March 12, 2002


Peabody Energy Corporation
701 Market Street
St. Louis, Missouri 63103

Ladies and Gentlemen:

                  We have acted as counsel to Peabody Energy Corporation, a
Delaware corporation (the "Company"), in connection with the Registration
Statement on Form S-3 (the "Registration Statement") filed by the Company with
the Securities and Exchange Commission (the "Commission") under the Securities
Act of 1933, as amended (the "Act"), relating to the sale by those selling
stockholders of the Company (the "Selling Stockholders") named in the
Registration Statement, of an aggregate of up to 10,000,000 shares of common
stock, par value $0.01 per share, which shares have been issued and are
outstanding (together with any additional shares of such stock that have been
issued and are outstanding that may be sold by the Selling Stockholders
pursuant to Rule 462(b) (as prescribed by the Commission pursuant to the Act),
in connection with the offering described in the Registration Statement, the
"Shares").

                  We have examined the Registration Statement and a form of
the share certificate, which has been filed with the Commission as an exhibit
to the Registration Statement. We also have examined originals, or duplicates
or certified or conformed copies, of such records, agreements, instruments and
other documents and have made such other and further investigations as we have
deemed relevant and necessary in connection with the opinion




Peabody Energy Corporation           -2-                        March 12, 2002



expressed herein. As to questions of fact material to this opinion, we have
relied upon certificates of public officials and of officers and
representatives of the Company.

                  In such examination, we have assumed the genuineness of all
signatures, the legal capacity of natural persons, the authenticity of all
documents submitted to us as originals, the conformity to original documents
of all documents submitted to us as duplicates or certified or conformed
copies, and the authenticity of the originals of such latter documents.

                  Based upon the foregoing, and subject to the qualifications
and limitations stated herein, we are of the opinion that the Shares have been
duly authorized and are validly issued, fully paid and non-assessable.

                  We are members of the Bar of the State of New York, and we
do not express any opinion herein concerning any law other than the Delaware
General Corporation Law (including the statutory provisions, all applicable
provisions of the Delaware Constitution and reported judicial decisions
interpreting the foregoing).

                  We hereby consent to the filing of this opinion letter as
Exhibit 5.1 to the Registration Statement and to the use of our name under the
caption "Legal Matters" in the Prospectus included in the Registration
Statement.

                                                 Very truly yours,

                                                 /s/ Simpson Thacher & Bartlett

                                                 SIMPSON THACHER & BARTLETT