The sun might seem like a pretty huge galactic blind spot, but we've already managed to glimpse behind it, and there's nothing there in the way of another Earth, says NASA scientist Michael Kaiser, "unless it's awfully tiny."
Kaiser is the project scientist for NASA's Solar Terrestrial Relations Observatory (STEREO) mission, which in 2006 sent two golf-cart-size satellites into orbit around the sun to study the explosions on the solar surface that are a major factor in space weather. A few months after their launch, the two probes were angled such that they could see beyond the sun, but they found no planets lurking behind the big star.
Even if we couldn't see behind the sun, the gravitational pull of a roughly 100-mile-wide planet hiding there would noticeably affect the orbits of the other planets. And if astronomers had somehow missed that detail, Kaiser says, an unaccounted-for tug of gravity in the solar system would have disrupted the orbit of man-made satellites circling the Earth or interfered with intra-solar-system spacecraft. That hasn't happened, so unless beings on a hidden planet have invented both an invisibility cloak and a gravity-masking device, the other side of the sun is almost certainly just empty space.
Srimad Bhagavatam Summary of Canto 5 Chapter Twenty-four
The Subterranean Heavenly Planets
This chapter describes the planet Rahu, which is 10,000 yojanas (80,000 miles) below the sun, and it also describes Atala and the other lower planetary systems. Rahu is situated below the sun and moon. It is between these two planets and the earth. When Rahu conceals the sun and moon, eclipses occur, either total or partial, depending on whether Rahu moves in a straight or curving way.
Below Rahu by another 1,000,000 yojanas are the planets of the Siddhas, Caranas and Vidyadharas, and below these are planets such as Yaksaloka and Raksaloka. Below these planets is the earth, and 70,000 yojanas below the earth are the lower planetary systems--Atala, Vitala, Sutala, Talatala, Mahatala, Rasatala and Patala. Demons and Raksasas live in these lower planetary systems with their wives and children, always engaged in sense gratification and not fearing their next births. The sunshine does not reach these planets, but they are illuminated by jewels fixed upon the hoods of snakes. Because of these shining gems there is practically no darkness. Those living in these planets do not become old or diseased, and they are not afraid of death from any cause but the time factor, the Supreme Personality of Godhead.
In the planet Atala, the yawning of a demon has produced three kinds of women, called svairini (independent), kamini (lusty) and pumscali (very easily subdued by men). Below Atala is the planet Vitala, wherein Lord Siva and his wife Gauri reside. Because of their presence, a kind of gold is produced called hataka. Below Vitala is the planet Sutala, the abode of Bali Maharaja, the most fortunate king. Bali Maharaja was favored by the Supreme personality of Godhead, Vamanadeva, because of his intense devotional service. The Lord went to the sacrificial arena of Bali Maharaja and begged him for three paces of land, and on this plea the Lord took from him all his possessions. When Bali Maharaja agreed to all this, the Lord was very pleased, and therefore the Lord serves as his doorkeeper. The description of Bali Maharaja appears in the Eighth Canto of Srimad-Bhagavatam.
When the Supreme Personality of Godhead offers a devotee material happiness, this is not His real favor. The demigods, who are very puffed up by their material opulence, pray to the Lord only for material happiness, not knowing anything better. Devotees like Prahlada Maharaja, however, do not want material happiness. Not to speak of material happiness, they do not want even liberation from material bondage, although one can achieve this liberation simply by chanting the holy name of the Lord, even with improper pronunciation.
Below Sutala is the planet Talatala, the abode of the demon Maya. This demon is always materially happy because he is favored by Lord Siva, but he cannot achieve spiritual happiness at any time. Below Talatala is the planet Mahatala, where there are many snakes with hundreds and thousands of hoods. Below Mahatala is Rasatala, and below that is Patala, where the serpent Vasuki lives with his associates.