth's transit from distant civilizations.
• To address the challenge of inefficient broad-sky radio searches by focusing on regions where Earth would be most likely to be discovered via transit observations.
• To evaluate the feasibility and implications of targeting these directions for increased signal detection probability.
• To assess the impact of solar activity and planetary orbital inclinations on detectability via transit methods.
• To explore whether the low detection rate of Earth by extraterrestrial civilizations could explain the Fermi paradox.

Proposed method

• Identify stars from which Earth's transit across the Sun is visible, defined by the angular width of the ecliptic plane relative to the Sun’s radius.
• Calculate the solid angle of the 'transit visibility stripe' for each inner planet (Mercury, Venus, Earth, Mars) based on orbital inclination and solar angular radius.
• Estimate the fraction of the sky covered by the combined visibility stripes of all inner planets, including the broader +/- 3.4° stripe.
• Use the transit depth (e.g., 77 ppm for Earth) and duration (up to 13 hours) to assess detectability by advanced civilizations.
• Model the effect of solar surface noise (sunspots, granulation) on transit detection, and identify 'quiet Sun' periods as optimal observation windows.
• Account for perturbations from moons (e.g., Earth’s Moon) and the asteroid belt, showing they do not significantly interfere with transit detection.

Experimental results

Research questions

• RQ1 Which directions in the sky are most likely to contain civilizations that have observed Earth via transit events?
• RQ2 What fraction of the sky constitutes the combined visibility stripes for transits of Mercury, Venus, Earth, and Mars?
• RQ3 How does solar activity (e.g., sunspots) affect the detectability of Earth’s transit by extraterrestrial observers?
• RQ4 Could the low number of civilizations detecting Earth via transits explain the Fermi paradox?
• RQ5 To what extent do orbital inclinations and planetary system geometry limit the number of civilizations capable of detecting Earth?

Key findings

• The combined visibility stripes for transits of Mercury, Venus, Earth, and Mars cover approximately 1.5% to 2.5% of the celestial sphere, increasing to about 7% when including a broader +/- 3.4° stripe.
• Earth’s transit is detectable from a stripe of angular width ±0.28° (1/200 of the sky), corresponding to a solid angle of 2π × 2θ(Earth) ≈ 0.03 steradians.
• Venus transits are visible from a stripe of width ±0.36° (1/140 of the sky), and Mars transits from a stripe of width ±0.5°, due to their orbital distances.
• Jupiter and Saturn transits, though deeper (1% dimming), are observable from much narrower stripes and occur only every 12 and 20 years, respectively.
• The total solid angle covered by all inner planet transits (excluding Mercury) is ~1.5% of the sky, rising to ~2.5% when Mercury is included.
• Quiet solar conditions—free of sunspots and granulation noise—occur periodically and could extend the detection window for transits by up to 300 years, due to light-travel time effects.